In physics and chemistry, melting is the process of converting a solid substance to its liquid form, typically by heating the substance to a temperature called its melting point. In the reverse process, called freezing, the liquid substance is cooled to the temperature called the freezing point, when it changes to the solid form. The melting point and freezing point are usually the same temperature.
The melting or freezing of a substance is known as a phase change or phase transition. Chemically, the substance remains the same, but its physical state (or "phase") changes. Unlike the boiling point of a substance, its melting point is relatively insensitive to pressure.
Our knowledge of melting and freezing can be applied in various ways. For instance, the purity of many solid compounds can be determined by checking their melting points. Also, freezing is a familiar method of preserving food. Melting a solid, such as a metal or plastic piece, is an extremely valuable means of molding it into a desired shape. The preparation of alloys involves the combining of metals after they have been melted. Given a mixture of several substances in the solid state, melting is a way by which some of these substances can be separated from others (with higher melting points). Materials with high melting points are valuable for making products that need to resist high heat.
Some substances, by contrast, possess differing solid-liquid transition temperatures. For example, agar melts at 85 °C (185 °F) and solidifies from 32 °C to 40 °C (89.6 °F to 104 °F)—this process is known as "hysteresis." Certain materials, such as glass, may harden without crystallizing; these are called "amorphous" solids.
The melting point of water at 1 atmosphere pressure is 0 °C (32 °F, 273.15 K), also known as the ice point. In the presence of nucleating agents such as dust particles, the freezing point of water is the same as the melting point. On the other hand, in the absence of such agents, water can be supercooled to −42 °C (−43.6 °F, 231 K) before it freezes. As nucleating agents are commonly present in the environment, rainwater and tap water normally freeze at (or close to) the melting point of pure water.
Also, if a substance is rapidly cooled by sudden exposure to cryogenic (extremely low) temperatures, it will freeze below its melting point. This process is known as "flash freezing."
If a substance contains a small amount of impurity, its melting point (and freezing point) becomes lower than that of the pure substance. The greater the amount of impurity present, the lower the melting point. Eventually, at a certain composition of the mixture, a minimum melting point will be reached. At that stage, the mixture is said to be at its "eutectic point," or it is called a "eutectic mixture."
If a solid substance is heated, its temperature will rise until it reaches its melting point. If additional heat is supplied, the substance starts melting, and the temperature remains the same until the material is entirely melted.
The amount of heat absorbed by 1 gram of a substance at its melting point when changing from the solid state to the liquid state is called the latent heat of fusion or the enthalpy of fusion (symbol: ). For example, the latent heat of fusion of ice is approximately 80 calories per gram. This means that 1 gram of ice at its melting temperature (0 °C) will absorb 80 calories of heat before it melts completely.
For the reverse process, when 1 gram of the same substance in the liquid state is frozen to its solid state, it releases the same amount of heat.
The heat of fusion of water is 79.72 calories per gram, or 334.5 joules per gram. The heat of fusion of some other common substances are given in the table below.
|Substance||Heat of fusion
|Heat of fusion
Source: CRC Handbook of Chemistry and Physics, 62nd edition.
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