 The specific heat of a substance, especially a gas, may be significantly higher when it is allowed to expand as it is heated (specific heat "at constant pressure") than when is heated in a closed vessel that prevents expansion (specific heat "at constant volume").These two values are usually denoted by formula_1 and formula_2, respectively; their quotient formula_3is the heat capacity ratio.In some contexts, however, the term "specific heat capacity" (or "specific heat") may refer to the ratio between the specific heats of a substance at a given temperature and of a reference substance at a reference temperature, such as water at 15 °C; much in the fashion of specific gravity.