Unique Properties of Water

Polar Covalent Bonds Polar covalent bond: unequal sharing of electrons

Hydrogen Bond Attraction between a hydrogen atom and an atom such as oxygen or nitrogen that is either part of another molecule or located at a distant site on the same molecule. Both the hydrogen atom and the other atom must be involved in a polar covalent bond

Unique Properties of Water This is the chemical basis for the unique properties of water. Life as we know it would not exist without these properties Before exploring more of the unique properties of water, please take the following Water Quiz to test how much you understand already.

Adhesion & Cohesion Cohesion: attraction of water for itself Adhesion: attraction for water to other polar or charged materials Surface tension: Because of the hydrogen-bonding, water acts as if it were coated with a film. This surface tension (the tough "skin" formed on the surface of water) causes water to "bead-up" on a hard, unwettable surface. Surface tension allows us to skip rocks on water and it allows small organisms like the water strider to "walk on water".

High specific heat Specific heat: the amount of heat required to raise or lower the temperature of 1 gram of substance 1 degree C. a lot of heat energy is required to break hydrogen bonds, water resists temperature change. When water is heated, most of the heat is used to break hydrogen bonds and not much is left over to raise the temperature of the water. Because water can absorb a lot of heat with little change in temperature, it acts as a thermal buffer. On a small scale, water in a cell can absorb much heat with little change in temperature. On a larger scale, the water in the ocean acts as a thermal buffer for the earth, resisting temperature change and creating a hospitable environment for life.

High heat of vaporization Heat of vaporization: The amount of heat required for 1 gram of a substance to be converted from a liquid to a gas. Hydrogen bonds make it difficult for water molecules to escape the liquid state and are responsible for water's high heat of vaporization. The hydrogen bonds must be broken before water can evaporate and this requires considerable energy. Because water has a high heat of vaporization Helps our bodies and our planet to maintain our temperature within a tolerable range. When we get hot and sweat, water evaporates from our skin and cools us. Since the evaporation of water requires a considerable amount of energy, it is very effective in cooling us. Water also helps moderate global climate by absorbing solar radiation and dissipating the heat by evaporation of surface water.

Density Water is also unique in the way temperature affects its density. Most substances increase in density as temperature decreases because the molecules making up the substance begin to move more slowly and get closer together. The density of water increases as it is cooled to 4 degrees C and then its density begins to decrease as the temperature decreases to 0 degrees C, the freezing point of water. As the freezing point is approached, hydrogen bonds relax and form a crystal lattice that keeps molecules further apart than they are in liquid water. This is the reason why ice floats and ponds don't freeze solid.