The Properties of Water

What is so special about the properties of water that life on earth could not exist without it? There are several very important things about the properties of water that we need to understand to truly appreciate it's role in biology.

Water is a polar molecule. Water is a polar molecule because of a slightly negative charge at the oxygen end and a slightly positive charge at the hydrogen end. Water molecules can form hydrogen bonds with each other. Polar substances are hydrophilic (water loving). Nonpolar substances are hydrophobic (water hating) and are repelled by water.

Water's ability to expand as it freezes is related to hydrogen bonding. In freezing, hydrogen bonds resist breaking and lock the water molecules in the bonding patterns of ice. Water is one of the few substances that is less dense as a solid than it is as a liquid. If this were not the case, ponds, lakes, and oceans would freeze solid. Instead, they form an insulating ice layer on the surface.

Water is cohesive. Cohesion is the capacity to resist rupturing. Because of hydrogen bonding, the molecules of water stick together. This cohesion makes surface tension possible. Wherever there is an interface between water and air, such as in a glass of water, or the surface of the ocean, there is an ordered arrangment of water molecules. Hydrogen molecules are bonded to each other and to the water below. The water acts as though it were coated with a film. This makes it possible for some small insects to literally walk on the surface of the water. When we study plants, we will learn more about how cohesion helps to move water through the plant.

Water has the ability to stabilize temperature. It can absorb a large amount of heat before its temperature changes. Both living organisms and the ocean, for example, are resistant to changes in temperature. This is due to hydrogen bonding. Considerable heat input is required to break hydrogen bonds. Water also has a high heat of vaporization. This is the quantity of heat that a liquid must absorb for one gram of it to be converted from the liquid to the gaseous state. The heat of vaporization helps the body to cool by sweating. In evaporation, the input of heat energy increases the molecular motion so much that the hydrogen bonds are broken and water molecules escape into the air, thus cooling the surface.

Water is the universal solvent. Charged regions of polar water molecules are attracted to ions and a wide variety of polar compounds. This make water an effective medium for complex chemical reactions in organisms. Because of the large variety of compounds that will dissolve and stay in solution in water, it is often called the universal solvent. "Spheres of hydration" are formed around the solute (dissolved) molecules.

This animation is a good review of the polarity of water.

REVIEW: Water is an excellent solvent because
a. it forms spheres of hydration around charged substances and can form hydrogen bonds with many nonpolar substances.
b. it has a high heat of fusion.
c. of its cohesive properties.
d. it is a liquid at room temperature.
e. all of these
Answer e