5.3 Other Properties of Water
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 constantly form and break as the water molecules slide past each other. The water molecules’ motion causes the bonds to break due to the heat contained in the system. When the heat rises as water boils, the water molecules’ higher kinetic energy causes the hydrogen bonds to break completely and allows water molecules to escape into the air as gas (steam or water vapor). Alternatively, when water temperature reduces 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 that we do not see when other liquids solidify.
The lower density of ice causes it to float at the surface of liquid water, such as in an iceberg or ice cubes in a glass of water. In lakes and ponds, ice will form on the water’s surface creating an insulating barrier that protects the animals and plant life in the pond from freezing. Without this insulating ice layer, plants and animals living in the pond would freeze in the solid block of ice and could not survive.
Water’s High Heat Capacity
Water’s high heat capacity is a property that hydrogen bonding among water molecules causes. Water has the highest specific heat capacity of any liquids. It takes water a long time to heat and a long time to cool. In fact, water’s specific heat capacity 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, warm blooded animals use water 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 is required to accomplish this change in water. As liquid water heats up, hydrogen bonding makes it difficult to separate the liquid water molecules from each other. As a result, water 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. 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: we call this process evaporation.
The fact that hydrogen bonds need to be broken for water to evaporate means that bonds use a substantial amount of energy 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 it can maintain homeostasis of body temperature.
