Properties of Water

Water moderates temperature through two properties: its high specific heat and its high heat of vaporization. Water also has the unusual property of being less dense in its solid form, ice, than as a liquid. In addition, water acts as a solvent for polar molecules and exerts an organizing effect on nonpolar molecules. Water can also dissociate to form ions. All these proper-ties result from its polar nature. 

Water's high specific heat helps maintain temperature

 The temperature of any substance is a measure of how rapidly its individual molecules are moving. In the case of water, a large input of thermal energy is required to break the many hydrogen bonds that keep individual water molecules from moving about Therefor; water is said to have a high specific heat, which is defined as the amount of heat that must be absorbed or lost by 1 gram of a substance to change its temperature by 1 degree Celsius (°C). Specific heat measures the extent to which a substance resists changing its temperature when it absorbs or loses heat. Because polar substances tend to form hydrogen bonds, the more polar a substance is, the higher is its specific heat. The specific heat of water (1 calorie/gram/°C) is twice that of most carbon compounds and nine times that of iron. Only ammonia, which is more polar than water and forms very strong hydrogen bonds, has a higher specific heat than water (1.23 calories/gram/°C). Still, only 20% of the hydrogen bonds are broken as water heats from 0° to 100°C. 

Because of its high specific heat, water heats up more slowly than almost any other compound and holds its temperature longer when heat is no longer applied. This characteristic enables organisms, which have a high water content, to maintain a relatively constant internal temperature. The heat generated by the chemical reactions inside cells would destroy the cells if not for the absorption of this heat by the water within them.

 Water's high heat of vaporization facilitates cooling 

The heat of vaporization is defined as the amount of energy required to change 1 gram of a substance from a liquid to a gas. A considerable amount of heat energy (586 calories) is required to accomplish this change in water. Because the transition of water from a liquid to a gas requires the input of energy to break its many hydrogen bonds, the evaporation of water from a surface causes cooling of that surface. Many organisms dis-pose of excess body heat by evaporative cooling, for example, through sweating in humans and many other vertebrates. 

Solid water is less dense than liquid water 

At low temperatures, water molecules are locked into a crystal-like lattice of hydrogen bonds, forming solid ice . Interestingly, ice is less dense than liquid water because the hyrogen bonds in ice space the water molecules relatively far apart. is unusual feature enables icebergs to float. If water did not have this property, nearly all bodies of water would be ice, with only the shallow surface melting annually. The buoyancy of ice is important ecologically because it means bodies of water freeze from the top down and not the bottom up. Liquid water beneath the surface of ice that covers most lakes in the winter allows fish and other animals to overwinter without being frozen. 

The solvent properties of water help move ions and polar molecules 

Water molecules gather closely around any substance that bears an electrical charge, whether that substance carries a full charge (ion) or a charge separation (polar molecule). For example, sucrose (table sugar) is composed of molecules that contain polar hydroxyl (OH) groups. A sugar crystal dissolves rapidly in water because water molecules can form hydrogen bonds with individual hydroxyl groups of the sucrose molecules. Therefore, sucrose is said to be solubk in water. Water is termed the solvent, and sugar is called the solute. Every time a sucrose molecule dissociates, or breaks away, from a solid sugar crystal, water molecules surround it in a cloud, forming a hydration shell that prevents it from associating with other sucrose molecules. Hydration shells also form around ions such as Na. and Cl-.