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For freezing as a method of food preservation, see frozen food.

In physics and chemistry, freezing is the process whereby a liquid turns to a solid. The freezing point is the temperature at which this happens. Melting, the process of turning a solid to a liquid, is the opposite of freezing. For most substances, the melting and freezing points are equal.

The melting point and freezing point of the element mercury are the same. Rapid cooling by exposure to cryogenic temperatures can cause a substance to freeze below its melting point, a process known as flash freezing. For some pure substances, such as pure water, the freezing temperature is lower than the melting temperature. The freezing point for water is only the same temperature as the melting point when nucleators are present to prevent supercooling. The freezing point of water is 0°C (32°F, 273 K). In the absence of nucleators water will supercool to −42°C (−43.6°F, 231 K) before freezing. But in the presence of nucleating substances the freezing point of water is the same as the melting point. Nucleating agents, such as dust, are commonly present in the environment, which is why rain water and tap water will normally freeze at the melting point of water.

Main article: Frozen food

Freezing is a common method of food preservation which slows both food decay and the growth of micro-organisms. Besides the effect of lower temperatures on reaction rates, freezing makes water less available for bacterial growth.

Why does freezing always start from the lake surface? How does this phenomenon affect the fish?

Water density changes with the water temperature. The water density is highest at 3.98°C, therefore water molecules at 3.98°C are more likely to sink than the ones at other temperatures.^{[citation needed]}

Let us consider the situation with a water temperature higher than 3.98°C. In Fig.1, the lake's surface temperature is 20°C, the air temperature is 5°C lower than the water temperature, that is, 15°C. Part of the thermal energy of the water molecules on the lake surface will flow to the cooler air. As a result, the temperature of the water molecules drops and the density rises and therefore these water molecules sink. The warmer water under the surface will rise gradually because of its smaller density, convection thus takes place and the whole lake will be cooled down.

Let us see the situation with a water temperature lower than 3.98°C (Fig. 2). As water with a lower temperature has a lower density, the cooler water molecules will flow on the water surface. On the contrary, the molecules that are closer to 3.98°C will sink due to their higher density. When the water temperature is lower than 3.98°C, water with a lower temperature on the lake surface will freeze first and then the freezing will spread to the lake bottom. Because heat is transmitted by conduction, the cooling rate is very slow. The water temperature under the ice layer usually maintains at 3.98°C. Fish can still survive. If water does not have this special property, we can imagine that water will start to freeze from the bottom and convection will increase the rate of freezing. After a very cold winter, we are afraid that all fish and other living things in the lake will all be dead.

• Flash freezing
• Nucleation
• Supercooling
• Melting point