How Substances Are Changed By Heat

The main changes that substances undergo when they are heated are (1) increase in temperature, (2) change of state, and (3) expansion. Each of these changes depends on properties that differ from one substance to another. The rate of temperature change depends on the specific heat of the substance. Change of state—from solid to liquid, or from liquid to gas—occurs only when the latent heat requirements of the substance have been met. Expansion of solids and liquids takes place in accordance with the coefficient of expansion of the substance.

Specific Heat

The amount of heat required to raise the temperature of a unit mass of a substance by a certain amount is called that substance's specific heat. It is expressed as a ratio to the specific heat of water, which by definition is 1.

Latent Heat

is the amount of heat energy that must be absorbed or released by a given quantity of a substance to bring about a complete change of state in the substance. As the latent heat is absorbed or released, the temperature of the substance remains the same. Latent heat is usually referred to in terms of the type of change of state involved.

Heat of Fusion is the latent heat needed to change a substance from a solid to a liquid. The change is called melting.

Heat of Vaporization is the latent heat needed to change a substance from a liquid to a gas (vapor). The change is called boiling if the vapor forms within the liquid, or evaporation if the vapor forms only at the surface.

Heat of Sublimation. At atmospheric pressure some substances when heated change directly from a solid to a gas. This change of state is called sublimation, and the latent heat needed for it is called the heat of sublimation.

Heat of Condensation is the latent heat given up by a substance in changing from a gas to a liquid. The change of state is sometimes called liquefaction. For any given substance, the heat of condensation is equal to the heat of vaporization.

Heat of Solidification is the latent heat given up by a substance in changing from a liquid to a solid or, for those substances that undergo sublimation, in changing from a gas into a solid. In the first case, the heat of solidification is equal to the heat of fusion and the change of state is commonly called freezing. In the second case, the heat of solidification is equal to the heat of sublimation.

Expansion

As a substance gains heat energy, its molecules push farther apart, causing the substance to occupy more space. This increase in size is called expansion. Solids and liquids expand in the same way. Gases are subject to different laws.