Freezing
Freezing, the process of changing a liquid into a solid by cooling below a certain temperature called the freezing point. The freezing point is the temperature at which the liquid and solid forms of a substance are in equilibrium (balance). At this point, if heat is neither added nor taken away, the liquid will not change into a solid, nor the solid into a liquid. Thus, the freezing point and the melting point of a substance are the same.
About 80 calories of heat are required to convert a gram of ice into water. Conversely, water liberates about 80 calories of heat per gram into the air when it freezes. This heat is called the heat of fusion, or latent heat. The temperature of both the ice and the water from melting ice is 32 F., or 0 C.
The freezing points of substances vary widely. Freezing points are governed largely by the nature of the substances themselves, but may be changed by the addition of other substances and by pressure. The freezing point of a substance often determines how it can be used. Ethyl alcohol, for example, freezes at -179 F. (-117 C.), mercury at -38 F. (-39 C.). For this reason, alcohol is used instead of mercury in thermometers in colder regions.
A familiar example of changing the freezing point of a liquid by adding another substance is the use of antifreeze. Automobile radiators are commonly protected against freezing during winter by adding ethylene glycol or other antifreezes to the water to lower its freezing point.
The freezing point of a substance is also lowered by pressure. An ice skater, for example, does not actually travel on ice, but rather on a thin layer of water melted from the ice by the pressure of his skate blades. This water refreezes almost instantly when the pressure is removed.
Most substances contract and become denser upon freezing, but water expands and becomes less dense. It is this expansion that causes pipes and bottles to crack when their contents freeze, and rocks to split open when water freezes in their crevices. Icebergs and blocks of ice float in water because they are less dense than the water from which they were frozen.