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 HOW DOES FREEZING RELEASE HEAT ENERGY?

METEOROLOGIST JEFF HABY

When water is freezing it makes sense to think that the water is getting colder. When you think of the word freezing you think of it being cold. Actually, when water freezes it stays at a constant temperature. That constant temperature is the freezing point of 32 F (0 C). Suppose you have a small cup of water that has a temperature of 50 F. You place that cup of water outside where the temperature is 20 F. There is also a thermometer in the water to record the temperature. When the water is placed outside the temperature of the water will gradually cool. The thermometer will start at 50 F and it will gradually cool. Once the temperature reaches 32 F the temperature will stop cooling. At that point the liquid will begin to turn to ice. During the entire time the liquid is converting to ice the temperature will stay at a constant of 32 F. Once all the liquid is frozen then the temperature will resume the decrease. The temperature of the ice will continue to decrease until it reaches the same temperature as the outside air which is 20 F.

Liquid water has more energy than frozen water. When water freezes it gives up some of the water's energy. This energy that is given up is the latent heat of freezing. When the water was freezing latent heat of freezing energy was being released. Heat energy was actually being released. It is this heat energy that prevented the temperature from continuing to cool once the temperature reached 32 F. The way to think of this is that the heat energy does not warm the temperature but rather stops the cooling. The cooling that would have occurred is perfectly offset by the latent heat energy release and thus the temperature remains constant. When a latent heat process occurs the temperature remains constant. Energy is being transferred even through the temperature is constant. A mixture of water and ice will always have an equilibrium temperature of 32 F. Only when it is only liquid or only ice will the temperature increase or decrease.