This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
A pure liquid has a definite freezing point. In freezing the fluid changes from a liquid to a solid state. Water freezes at 0°C. or 32°F. Just as sugar solutions in cooling may give supersaturated solutions before crystallization starts, so water may be supercooled before it freezes. Furthermore, the conditions for supersaturation and supercooling are similar. For supersaturation the solution must not be agitated and no crystals must be added. If a test tube of water containing a thermometer is immersed in a mixture of ice and salt, the temperature of the water will drop to - 4°C. or lower. If the slightest movement is made or if a small crystal of ice is added the water will crystallize quickly, and the temperature rises to 0°C, for water in freezing gives off heat. A gram of water changing from 0° to ice at 0°C. gives off 79.9 (or about 80) calories of heat. When the water has been supercooled, this heat of solidification elevates the temperature of the ice and liquid to 0°C. The heat of crystallization may be absorbed by the liquid or given off to the surroundings if the freezing liquid is not insulated.