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2.5: Properties of Water

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• Page ID 74484

Skills to Develop

• Describe the properties of water that are critical to maintaining life
• Explain why water is an excellent solvent
• Provide examples of water’s cohesive and adhesive properties
• Discuss the role of acids, bases, and buffers in homeostasis

Why do scientists spend time looking for water on other planets? Why is water so important? It is because water is essential to life as we know it. Water is one of the more abundant molecules and the one most critical to life on Earth. Approximately 60–70 percent of the human body is made up of water. Without it, life as we know it simply would not exist.

The polarity of the water molecule and its resulting hydrogen bonding make water a unique substance with special properties that are intimately tied to the processes of life. Life originally evolved in a watery environment, and most of an organism’s cellular chemistry and metabolism occur inside the watery contents of the cell’s cytoplasm. Special properties of water are its high heat capacity and heat of vaporization, its ability to dissolve polar molecules, its cohesive and adhesive properties, and its dissociation into ions that leads to the generation of pH. Understanding these characteristics of water helps to elucidate its importance in maintaining life.

Water’s Polarity

One of water’s important properties is that it is composed of polar molecules: the hydrogen and oxygen within water molecules (H 2 O) form polar covalent bonds. While there is no net charge to a water molecule, the polarity of water creates a slightly positive charge on hydrogen and a slightly negative charge on oxygen, contributing to water’s properties of attraction. Water’s charges are generated because oxygen is more electronegative than hydrogen, making it more likely that a shared electron would be found near the oxygen nucleus than the hydrogen nucleus, thus generating the partial negative charge near the oxygen.

As a result of water’s polarity, each water molecule attracts other water molecules because of the opposite charges between water molecules, forming hydrogen bonds. Water also attracts or is attracted to other polar molecules and ions. A polar substance that interacts readily with or dissolves in water is referred to as hydrophilic (hydro- = “water”; -philic = “loving”). In contrast, non-polar molecules such as oils and fats do not interact well with water, as shown in Figure \(\PageIndex{1}\) and separate from it rather than dissolve in it, as we see in salad dressings containing oil and vinegar (an acidic water solution). These nonpolar compounds are called hydrophobic (hydro- = “water”; -phobic = “fearing”).

Water’s States: Gas, Liquid, and Solid

The formation of hydrogen bonds is an important quality of the liquid water that is crucial to life as we know it. As water molecules make hydrogen bonds with each other, water takes on some unique chemical characteristics compared to other liquids and, since living things have a high water content, understanding these chemical features is key to understanding life. In liquid water, hydrogen bonds are constantly formed and broken as the water molecules slide past each other. The breaking of these bonds is caused by the motion (kinetic energy) of the water molecules due to the heat contained in the system. When the heat is raised as water is boiled, the higher kinetic energy of the water molecules causes the hydrogen bonds to break completely and allows water molecules to escape into the air as gas (steam or water vapor). On the other hand, when the temperature of water is reduced and water freezes, the water molecules form a crystalline structure maintained by hydrogen bonding (there is not enough energy to break the hydrogen bonds) that makes ice less dense than liquid water, a phenomenon not seen in the solidification of other liquids.

Water’s lower density in its solid form is due to the way hydrogen bonds are oriented as it freezes: the water molecules are pushed farther apart compared to liquid water. With most other liquids, solidification when the temperature drops includes the lowering of kinetic energy between molecules, allowing them to pack even more tightly than in liquid form and giving the solid a greater density than the liquid.

The lower density of ice, illustrated and pictured in Figure \(\PageIndex{2}\), an anomaly, causes it to float at the surface of liquid water, such as in an iceberg or in the ice cubes in a glass of ice water. In lakes and ponds, ice will form on the surface of the water creating an insulating barrier that protects the animals and plant life in the pond from freezing. Without this layer of insulating ice, plants and animals living in the pond would freeze in the solid block of ice and could not survive. The detrimental effect of freezing on living organisms is caused by the expansion of ice relative to liquid water. The ice crystals that form upon freezing rupture the delicate membranes essential for the function of living cells, irreversibly damaging them. Cells can only survive freezing if the water in them is temporarily replaced by another liquid like glycerol.

Link to Learning

Video: Click here to see a 3-D animation of the structure of an ice lattice. (Image credit: Jane Whitney. Image created using Visual Molecular Dynamics VMD software. 2 )

Water’s High Heat Capacity

Water’s high heat capacity is a property caused by hydrogen bonding among water molecules. Water has the highest specific heat capacity of any liquids. Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius. For water, this amount is one calorie . It therefore takes water a long time to heat and long time to cool. In fact, the specific heat capacity of water is about five times more than that of sand. This explains why the land cools faster than the sea. Due to its high heat capacity, water is used by warm blooded animals to more evenly disperse heat in their bodies: it acts in a similar manner to a car’s cooling system, transporting heat from warm places to cool places, causing the body to maintain a more even temperature.

Water’s Heat of Vaporization

Water also has a high heat of vaporization , the amount of energy required to change one gram of a liquid substance to a gas. A considerable amount of heat energy (586 cal) is required to accomplish this change in water. This process occurs on the surface of water. As liquid water heats up, hydrogen bonding makes it difficult to separate the liquid water molecules from each other, which is required for it to enter its gaseous phase (steam). As a result, water acts as a heat sink or heat reservoir and requires much more heat to boil than does a liquid such as ethanol (grain alcohol), whose hydrogen bonding with other ethanol molecules is weaker than water’s hydrogen bonding. Eventually, as water reaches its boiling point of 100° Celsius (212° Fahrenheit), the heat is able to break the hydrogen bonds between the water molecules, and the kinetic energy (motion) between the water molecules allows them to escape from the liquid as a gas. Even when below its boiling point, water’s individual molecules acquire enough energy from other water molecules such that some surface water molecules can escape and vaporize: this process is known as evaporation .

The fact that hydrogen bonds need to be broken for water to evaporate means that a substantial amount of energy is used in the process. As the water evaporates, energy is taken up by the process, cooling the environment where the evaporation is taking place. In many living organisms, including in humans, the evaporation of sweat, which is 90 percent water, allows the organism to cool so that homeostasis of body temperature can be maintained.

Water’s Solvent Properties

Since water is a polar molecule with slightly positive and slightly negative charges, ions and polar molecules can readily dissolve in it. Therefore, water is referred to as a solvent , a substance capable of dissolving other polar molecules and ionic compounds. The charges associated with these molecules will form hydrogen bonds with water, surrounding the particle with water molecules. This is referred to as a sphere of hydration , or a hydration shell, as illustrated in Figure \(\PageIndex{3}\) and serves to keep the particles separated or dispersed in the water.

When ionic compounds are added to water, the individual ions react with the polar regions of the water molecules and their ionic bonds are disrupted in the process of dissociation . Dissociation occurs when atoms or groups of atoms break off from molecules and form ions. Consider table salt (NaCl, or sodium chloride): when NaCl crystals are added to water, the molecules of NaCl dissociate into Na + and Cl – ions, and spheres of hydration form around the ions, illustrated in Figure \(\PageIndex{3}\). The positively charged sodium ion is surrounded by the partially negative charge of the water molecule’s oxygen. The negatively charged chloride ion is surrounded by the partially positive charge of the hydrogen on the water molecule.

Water’s Cohesive and Adhesive Properties

Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass. This water can stay above the glass because of the property of cohesion . In cohesion, water molecules are attracted to each other (because of hydrogen bonding), keeping the molecules together at the liquid-gas (water-air) interface, although there is no more room in the glass.

Cohesion allows for the development of surface tension , the capacity of a substance to withstand being ruptured when placed under tension or stress. This is also why water forms droplets when placed on a dry surface rather than being flattened out by gravity. When a small scrap of paper is placed onto the droplet of water, the paper floats on top of the water droplet even though paper is denser (heavier) than the water. Cohesion and surface tension keep the hydrogen bonds of water molecules intact and support the item floating on the top. It’s even possible to “float” a needle on top of a glass of water if it is placed gently without breaking the surface tension, as shown in Figure \(\PageIndex{4}\).

These cohesive forces are related to water’s property of adhesion , or the attraction between water molecules and other molecules. This attraction is sometimes stronger than water’s cohesive forces, especially when the water is exposed to charged surfaces such as those found on the inside of thin glass tubes known as capillary tubes. Adhesion is observed when water “climbs” up the tube placed in a glass of water: notice that the water appears to be higher on the sides of the tube than in the middle. This is because the water molecules are attracted to the charged glass walls of the capillary more than they are to each other and therefore adhere to it. This type of adhesion is called capillary action , and is illustrated in Figure \(\PageIndex{5}\).

Why are cohesive and adhesive forces important for life? Cohesive and adhesive forces are important for the transport of water from the roots to the leaves in plants. These forces create a “pull” on the water column. This pull results from the tendency of water molecules being evaporated on the surface of the plant to stay connected to water molecules below them, and so they are pulled along. Plants use this natural phenomenon to help transport water from their roots to their leaves. Without these properties of water, plants would be unable to receive the water and the dissolved minerals they require. In another example, insects such as the water strider, shown in Figure \(\PageIndex{6}\), use the surface tension of water to stay afloat on the surface layer of water and even mate there.

pH, Buffers, Acids, and Bases

The pH of a solution indicates its acidity or alkalinity.

litmus or pH paper, filter paper that has been treated with a natural water-soluble dye so it can be used as a pH indicator, to test how much acid (acidity) or base (alkalinity) exists in a solution. You might have even used some to test whether the water in a swimming pool is properly treated. In both cases, the pH test measures the concentration of hydrogen ions in a given solution.

Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules into equal numbers of hydrogen (H + ) ions and hydroxide (OH - ) ions. While the hydroxide ions are kept in solution by their hydrogen bonding with other water molecules, the hydrogen ions, consisting of naked protons, are immediately attracted to un-ionized water molecules, forming hydronium ions (H 3 0 + ). Still, by convention, scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water.

The concentration of hydrogen ions dissociating from pure water is 1 × 10 -7 moles H + ions per liter of water. Moles (mol) are a way to express the amount of a substance (which can be atoms, molecules, ions, etc), with one mole being equal to 6.02 x 10 23 particles of the substance. Therefore, 1 mole of water is equal to 6.02 x 10 23 water molecules. The pH is calculated as the negative of the base 10 logarithm of this concentration. The log10 of 1 × 10 -7 is -7.0, and the negative of this number (indicated by the “p” of “pH”) yields a pH of 7.0, which is also known as neutral pH. The pH inside of human cells and blood are examples of two areas of the body where near-neutral pH is maintained.

Non-neutral pH readings result from dissolving acids or bases in water. Using the negative logarithm to generate positive integers, high concentrations of hydrogen ions yield a low pH number, whereas low levels of hydrogen ions result in a high pH. An acid is a substance that increases the concentration of hydrogen ions (H + ) in a solution, usually by having one of its hydrogen atoms dissociate. A base provides either hydroxide ions (OH – ) or other negatively charged ions that combine with hydrogen ions, reducing their concentration in the solution and thereby raising the pH. In cases where the base releases hydroxide ions, these ions bind to free hydrogen ions, generating new water molecules.

The stronger the acid, the more readily it donates H + . For example, hydrochloric acid (HCl) completely dissociates into hydrogen and chloride ions and is highly acidic, whereas the acids in tomato juice or vinegar do not completely dissociate and are considered weak acids. Conversely, strong bases are those substances that readily donate OH – or take up hydrogen ions. Sodium hydroxide (NaOH) and many household cleaners are highly alkaline and give up OH – rapidly when placed in water, thereby raising the pH. An example of a weak basic solution is seawater, which has a pH near 8.0, close enough to neutral pH that marine organisms adapted to this saline environment are able to thrive in it.

The pH scale is, as previously mentioned, an inverse logarithm and ranges from 0 to 14 (Figure \(\PageIndex{7}\)). Anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is alkaline. Extremes in pH in either direction from 7.0 are usually considered inhospitable to life. The pH inside cells (6.8) and the pH in the blood (7.4) are both very close to neutral. However, the environment in the stomach is highly acidic, with a pH of 1 to 2. So how do the cells of the stomach survive in such an acidic environment? How do they homeostatically maintain the near neutral pH inside them? The answer is that they cannot do it and are constantly dying. New stomach cells are constantly produced to replace dead ones, which are digested by the stomach acids. It is estimated that the lining of the human stomach is completely replaced every seven to ten days.

Watch this video for a straightforward explanation of pH and its logarithmic scale.

So how can organisms whose bodies require a near-neutral pH ingest acidic and basic substances (a human drinking orange juice, for example) and survive? Buffers are the key. Buffers readily absorb excess H + or OH – , keeping the pH of the body carefully maintained in the narrow range required for survival. Maintaining a constant blood pH is critical to a person’s well-being. The buffer maintaining the pH of human blood involves carbonic acid (H 2 CO 3 ), bicarbonate ion (HCO 3 – ), and carbon dioxide (CO 2 ). When bicarbonate ions combine with free hydrogen ions and become carbonic acid, hydrogen ions are removed, moderating pH changes. Similarly, as shown in Figure \(\PageIndex{8}\), excess carbonic acid can be converted to carbon dioxide gas and exhaled through the lungs. This prevents too many free hydrogen ions from building up in the blood and dangerously reducing the blood’s pH. Likewise, if too much OH – is introduced into the system, carbonic acid will combine with it to create bicarbonate, lowering the pH. Without this buffer system, the body’s pH would fluctuate enough to put survival in jeopardy.

Other examples of buffers are antacids used to combat excess stomach acid. Many of these over-the-counter medications work in the same way as blood buffers, usually with at least one ion capable of absorbing hydrogen and moderating pH, bringing relief to those that suffer “heartburn” after eating. The unique properties of water that contribute to this capacity to balance pH—as well as water’s other characteristics—are essential to sustaining life on Earth.

To learn more about water. Visit the U.S. Geological Survey Water Science for Schools All About Water! website.

Water has many properties that are critical to maintaining life. It is a polar molecule, allowing for the formation of hydrogen bonds. Hydrogen bonds allow ions and other polar molecules to dissolve in water. Therefore, water is an excellent solvent. The hydrogen bonds between water molecules cause the water to have a high heat capacity, meaning it takes a lot of added heat to raise its temperature. As the temperature rises, the hydrogen bonds between water continually break and form anew. This allows for the overall temperature to remain stable, although energy is added to the system. Water also exhibits a high heat of vaporization, which is key to how organisms cool themselves by the evaporation of sweat. Water’s cohesive forces allow for the property of surface tension, whereas its adhesive properties are seen as water rises inside capillary tubes. The pH value is a measure of hydrogen ion concentration in a solution and is one of many chemical characteristics that is highly regulated in living organisms through homeostasis. Acids and bases can change pH values, but buffers tend to moderate the changes they cause. These properties of water are intimately connected to the biochemical and physical processes performed by living organisms, and life would be very different if these properties were altered, if it could exist at all.

• 1 W. Humphrey W., A. Dalke, and K. Schulten, “VMD—Visual Molecular Dynamics,” Journal of Molecular Graphics 14 (1996): 33-38.
• 2 W. Humphrey W., A. Dalke, and K. Schulten, “VMD—Visual Molecular Dynamics,” Journal of Molecular Graphics 14 (1996): 33-38.

• Physical And Chemical Properties Of Water

Properties Of Water: Physical And Chemical

What is water.

Water is the chemical substance with chemical formula H 2 O , one molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom. Let us learn about the physical and chemical properties of water.

Table of Content

Properties of water, physical properties of water, chemical properties of water.

• Frequently Asked Questions – FAQs

A glance of earth taken from space will depict it blue. This blue colour is actually water, the major part of the earth is covered with water. We need water for almost everything, for example- drinking, bathing, cooking etc and therefore we should know about the properties of water. 65 % human body is composed of water. Water is essential for the survival of life on earth. Water is distributed unevenly on the earth’s surface. It forms a major solvent and dissolves almost every polar solute. So, let us have a look at its properties and understand the reason for its significance:

Water is a colourless and tasteless liquid. The molecules of water have extensive hydrogen bonds resulting in unusual properties in the condensed form. This also leads to high melting and boiling points . As compared to other liquids, water has a higher specific heat , thermal conductivity, surface tension, dipole moment, etc. These properties form the reason for its significance in the biosphere. Water is an excellent solvent and therefore it helps in the transportation of ions and molecules required for metabolism. It has a high latent heat of vaporization which helps in the regulation of body temperature.

Water reacts with a lot of substances to form different compounds. Some significant reactions are as follows:

1. Amphoteric nature :

Water can act as both acid and base, which means that it is amphoteric in nature. Example:

Acidic Behaviour : 

\(\begin{array}{l}H_2O (l)   +  NH_3 (aq)  \end{array} \)  ⇌  \(\begin{array}{l}   NH^+_4 (aq) + OH^- (aq)\end{array} \)

Basic Behavior:

\(\begin{array}{l}H_2O (l)   +  H_2S (aq) \end{array} \)    ⇌     \(\begin{array}{l}H_3O^+ (aq)     +   HS^-   (aq)\end{array} \)

2. Redox reactions :

Electropositive elements reduce water to hydrogen molecule. Thus, water is a great source of hydrogen. Let us see an example in this case:

\(\begin{array}{l}2H_2O(l) + 2Na(s) \rightarrow 2NaOH(aq) + H_2(g) \end{array} \)

During the process of photosynthesis, water is oxidized to O 2 . As water can be oxidized and reduced, it is very useful in redox reactions.

3. Hydrolysis reaction

Water has a very strong hydrating tendency due to its dielectric constant. It dissolves many ionic compounds. Some covalent and ionic compounds can be hydrolyzed in water.

Frequently Asked Questions – FAQs

What is the melting point.

The temperature at which a solid becomes a liquid due to enough heat. For a given substance, its solid form’s melting point is the same as its liquid form’s freezing point and depends on factors such as the substance’s purity and surrounding pressure.

How does boiling point work?

A liquid‘s boiling point is the temperature at which its vapor pressure is equal to that of the atmospheric pressure. The normal boiling point of a liquid is the temperature at which one atmosphere (760 torr) is equal to its vapor pressure. The normal boiling point of water is 100 degree celcius.

What affects the boiling point of water?

The surrounding pressures are the greatest determinant of the boiling point of a liquid. The air pressure in an open system is most definitely the atmosphere on earth. For instance, water reaches the standard atmospheric pressure at 100 degrees centigrade. Water can boil at a lower temperature as elevation increases.

What is specific heat in chemistry?

The amount of energy needed to increase the temperature of 1 gram of a material by 1 °C is known as its specific heat.

Why is specific heat important?

Water’s high specific heat capacity makes it suitable for central heating systems because it can transfer a lot of energy by heating while its temperature changes just slightly.

We have seen the physical and chemical properties of water and understood its importance. There is a lot more to explore and learn about water. If you are curious to know further, kindly install BYJU’S – The Learning App.

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Physical & Chemical Properties of Water

Core Concepts

In this tutorial on the properties of water, you will learn about the physical and chemical properties of water. You will also learn about the structure of a water molecule.

Topics Covered in Other Articles

Polarity of Water

Electronegativity

Solvent v.s. Solute

Specific Heat

K w of Water

Density- Mass per unit volume

Specific Heat Capacity- Amount of energy required to raise the temperature of 1 kg of a material by 1 C

Heat of Vaporization- Amount of energy required to transform a quantity of a liquid into a gas

Polar Molecule- A molecule with a partially negative charged end and a partially positive charged end

Electronegativity- Tendency for an atom to attract shared electrons in a chemical bond

Introduction to Properties of Water

Water (H20) is the “universal solvent” and the most abundant surface on Earth. It is also the only common substance to exist as a solid, liquid, and gas naturally. Water molecules form hydrogen bonds and are extremely polar. The five main properties of water are its high polarity, high specific heat, high heat of vaporization, low density as a solid, and attraction to other polar molecules.

Polarity and Structure

One oxygen atom and two hydrogen atoms make a water molecule. It has a bent molecular geometry with the oxygen having two lone pairs of electrons. The difference in electronegativity between the oxygen and hydrogens causes the oxygen to have a partial negative charge and the hydrogen to have a partial positive charge. This difference in charge causes polarity. The partial positive charge of the hydrogen of one water molecule attracts the partial negative charge of the oxygen of another water molecule. This attraction is called hydrogen bonding. 

Hydrogen bonding is weaker than the covalent bonds between the oxygen and hydrogens of the same molecule but causes many of water’s unique properties. For example, more energy is required to break hydrogen bonds, so water has a higher melting and boiling point .

Universal Solvent

Water is the solvent of life. Hydrophilic substances are those that dissolve in water, while hydrophobic substances do not mix well with water. Substances can dissolve in water if they can match or overcome the hydrogen bonding between water molecules. If they cannot, the substance forms a precipitate. Acids , alcohols, and salts are rather soluble in water while fats and oils are hydrophobic. 

An ionic solute dissolved in water becomes separated. For example, NaCl separates into Na+ cations and Cl- anions surrounded by water molecules. Water is amphoteric, meaning it can act as either an acid or a base depending on the solution. It can produce both H+ and OH- ions. 

Specific Heat Capacity

Water has a high specific heat of 4184J/(kg x K) at 20 C and high heat of vaporization because of hydrogen bonding. This allows bodies of water to have minimal fluctuations in temperature to regulate climate.

Water’s high heat of vaporization allows humans to use sweat to cool off. Sweat is mostly made of water. It absorbs excess body heat as it evaporates. This process is known as evaporative cooling.

Water’s density is 1 gram per cubic cm. This is used to define the gram. Instead of undergoing thermal expansion, the density rises with temperature to a peak of 3.98 C and then decreases. Negative thermal expansion is the increase in density between 32 and 39.16 F. As a result, ice is less dense than water, which has a decrease in density by about 10%. This is why bodies of water may have a layer of ice on the surface but contain liquid underneath. This allows fish and marine life to survive under the ice. High specific heat keeps the temperature of the water relatively stable through the winter so that marine life can survive.

Salt content lowers the freezing point of the ocean by almost 2 C. Ice still floats on the ocean, but the ice is near salt-free with a similar density to ice on bodies of freshwater. The salt adds to the salinity and density of the remaining water which sinks by convection. This process is called brine rejection. 

Compressibility

Compressibility is a result of pressure and temperature. The compressibility of water is so low that is often assumed to be incompressible. Low compressibility allows water in deep oceans with high pressure to only decrease by 1.8% in volume.

Electrical Conductivity

Pure water is a good insulator, but deionized water is never completely free of ions. Water undergoes a process called autoionization as a liquid. This means that two water molecules can form one hydroxide anion (OH-) and one hydronium cation (H 3 0+).

Cohesion and Adhesion

Hydrogen bonds between water molecules are constantly breaking and reforming with other water molecules. Cohesion is the ability of water molecules to stick together. Water’s polarity also gives water high adhesion: the ability to stick to other surfaces. The adhesive forces are stronger than cohesive forces.

Strong cohesion and adhesion cause water to exhibit capillary action. Capillary action is the process of liquid flowing through a narrow space without and often against gravity. Water adheres to the walls of plants’ roots and rises up into the plant. Porous materials such as water also exhibit capillary action. Trees can transport water through capillary action over 100 meters.

Surface Tension

The hydrogen bonding also causes water to have a high surface tension of 71.99mN/m at 25. The surface tension is high enough for insects to walk on water. Surface tension is a result of water’s cohesive properties. Water droplets and water rising above the rim of a glass show water’s high surface tension. Learn more about the properties of water here.

The teacher will help to clear any misconceptions about properties of water. Some major misconceptions are students think objects float in water because they are lighter than water. Not always – think surface tension. Also, students think ice molecules are colder than water molecules. Really ice molecules have less kinetic energy than water molecules.

Estimated Class Time for the Engagement: 20-30 minutes

EXPLORATION

This student-centered station lab is set up so students can begin to explore the properties of water. Four of the stations are considered input stations where students are learning new information about the properties of water and four of the stations are output stations where students will be demonstrating their mastery of the input stations.  Each of the stations is differentiated to challenge students using a different learning style.  You can read more about   how I set up the station labs here .

EXPLORE IT!

Students will be working in pairs to better understand what it means when water is described as having surface tension. Students will follow the directions on the task cards as they conduct a mini lab about surface tension with a water dropper and a penny.

WATCH IT!

At this station, students will be watching a seven-minute video explaining the properties of water. Students will then answer questions related to the video and record their answers on their lab station sheet. For example: Describe what a hydrogen bond is. What is surface tension? List two examples from the video of how the properties of water are important to life on Earth.

RESEARCH IT!

The research station will allow students to interact with a simulation that allows them to create water molecules. It will teach the students about polarity and how molecules can be fused by chemical bonding. Students will then be directed to answer a few questions based on the research they conducted.

READ IT!

This station will provide students with a one page reading about capillary action. In the reading, students will discover what capillary action is and why it is important in our everyday lives. There are 4 follow-up questions that the students will answer to show reading comprehension of the subject.

ASSESS IT!

The assess it station is where students will go to prove mastery over the concepts they learned in the lab.  The questions are set up in a standardized format with multiple choice answers.  Some questions include: Which best illustrates the electrical charge of a water molecule? Which property of water is responsible for capillary action? Why is a good example of the property of water called surface tension? Which answer best describes cohesion?

WRITE IT!

Students who can answer open-ended questions about the lab truly understand the concepts that are being taught.  At this station the students will be answering three task cards: Describe what polarity means in terms of being a property of water. What is evaporation and how does it contribute to the survival of species? What properties of water allows the organisms to survive underneath the ice?

ILLUSTRATE IT!

Your visual students will love this station. Students will need to have completed the research station portion first because students will need to draw a model showing 4 water molecules bonded together. Students may use a computer device for reference.

ORGANIZE IT!

The organize it station allows your students to place cards containing the definitions to the correct vocabulary word that it is describing.

Estimated Class Time for the Exploration: 1-2, 45 minute class periods

EXPLANATION

The explanation activities will become much more engaging for the class once they have completed the exploration station lab.  During the explanation piece, the teacher will be clearing up any misconceptions about the properties of water with an interactive PowerPoint, anchor charts, and interactive notebook activities. The properties of water lesson includes a PowerPoint with activities scattered throughout to keep the students engaged.

The students will also be interacting with their journals using INB templates for properties of water.  Each INB activity is designed to help students compartmentalize information for a greater understanding of the concept.  The properties of water INB template allow students to focus their notes on learning the vocabulary and their definitions.

Estimated Class Time for the Exploration: 2-3, 45 minute class periods

ELABORATION

The elaboration section of the 5E method of instruction is intended to give students choice on how they can prove mastery of the concept.  When students are given choice the ‘buy-in’ is much greater than when the teacher tells them the project they will have to create.  The elaboration project will allow students to create a presentation to teach about the properties of water.

Estimated Class Time for the Elaboration: 2-3, 45 minute class periods (can also be used as an at-home project)

The final piece of the 5E model is to evaluate student comprehension.  Included in every 5E lesson is a homework assignment, assessment, and modified assessment.  Research has shown that homework needs to be meaningful and applicable to real-world activities in order to be effective.  When possible, I like to give open-ended assessments to truly gauge the student’s comprehension.

Estimated Class Time for the Elaboration: 1, 45 minute class period

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Testing The Properties Of Water

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Testing the Properties of Water

Before testing the properties of water, it’s important for students to know the basic properties of water. The main ones we discuss here are: polarity, surface tension, adhesion, cohesion, and capillary action.

Below is an explanation of 5 properties of water, followed by an easy-to-do activity.

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Table of Contents

A water molecule is made up of two hydrogen atoms and one oxygen atom. There are two ends to a water molecule. The two positively charged hydrogen atoms at one end and the negatively charged oxygen atom at the other end give the water molecules two poles.

This combination of atoms makes water a polar molecule. Polar molecules have a negative side and a positive side

Pictured above are two water molecules. Have your student identify the two hydrogen atoms and one oxygen atom in each of the two molecules.

Water is also neutrally charged. This means it has an equal number of protons and electrons.

However, even though a water molecule has an equal number of protons and electrons, there is an uneven distribution of the electrons within each water molecule that makes it polar.

So, polarity applies to the distribution of electrical charges around a molecule. In a polar bond, one atom has a partial positive electrical charge, and the other atom has a partial negative electrical charge.

This polarity of water gives it some special properties, like cohesion and adhesion , that can be easily demonstrated and observed in the activities below.

Some examples of other polar molecules are ozone, carbon monoxide, hydrochloric acid, ammonia, and others in the chart below. (This is not the exhaustive list of polar molecules.)

Nonpolarity

Before we move on, let’s talk about nonpolar molecules. Nonpolar molecules have a more even distribution of charge. (Unlike water that is positively charged on one end and negatively charged on the other.)

In a nonpolar bond, atoms share electrons equally, thus there is no partial positive or negative charge between the atoms.

The Behavior Between Polar and Nonpolar Molecules

Polar and nonpolar molecules tend to not be attracted to each other. In other words, polar and nonpolar molecules repel each other.

Polar molecules are attracted to other polar molecules.

Nonpolar molecules tend to be attracted to other nonpolar molecules.

In the oil and water activity below, we’ll be able to see the repelling and attraction of polar and nonpolar molecules.

Cohesion and Adhesion Properties of Water

  Molecules that are polar will mix with each other. This is called cohesion. This happens in water because the negative charge of the oxygen atom in a water molecule is attracted to the positive charge of the hydrogen atoms in another water molecule.

 Cohesion simply means that water molecules like to stick to each other. This is caused by the slightly negative charge of the oxygen atom of one water molecule being attracted to the slightly positive charge of the hydrogen atoms of another water molecule.

The hydrogen bonds between water molecules is illustrated below.

If you have two magnets handy, a great way to demonstrate the concept of a negative end of a water molecule being attracted to a positive end of another water molecule is to hold the north pole of a magnet to the south pole of a magnet. They are attracted to each other and “stick” together.

Related post: Properties Of Liquids Worksheet

Surface Tension

Milk is made primarily of water molecules and water molecules like to stick together. On the surface, where the water meets the air, water molecules cling even more tightly to each other. This causes a “skin” to form on the surface of the water. This skin is so strong that it can hold a weight that normally would sink in water. This is called surface tension .

When the surface tension is disrupted, the heavy object that is floating on the skin will sink. A light object on the surface will be pulled by the attraction of the water molecules if the surface tension is disrupted. This easy experiment will demonstrate that phenomenon.

The surface tension of water is caused by cohesion .   Cohesion means that the water molecules like to stick to each other. This is caused by the slightly negative charge of the oxygen atom of one water molecule being attracted to the slightly positive charge of the hydrogen atoms of another water molecule. You can, also, test the cohesion properties of water using an eyedropper, water and a coin.

Capillary Action

The capillary property of water is the ability of water to move through narrow spaces, such as thin tubes or porous materials, against the force of gravity. This behavior is primarily attributed to the combined effects of cohesion and adhesion among water molecules.

As mentioned above, water molecules like to stick together (cohesion), and they also like to stick to other things, like the sides of the straw or the tiny spaces in soil (adhesion).

When water encounters a narrow space, like a thin tube, and the diameter is sufficiently small, the cohesive forces within the liquid, coupled with the adhesive forces between the liquid and the container’s surface, work together. This collaboration overcomes the force of gravity and propels the water upward. It’s like a delicate balance of attractive forces that defies the natural tendency of water to be pulled downward by gravity.

Check out these other activities that demonstrate capillary action:

• Get Growing free printable – 50+ page printable covering plant life cycles, photosynthesis, capillary action, and more.
• Color-changing flowers activity .
• Coffee filter painting

Activities for Testing the Properties of Water

Testing the properties of water activity 1:  surface tension with soap and pepper.

A second surface tension activity is listed below.

Testing the Properties of Water Activity 2:  Surface Tension – How to Make a Paper Clip Float

You’ll need to gather:

• A paper towel
• A paper clip
• A small bowl filled with water
• What do you think will happen when you drop the paper clip in the water?
• How can we make the paper clip float?
• Now, we’re going to make the paper clip float. Remove the paper clip from the bowl, dry it off.
• Fold the paper towel lengthwise. We used a half-sheet piece of paper towel.
• Place the paper clip on the folded paper towel.
• Hold the paper towel at each end and gently and slowly place it on the surface of the water in the bowl.
• The paper towel will begin to absorb the water and sink into the water.
• Once the paper towel separates from the paper clip, very carefully remove the paper towel from the bowl, leaving the paper clip on the surface.

Explanation of what happened:

Remember, water molecules are polar. This means they have a positive end and a negative end.

The negative ends stick to the positive ends, and the positive ends stick to the negative ends. This creates surface tension.

When the paper clip was dropped into the water, the “skin” on the water wasn’t strong enough to hold the weight and force. When the paper towel was used, the weight of the paper clip was evenly distributed on the surface of the water as the paper towel absorbed water and began to sink.

Testing the Properties of Water Activity 3:  Cohesion Will Cost You a Penny!

The surface tension of water is caused by cohesion .   Cohesion means that the water molecules like to stick to each other. This is caused by the slightly negative charge of the oxygen atom of one water molecule being attracted to the slightly positive charge of the hydrogen atoms of another water molecule. You can test the cohesion properties of water using an eyedropper, water, and a coin.

• A coin (penny works well)

Slowly, drop water onto a coin. Watch as the drops of water stick together to form a larger drop.

Explanation of What Happened:

The water molecules will stick together and form a dome over the coin. Keep adding drops until the drop breaks and spills off of the coin. This is caused by gravity overcoming the force of the cohesion.   How many drops of water can you fit on a coin?

Testing the Properties of Water Activity 4: Cohesion in a Glass

What You’ll Need:

• Glass of water filled to the top
• Small bowl of water

You can also observe cohesion by filling a glass of water to the top. Next, use a dropper to very carefully add more water until the water is forming an arc of water slightly over the top of the glass. The cohesive properties of the water are holding the molecules together so that they will not spill over the top of the glass. There will come a point when the weak hydrogen bonds can no longer hold and the water will spill over.

Testing the Properties of Water Activity 5: Polarity 

In this activity, your student will be able to demonstrate and observe the interaction of polar and nonpolar molecules. We’ll be using water (polar) and vegetable oil (nonpolar).

What you need to gather:

• Some water in a measuring cup (a 1/4 cup should do)
• Some vegetable oil (a 1/4 cup should do)
• Food coloring
• Dawn dishwashing liquid
• Mix food coloring in with the water and stir.
• Pour 3 tablespoons of oil onto the middle of the plate.
• Ask your student to observe what happened? Were the drops of oil attracted to each other? Why do you think that happened?
• Next pour the water onto a different section of the plate, but near the oil.
• Pick up each side of the plate slightly off the surface and move the water around the perimeter of the oil. What happened? Are they mixing?
• Knowing what we learned about polar and nonpolar molecules, explain what you are observing.
• Put the plate down on the table and drizzle the Dawn dish soap over the water and oil.
• Pick up the plate carefully on each side and move it around so the soap, water, and oil mingle. What starts to happen? Why do you think this happens?

Explanation of What Happened

The nonpolar molecules of the oil are attracted to each other, so the oil drops come together to form one big “puddle.”

The polar molecules of the water are attracted to each other. But, when the water comes up against the oil, the polar molecules of the water are not attracted to the nonpolar molecules of the oil.

Soap molecules are a bit different. They have an elongated shape. One end is polar and the other is nonpolar. When the soap was added, the polar end of the soap molecule was attracted to the water and the nonpolar end was attracted to the oil.

Now, you can see why soap and water is used to wash dishes and how soap works on grease.

Testing the Properties of Water Activity 6 : Capillary Action

Make a rainbow with capillary action.

• Paper towels
• Box of 4 food coloring bottles
• 4 small jars or clear plastic cups
• 1 white crayon
• 1 water
• 1 baking sheet or foil baking sheet from the dollar store

Instructions

• Fill each jar with equal amounts of water
• Pour 10 drops of red in one jar of water, 10 drops of yellow in another jar of water, 10 drops of blue in another jar of water, and 5 drops of blue and 5 drops of red in the last jar of water.

• Place the jars of water from step 2 on a baking sheet.

What happened?

The water travels through the paper towel for two reasons. First, all paper is made of a sugar molecule call cellulose. Water is highly attracted to cellulose and wants to bond (or stick) to it. Second, the cellulose fibers in a paper towel are made with spaces between them. Since water likes to stick together, the water fills these spaces as it follows the water attracted to the cellulose. More spaces allow more water to be absorbed.

Other Resources Related to the Properties of Water

Make a vocabulary list related to a study of water by using this list of terms and definitions from the USGS .

More Water Experiments

Water Quality Experiment

Charcoal Water Purifying Experiment

Learn About The Water Cycle (and an experiment)

Take the USGS Water Properties quiz!

I hold a master’s degree in child development and early education and am working on a post-baccalaureate in biology. I spent 15 years working for a biotechnology company developing IT systems in DNA testing laboratories across the US. I taught K4 in a private school, homeschooled my children, and have taught on the mission field in southern Asia. For 4 years, I served on our state’s FIRST Lego League tournament Board and served as the Judging Director.  I own thehomeschoolscientist and also write a regular science column for Homeschooling Today Magazine. You’ll also find my writings on the CTCMath blog. Through this site, I have authored over 50 math and science resources.

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Properties of Water

Lexile Levels

Several times each day, your body takes in a chemical called hydrogen oxide.

It has the power to eat away solid rock over time. In its frozen state, mighty hydrogen oxide can crack concrete! It can also destroy the cells of plants and animals.

Hydrogen oxide is the chemical name for water. The chemical symbol is so familiar that it is a nickname for water: H 2 O. The H stands for an atom of hydrogen, which is positively charged. The 2 indicates that there are two atoms of hydrogen. The O stands for an atom of oxygen, which is negatively charged. So, water is a bond between three atoms: two positively (+) charged hydrogen atoms and a negatively (-) charged oxygen atom—H-O-H. That is true whether the water is a liquid, solid (ice), or gas (water vapor).

Water may seem ordinary, but it has some properties no other substance has. Water is a supersolvent! Water has the power to dissolve things like no other substance can. To find out more about what water can do, you can test many of its properties right in your own kitchen.

Rising Dome

The components of water stick together, forming drops. The ability of the components to stick together (called cohesion) causes water to form a “skin” on its surface. The force holding this “skin” together is called surface tension. With a straw or an eyedropper, put drops of water on a coin. Do you see a dome of water rising above the coin? Surface tension keeps the dome intact—up to a point. How many drops can you add before the dome collapses and the water spills?

Breaking the Tension

Soap breaks the surface tension of water. Soap attracts dirt and other particles, helping to clean dishes and other items. Set a dish of water on a level counter. Carefully place a small paper clip on top of the water’s “skin.” (This will take some practice.) Then try to float a paper clip in a dish of soapy water. Without a “skin,” the soapy water cannot hold up the paper clip.

Supersaturated

Water, our supersolvent, dissolves salt—up to a point. What is this point, and how much salt can water dissolve? Add a teaspoon of salt to a glass of water and stir it in, then look at how the water has changed. Keep adding a teaspoon of salt and stirring until the salt no longer dissolves and the grains of salt fall to the bottom of the glass. Let the water completely evaporate over several days. What is left in the glass?

Human Water Walker!

Can humans walk on water? Not usually, but Frenchman Rémy Bricka did. In 1988, he “ski-walked” across the Atlantic Ocean on water skis that were a little shorter than a compact car. The long skis helped to spread Bricka’s weight over the surface of the water, and that kept him from sinking. His watery stroll took two months.  ▶

In prehistoric times, Lake Bonneville covered most of the area that is now Utah. More water evaporated from the lake than flowed in from rivers and rain. So, in time, most of the lake dried up. Salt that had dissolved in the water remained, forming a vast white plain. The Bonneville Salt Flats are now a famous site for car races.

▲  A whirlpool in the ocean is a giant version of water that swirls down a drain. It spins tighter and tighter, and faster and faster, toward a hole in the center. Whirlpools can pull whole ships toward the bottom of the ocean. Then they spit the ships back up. Luckily for us, ocean waves near the shore are too random for whirlpools to form.

▲  The wind creates waves on the ocean’s surface. Below the surface, though, other forces are at work. Ocean currents are strong channels of flowing water caused in part by the meeting of warm and cold water. Warm water rises, and cold water sinks. In this satellite photo, the warm current of the Gulf Stream (red) flows up the eastern coast of the U.S. Cold currents from the Arctic (blue) flow down to meet it.

People who live in desert areas or go through droughts know water is valuable. But everyone should realize that it’s precious—and worth saving!  ▼

Toilets may use as much as five gallons per flush. Newer, low-flush toilets use 3.5 gallons. But you may have to flush more often. That does not save any water and may make the situation worse.

Some restaurants don’t give you water unless you ask. Every glass of water on a table needs two more glasses of water to wash and rinse the glass.

Did you know that 75 percent of water used in a house is used in the bathroom? Try not to let the faucet run while washing your face or brushing your teeth. You could save about 10 to 20 gallons of water a day!

At least 97 percent of the world’s water is salty and undrinkable. Another 2 percent is polluted, polar ice, or otherwise can’t be reached. That means people can only use 1 percent of all of Earth’s water!

Next topic in Water

The journey of a raindrop.

Which term means "ability of components to stick together"?

When salt water evaporates, the salt in the water remains behind.

Soap increases the surface tension of water.

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Properties of Water

Water (H2O) is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. With 70% of our earth being ocean water and 65% of our bodies being water, it is hard to not be aware of how important it is in our lives. It is by far the most studied chemical compound and is described as the “universal solvent” and the “solvent of life”.

Water is the most abundant compound on Earth’s surface. It is the most abundant substance on Earth and the only common substance to exist as a solid, liquid, and gas on Earth’s surface. In nature, water exists in the liquid, solid, and gaseous states. It is in dynamic equilibrium between the liquid and gas states at 0 degrees Celsius and 1 atm of pressure. It is also the third most abundant molecule in the universe (behind molecular hydrogen and carbon monoxide). The properties of water make it suitable for organisms to survive in during differing weather conditions.

The main properties of water are its polarity, cohesion, adhesion, surface tension, high specific heat, and evaporative cooling.

• Polarity – A water molecule is slightly charged on both ends. This is because oxygen is more electronegative than hydrogen.
• Cohesion – Cohesion creates surface tension which is why if you fill a spoon with water, drop by drop, the water volume will actually be bigger than the spoon’s surface before the waterfalls off.
• Adhesion – Similar to cohesion, but adhesion is when the hydrogen bonds in water allow for the water molecules to be held to another substance.
• High Specific Heat – Specific heat is the amount of heat absorbed or lost for 1g to change 1ºC, which in the case of water, is pretty high.

Water is also essential for life. Water molecules form hydrogen bonds with each other and are strongly polar. This polarity allows it to dissociate ions in salts and bond to other polar substances such as alcohols and acids, thus dissolving them. Its hydrogen bonding causes its many unique properties, such as having a solid form less dense than its liquid form, a relatively high boiling point of 100 °C for its molar mass, and high heat capacity.

Water is amphoteric, meaning that it can exhibit properties of an acid or a base, depending on the pH of the solution that it is in; it readily produces both H+and OH−ions. Related to its amphoteric character, it undergoes self-ionization. The product of the activities, or approximately, the concentrations of H+and OH−is a constant, so their respective concentrations are inversely proportional to each other. Water is the major constituent of almost all life forms. Most animals and plants contain more than 60% water by volume. Without water, life would probably never have developed on our planet.

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Common Core Standards

Life emerges due to the chemical organization of matter into cells.

Essential Questions:

How does life result from chemical structure and function?

Liquid water forms hydrogen bonds, is a solvent, and forms hydronium ions allowing a wide range of biochemical reactions to occur.

Competencies:

Provide examples for when it is correct to use the terms scientific principle, scientific theory, scientific law, fact, and belief.

Pose questions and provide evidence-based explanations about understanding and observations of biological phenomena and processes.

Select and use appropriate tools and techniques when designing and conducting experiments related to the biological sciences and then communicate an analysis of the findings using various types of media.

Identify and describe various ways models are used to explain, interpret, and predict, biological phenomena/systems.

Covalent bond

Electronegativity

Polar molecule

Hydrogen bonding

Specific heat

Surface tension

The student will be able to:

• explain why water is a polar molecule.
• describe how polarity leads to hydrogen bonding.
• explain how hydrogen bonding accounts for the unique properties of water.
• analyze why water's properties are important to maintaining life on Earth.
• use process skills to determine the pH of common household products.
• explain why pH is important to living things.

Lesson Essential Question(s)

3 days (after teaching polarity and hydrogen bonding between water molecules)

Poster board

Markers/Colored pencils

Handout and material list for pH Lab:  http://powerposse.aps.com/classroom/curriculums/pdf/Eighth/eiggactivity5.pdf

Suggested Instructional Strategies

Instructional procedures.

Days 1: Jigsaw activity

1. In their journals, students answer a review question about the previous day's material. "Draw 3 water molecules and show how/where they would hydrogen bond with each other.

2. The teacher monitors student progress. Students share their drawing with their desk partner to compare and discuss.

3. The teacher chooses one pair of students to draw their water molecules on the Promethean Board. The teacher reviews the drawing with the class.

4. The class is broken into four groups. Each group is assigned one of the properties of water (ability to make solutions/pH, cohesion/adhesion, high specific heat, ice is less dense than liquid water). Students read about their designated property out of their text book.

5. Students work with their group to create a poster summarizing the property. The poster must include a drawing representing how the property applies to water's importance to life and a short summary, in words, what the property is.

6. Each group gives a short presentation about their poster. The other groups will take notes in their notebooks with a graphic organizer. (the name of the property/what it means/what the drawing represents)

Day 2: pH Lab

1. Students will work in pairs to complete the activity. Students will work with their partners to answer warm up questions in their journals. "What is pH? Which of the four properties of water would pH be categorized under?"

2. Each pair receives 4 paper cups, each with a different house hold material (grape juice, stain remover, vinegar, and baking soda)2. Students will work with their partner to make predictions as to whether they think each substance is an acid or a base and what they think the pH is. This is recorded in the data chart. (see link in Materials section for the link to the lab handout)

3. Students use Litmus paper to test and record the pH for each substance. This is recorded in the data chart.

4. Students answer the Analysis Questions with their partner.

5. Students revisit the journal questions from the beginning of class, and using what they've learned in the lab, adjust their answers if necessary.

 Day 3: Formative Assessment

1. Working with the same partners from the previous day, the pair travels and meets with another pair to compare and share answers from the journal questions from day 2.

2. The pair finds at least two other pairs to repeat the discussion.

3. Students adjust their answers if necessary.

Formative Assessment:

4. Students first work individually to answer the formative assessment questions.

5. Students then exchange their answer with another student, who "grades" their response.

6. Students meet with their peer editor to discuss the grading.

7. The teacher displays an acceptable answer (answer key) on the board and discusses its evaluation.

Formative Assessment

AnswerKey Assessment 7/24/2012 12:50 PM.pdf StudentVersion Assessment 7/24/2012 12:50 PM.pdf

Related Materials & Resources

http://powerposse.aps.com/classroom/curriculums/pdf/Eighth/eiggactivity5.pdf - Handout and material list for pH Lab.

Other related resources:

Water Properties and Systems

Water, Uniquely Created for Life

Like Dissolves Like - Water the Universal Solvent

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12.3: Properties of Liquids

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Learning Objectives

By the end of this section, you will be able to:

• Distinguish between adhesive and cohesive forces
• Define viscosity, surface tension, and capillary rise
• Describe the roles of intermolecular attractive forces in each of these properties/phenomena

When you pour a glass of water, or fill a car with gasoline, you observe that water and gasoline flow freely. But when you pour syrup on pancakes or add oil to a car engine, you note that syrup and motor oil do not flow as readily. The viscosity of a liquid is a measure of its resistance to flow. Water, gasoline, and other liquids that flow freely have a low viscosity. Honey, syrup, motor oil, and other liquids that do not flow freely, like those shown in Figure 1, have higher viscosities. We can measure viscosity by measuring the rate at which a metal ball falls through a liquid (the ball falls more slowly through a more viscous liquid) or by measuring the rate at which a liquid flows through a narrow tube (more viscous liquids flow more slowly).

The IMFs between the molecules of a liquid, the size and shape of the molecules, and the temperature determine how easily a liquid flows. As Table 1 shows, the more structurally complex are the molecules in a liquid and the stronger the IMFs between them, the more difficult it is for them to move past each other and the greater is the viscosity of the liquid. As the temperature increases, the molecules move more rapidly and their kinetic energies are better able to overcome the forces that hold them together; thus, the viscosity of the liquid decreases.

The various IMFs between identical molecules of a substance are examples of cohesive forces . The molecules within a liquid are surrounded by other molecules and are attracted equally in all directions by the cohesive forces within the liquid. However, the molecules on the surface of a liquid are attracted only by about one-half as many molecules. Because of the unbalanced molecular attractions on the surface molecules, liquids contract to form a shape that minimizes the number of molecules on the surface—that is, the shape with the minimum surface area. A small drop of liquid tends to assume a spherical shape, as shown in Figure 2, because in a sphere, the ratio of surface area to volume is at a minimum. Larger drops are more greatly affected by gravity, air resistance, surface interactions, and so on, and as a result, are less spherical.

Surface tension is defined as the energy required to increase the surface area of a liquid, or the force required to increase the length of a liquid surface by a given amount. This property results from the cohesive forces between molecules at the surface of a liquid, and it causes the surface of a liquid to behave like a stretched rubber membrane. Surface tensions of several liquids are presented in Table 2. Among common liquids, water exhibits a distinctly high surface tension due to strong hydrogen bonding between its molecules. As a result of this high surface tension, the surface of water represents a relatively “tough skin” that can withstand considerable force without breaking. A steel needle carefully placed on water will float. Some insects, like the one shown in Figure 3, even though they are denser than water, move on its surface because they are supported by the surface tension.

The IMFs of attraction between two different molecules are called adhesive forces . Consider what happens when water comes into contact with some surface. If the adhesive forces between water molecules and the molecules of the surface are weak compared to the cohesive forces between the water molecules, the water does not “wet” the surface.

For example, water does not wet waxed surfaces or many plastics such as polyethylene. Water forms drops on these surfaces because the cohesive forces within the drops are greater than the adhesive forces between the water and the plastic. Water spreads out on glass because the adhesive force between water and glass is greater than the cohesive forces within the water. When water is confined in a glass tube, its meniscus (surface) has a concave shape because the water wets the glass and creeps up the side of the tube. On the other hand, the cohesive forces between mercury atoms are much greater than the adhesive forces between mercury and glass. Mercury therefore does not wet glass, and it forms a convex meniscus when confined in a tube because the cohesive forces within the mercury tend to draw it into a drop (Figure 4).

If you place one end of a paper towel in spilled wine, as shown in Figure 5, the liquid wicks up the paper towel. A similar process occurs in a cloth towel when you use it to dry off after a shower. These are examples of capillary action —when a liquid flows within a porous material due to the attraction of the liquid molecules to the surface of the material and to other liquid molecules. The adhesive forces between the liquid and the porous material, combined with the cohesive forces within the liquid, may be strong enough to move the liquid upward against gravity.

Towels soak up liquids like water because the fibers of a towel are made of molecules that are attracted to water molecules. Most cloth towels are made of cotton, and paper towels are generally made from paper pulp. Both consist of long molecules of cellulose that contain many −OH groups. Water molecules are attracted to these −OH groups and form hydrogen bonds with them, which draws the H 2 O molecules up the cellulose molecules. The water molecules are also attracted to each other, so large amounts of water are drawn up the cellulose fibers.

Capillary action can also occur when one end of a small diameter tube is immersed in a liquid, as illustrated in Figure 6. If the liquid molecules are strongly attracted to the tube molecules, the liquid creeps up the inside of the tube until the weight of the liquid and the adhesive forces are in balance. The smaller the diameter of the tube is, the higher the liquid climbs. It is partly by capillary action occurring in plant cells called xylem that water and dissolved nutrients are brought from the soil up through the roots and into a plant. Capillary action is the basis for thin layer chromatography, a laboratory technique commonly used to separate small quantities of mixtures. You depend on a constant supply of tears to keep your eyes lubricated and on capillary action to pump tear fluid away.

Biomedical Applications of Capillary Action

Many medical tests require drawing a small amount of blood, for example to determine the amount of glucose in someone with diabetes or the hematocrit level in an athlete. This procedure can be easily done because of capillary action, the ability of a liquid to flow up a small tube against gravity, as shown in Figure 7. When your finger is pricked, a drop of blood forms and holds together due to surface tension—the unbalanced intermolecular attractions at the surface of the drop. Then, when the open end of a narrow-diameter glass tube touches the drop of blood, the adhesive forces between the molecules in the blood and those at the glass surface draw the blood up the tube. How far the blood goes up the tube depends on the diameter of the tube (and the type of fluid). A small tube has a relatively large surface area for a given volume of blood, which results in larger (relative) attractive forces, allowing the blood to be drawn farther up the tube. The liquid itself is held together by its own cohesive forces. When the weight of the liquid in the tube generates a downward force equal to the upward force associated with capillary action, the liquid stops rising.

Key Concepts and Summary

The intermolecular forces between molecules in the liquid state vary depending upon their chemical identities and result in corresponding variations in various physical properties. Cohesive forces between like molecules are responsible for a liquid’s viscosity (resistance to flow) and surface tension (elasticity of a liquid surface). Adhesive forces between the molecules of a liquid and different molecules composing a surface in contact with the liquid are responsible for phenomena such as surface wetting and capillary rise.

Key Equations

• Explain their differences in viscosity in terms of the size and shape of their molecules and their IMFs.
• Explain their differences in surface tension in terms of the size and shape of their molecules and their IMFs.
• You may have heard someone use the figure of speech “slower than molasses in winter” to describe a process that occurs slowly. Explain why this is an apt idiom, using concepts of molecular size and shape, molecular interactions, and the effect of changing temperature.
• It is often recommended that you let your car engine run idle to warm up before driving, especially on cold winter days. While the benefit of prolonged idling is dubious, it is certainly true that a warm engine is more fuel efficient than a cold one. Explain the reason for this.
• As temperature increases, what happens to the surface tension of water? Explain why this occurs, in terms of molecular interactions and the effect of changing temperature.
• As temperature increases, what happens to the viscosity of water? Explain why this occurs, in terms of molecular interactions and the effect of changing temperature.

[reveal-answer q=”334226″]Show Selected Answers[/reveal-answer] [hidden-answer a=”334226″]

2. The water molecules have strong intermolecular forces of hydrogen bonding. The water molecules are thus attracted strongly to one another and exhibit a relatively large surface tension, forming a type of “skin” at its surface. This skin can support a bug or paper clip if gently placed on the water.

4. Temperature has an effect on intermolecular forces: the higher the temperature, the greater the kinetic energies of the molecules and the greater the extent to which their intermolecular forces are overcome, and so the more fluid (less viscous) the liquid; the lower the temperature, the lesser the intermolecular forces are overcome, and so the less viscous the liquid.

6. (a) As the water reaches higher temperatures, the increased kinetic energies of its molecules are more effective in overcoming hydrogen bonding, and so its surface tension decreases. Surface tension and intermolecular forces are directly related.

(b) The same trend in viscosity is seen as in surface tension, and for the same reason.

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adhesive force: force of attraction between molecules of different chemical identities

capillary action: flow of liquid within a porous material due to the attraction of the liquid molecules to the surface of the material and to other liquid molecules

cohesive force: force of attraction between identical molecules

surface tension: energy required to increase the area, or length, of a liquid surface by a given amount

viscosity: measure of a liquid’s resistance to flow

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