Water’s High Heat Capacity
Water’s high heat capacity is a property arising from hydrogen bonding among water molecules. Water has the highest specific heat capacity of any liquids. Specific heat is 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, warm blooded animals use water to more evenly disperse heat in their bodies. This is similar 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. This is the amount of energy required to change one gram of a liquid substance to a gas. You would require a considerable amount of heat energy (586 cal) 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. This makes it difficult for liquid water 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.
Hydrogen bonds need to be broken for water to evaporate. This fact means that the process uses a substantial amount of energy. 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 (see image above), 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 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 the image below. It serves to keep the particles separated or dispersed in the water.
When you add ionic compounds to water, the individual ions react with the polar regions of the water molecules. As a result, 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 you add NaCl crystals to water, the molecules of NaCl dissociate into Na+ and Cl– ions. Then, spheres of hydration form around the ions, as illustrated above. The partially negative charge of the water molecule’s oxygen surrounds the positively charged sodium ion. The partially positive charge of the hydrogen on the water molecule surrounds the negatively charged chloride ion.