|
The heat of fusion is the amount of heat energy needed to convert one gram of a solid into its liquid form. You can calculate this value for water by heating a mass of ice, and measuring how much heat is needed to melt it completely. The value is determined by dividing the energy (in joules) by the mass:  The heat of fusion for ice/water is 333 J/g Knowing this value, you can determine how much heat will be necessary to melt a piece of ice of known mass. The rearranged formula you would use for this is Q = m x Hfus, where Hfus is 333 J/g The heat of vaporization is the amount of heat energy needed to convert one gram of a liquid into a gas. You can calculate this value for water by heating a mass of water, and measuring how much heat is needed to turn it completely into water vapour. The value is determined by dividing the energy (in joules) by the mass:  The heat of vaporization for water is 2260 J/g Knowing this value, you can determine how much heat will be necessary to completely evaporate a quantity of water of known mass. The rearranged formula you would use for this is Q = m x Hvap, where Hvap is 2260 J/g Both quantities are much larger for water than other liquids. This makes water the ideal liquid to support life. For example, the heat of fusion for water (the heat needed to melt one gram of ice) also represents the amount of heat released by a gram of water when it turns into ice. This means that when a large body of water, like a lake, freezes over in the winter, it releases a lot of heat into the atmosphere. This warms the air, and makes the environment near a body of water less cold. Temperatures are never as cold in winter if you live near a large lake. Similarly, if it gets cold enough in the atmosphere to cause snow to form (solid water), the change of state of water into a solid releases a lot of heat into the atmosphere. So falling snow often means the air is warmer. Water's large heat of vaporization (the heat necessary to evaporate one gram of water) also has beneficial effects. When water is on your skin because of perspiration, the heat that evaporates that water comes from your body. Removing the heat from your body causes you to feel cooler.  Large bodies of water also keep the environment cool. Heat in the air is used to evaporate water from a lake, and as the heat is used up to do this, the air gets cooler. Similarly, heat is used up in evaporating water from the surfaces of leaves as they exude it through transpiration. This also cools the air.At night, when it is cooler, all this water vapour condenses back to water liquid. This releases much of the heat back into the air, making it warmer than it would have been otherwise. The combination of these two effects ... cooling the air in the day and warming it at night ... result in a much smaller difference in day and night temperatures, all because of the heat of fusion and vaporization of water. You can think of water in the environment as a bank account, where heat can be stored when it's not needed, (cooling the environment) and released when it is (warming the environment). Incidentally, it should now make sense that one of the things that makes deserts so hot is the lack of bodies of water or transpiring plants to provide this heat storage. |