Covalent bonds form between atoms when they share electrons to fill their valence electron shells. When the sharing of electrons between atoms is equal, such as O2 (oxygen) or CH4 (methane), the covalent bond is said to be nonpolar. However, when electrons are shared, but not equally due to differences in electronegativity (the tendency to attract electrons), the covalent bond is said to be polar. H (water) is an example of a polar molecule. Because oxygen is more electronegative than hydrogen, the electrons are drawn toward oxygen and away from the hydrogen atoms; consequently, the oxygen atom acquires a slight negative charge and each hydrogen atoms acquires a slightly positive charge. It is important to remember that the electrons are still shared, just not equally.

Water s polarity allows for the formation of hydrogen bonds between adjacent water molecules, resulting in many unique properties that are critical to maintaining life. For example, water is an excellent solvent because hydrogen bonds allow ions and other polar molecules to dissolve in water. Water s hydrogen bonds also contribute to its high heat capacity and high heat of vaporization, resulting in greater temperature stability. Hydrogen bond formation makes ice less dense as a solid than as a liquid, insulating aquatic environments. Water s cohesive and adhesive properties are seen as it rises inside capillary tubes or travels up a large tree from roots to leaves. The pH or hydrogen ion concentration of a solution is highly regulated to help organisms maintain homeostasis; for example, as will be explored in later chapters, the enzymes that catalyze most chemical reactions in cells are pH specific. Thus, the properties of water are connected to the biochemical and physical processes performed by living organisms. Life on Earth would be very different if these properties were altered if life could exist at all.

The information presented and the examples highlighted in this section support concepts and Learning Objectives outlined in Big Idea 2 of the AP Biology Curriculum. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP Biology course, an inquiry-based laboratory experience, instructional activities, and AP Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.

Growth, reproduction and maintenance of living systems require free energy and matter.

Organisms must exchange matter with the environment to grow, reproduce and maintain organization.

The student can justify the selection of the kind of data needed to answer a particular scientific question.

The student is able to justify the selection of data regarding the types of molecules that an animal, plant, or bacterium will take up as necessary building blocks and excrete as waste products.

Discuss with students why scientists use the criteria of the presence of liquid water to determine if an environment or planet can support life. More information on this topic is available at this site.

Have students create visual representations with annotations to explain how water s molecular structure and the resulting polarity results in its unique properties. Have the students describe how these properties are vital to life processes.

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.

One of water s important properties is that it is composed of polar molecules: the hydrogen and oxygen within water molecules (H2O) 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 2.14 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 ).

Oil and water do not mix. As this macro image of oil and water shows, oil does not dissolve in water but forms droplets instead. This is due to it being a nonpolar compound. (credit: Gautam Dogra).

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 2.15, 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.

Click here to see a 3-D animation of the structure of an ice lattice.

Identify the red and white balls in the model and explain how arrangement of the molecules supports the fact that ice floats on water.

Red and white balls represent oxygen and hydrogen, respectively, loose arrangement of molecules results in low density of ice

Red and white balls represent oxygen and hydrogen respectively, tightly packed arrangement of molecules results in a low density of ice

Red and white balls represent hydrogen and oxygen, respectively, loose arrangement of molecules results in low density of ice

Red and white balls represent oxygen and hydrogen, respectively, tightly packed arrangement of molecules results in high density of ice

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 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, 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.

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 2.16 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 2.16. 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.

When table salt (NaCl) is mixed in water, spheres of hydration are formed around the ions.

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 2.17.

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 2.18.

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 2.19, use the surface tension of water to stay afloat on the surface layer of water and even mate there.

During a process called transpiration, water evaporates through a plant s leaves. Water in the ground travels up from the roots to the leaves. Based on water s molecular properties, create a visual representation (e.g., diagrams or models) with annotations to explain how water travels up a 300-ft. California redwood tree. What other unique properties of water are attributed to its molecular structure, and how are these properties important to life?

This activity is an application of Learning Objectives 2.8 and Science Practice 4.1 and Learning Objectives 2.9 and Science Practices 1.1 and 1.4 because you are modeling the relationship between water s molecular structure and its unique properties that are essential to maintaining life, including capillary action.

The pH of a solution indicates its acidity or basicity.

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