11.2: Properties of Water
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The physical and chemical properties of water are what make it essential to life and useful to civilization. Water is a molecule made of one negatively charged (-2) oxygen ion and two positively-charged (+1) hydrogen ions, giving it the chemical formula H2O, with strong covalent bonds between the oxygen and two hydrogen ions. The shape of the water molecule allows for an uneven distribution of charge, where one side is slightly positive and one side is slightly negative. Because of this polarity, water molecules form hydrogen bonds with each other. Hydrogen bonds are electrostatic intermolecular bonds that are weaker than ionic and covalent bonds (see discussion in the Minerals chapter). Water is amphoteric, that is it can self-ionize, breaking down into an acidic hydrogen ion (H+) and a hydroxyl ion (OH–), chemically a base. Because of its polarity and its ability to be amphoteric, water is a universal solvent—a chemical that can dissolve a wide range of other chemicals.
Other side-effects of water’s polarity are cohesion (water likes to stick to itself) and adhesion (water likes to stick to other things). Water has the highest cohesion of all nonmetallic liquids. Cohesion gives water surface tension, allowing water glider insects to float on the water surface. Surface tension is what gives raindrops a spherical shape. Capillary action occurs when a combination of adhesive and cohesive forces causes water to move up narrow passages and tubes, rising higher than surrounding liquid. Capillary action happens when the adhesion of water to the tube is greater than the water’s internal cohesive forces. Paper towels have small pores that use capillary forces to clean up water spills. Plants use capillary forces to pump water into their tissues.
Water has a high specific heat capacity. Specific-heat is the amount of heat required to raise the temperature of a substance. Compared to many other substances, water requires a large amount of heat to raise its temperature. The high specific heat of water allows it to act as an energy buffer to extreme changes in air temperature. It also allows the oceans to soak up solar heat without changing temperature much and distribute that heat over the Earth by currents thus making the Earth habitable.
The density curve of water shows that as water is cooled, it becomes more dense, as do most other substances, but its greatest density occurs at about 4 degrees Celsius while most other substances continue to increase in density until they freeze. This unique density curve means that water is most dense just above its freezing point and sinks. Thus the oceans remain liquid. If water behaved like other substances, the oceans would be frozen.
When water freezes, the molecules arrange themselves in a well-ordered crystal structure, creating a spacing between molecules that is greater than if the water is in liquid form. The difference in molecular spacing causes ice to be less dense than water, making it more buoyant than liquid water, causing it to float on water. Because of its high specific heat capacity, ice floating on a lake’s surface insulates the liquid water beneath and keeps it from freezing.
Because of its hydrogen bonds, water also has a high heat of vaporization. A significant amount of energy is required to evaporate water. As water evaporates, energy is absorbed by the breaking of hydrogen bonds and the air around the evaporating water is cooled. This energy is stored in the water vapor.