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Tuesday, May 5, 2009

Properties of Water

Water has a number of unusual properites which come about because the atoms that comprise a water molecule form apolar covalent bonds. The main properties of water that are important for living things are the following:

Ice floats. Usually when a liquid freezes the solid ismore dense than the liquid. In water thesolidphase is less dense than the liquid phase under normal conditions. If ice sank, ponds and lakes would freeze solid to the bottom making things very difficult for fish and other creatures to obtain food and oxygen in the winter.

Water is a great solvent especially for polar molecules of ionically bonded substances like salt. That's because the charged ends of the water molecules are attracted to the charges on other molecules. Water molecules will surround soluable molecules or ions in a hydration shell. This helps keep the molecules in solution.

Many gases dissolve in water because the the combination of attractive and repulsive forces among water molecules leaves space for the gas molecules that in most other liquids would not be there. The two gasses we are most concerned about dissolving are carbon dioxide and oxygen.

Water has a high latent heat. Water can store more heat per volume than just about any common substance and releases the heat slowly. This helps moderate the climate.

Has a high heat of vaporization. It takes a lot of heat to break the hydrogen bonds in water and when they braek, the heat energy is carried off as water vapor. This makes water useful as a coolent for large critters like mammals.

Water is cohesive. Water molecules tend to stick together, up to a point. Many insects take advantage of the surface tension brought about at the surface of lakes and pond by cohesion to walk on the surface of the water.

Water is adhesive. Sicks to surfaces with polar groups such as cellulose.

Water disassociates to form hydrogen ions(H+) and hydroxyl (OH-) in small quanitities. This is important for living things because the number of ions in the solutions inside organisms affects the shape and functioning of many of the molecules. The concentration of hydrogen ions in a solution is so important that scientists use a special scale, the pH scale to measure this concentration.

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