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Heat

Temperature Scales, pure substances, specific heat of water, phase changes, building blocks of matter

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>  Heat Units

>  Specific Heat

>  Transfer of Heat

A number of physical changes are associated with the change of temperature of a substance. Almost all substances expand in volume when heated and contract when cooled. The behavior of water between 0 and 4 C (32 and 39 F) constitutes an important exception to this rule. The phase of a substance refers to its occurrence as either a solid, liquid, or gas, and phase changes in pure substances occur at definite temperatures and pressures. The process of changing from solid to gas is referred to as sublimation, from solid to liquid as melting, and from liquid to vapor as vaporization. If the pressure is constant, these processes occur at constant temperature. The amount of heat required to produce a change of phase is called latent heat, and hence, latent heats of sublimation, melting, and vaporization exist (see Distillation; Evaporation). If water is boiled in an open vessel at a pressure of 1 atm, the temperature does not rise above 100 C (212 F), no matter how much heat is added. The heat that is absorbed without changing the temperature of the water is the latent heat; it is not lost but is expended in changing the water to steam and is then stored as energy in the steam; it is again released when the steam is condensed to form water (see Condensation). Similarly, if a mixture of water and ice in a glass is heated, its temperature will not change until all the ice is melted. The latent heat absorbed is used up in overcoming the forces holding the particles of ice together and is stored as energy in the water. To melt 1 g of ice, 79.7 cal are needed, and to convert 1 g of water to steam at 100 C, 541 cal are needed.

Temperature

The heat capacity, or the measure of the amount of heat required to raise the temperature of a unit mass of a substance one degree is known as specific heat. If the heating process occurs while the substance is maintained at a constant volume or is subjected to a constant pressure the measure is referred to as a specific heat at constant volume or at constant pressure. The latter is always larger than, or at least equal to, the former for each substance. Because 1 cal causes a rise of 1 C in 1 g of water, the specific heat of water is 1 cal/g/ C. In the case of water and other approximately incompressible substances, it is not necessary to distinguish between the constant-volume and constant-pressure specific heats, as they are approximately equal. Generally, the two specific heats of a substance depend on the temperature.

Temperature Scales

The physical methods by which energy in the form of heat can be transferred between bodies are conduction and radiation. A third method, which also involves the motion of matter, is called convection. Conduction requires physical contact between the bodies or portions of bodies exchanging heat, but radiation does not require contact or the presence of any matter between the bodies. Convection occurs when a liquid or gas is in contact with a solid body at a different temperature and is always accompanied by the motion of the liquid or gas. The science dealing with the transfer of heat between bodies is called heat transfer.

Latent Heat

To understand the nature of light and how it is normally created, it is necessary to study matter at its atomic level. Atoms are the building blocks of matter, and the motion of one of their constituents, the electron, leads to the emission of light in most sources.



Article key phrases:

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