These two values are usually denoted by c p. The specific heat capacity of a substance, especially a gas, may be significantly higher when it is allowed to expand as it is heated (specific heat capacity at constant pressure) than when it is heated in a closed vessel that prevents expansion (specific heat capacity at constant volume). While the substance is undergoing a phase transition, such as melting or boiling, its specific heat capacity is technically undefined, because the heat goes into changing its state rather than raising its temperature. The specific heat capacities of iron, granite, and hydrogen gas are about 449 J⋅kg −1⋅K −1, 790 J⋅kg −1⋅K −1, and 14300 J⋅kg −1⋅K −1, respectively. Liquid water has one of the highest specific heat capacities among common substances, about 4184 J⋅kg −1⋅K −1 at 20 ☌ but that of ice, just below 0 ☌, is only 2093 J⋅kg −1⋅K −1. Specific heat capacity often varies with temperature, and is different for each state of matter. For example, the heat required to raise the temperature of 1 kg of water by 1 K is 4184 joules, so the specific heat capacity of water is 4184 J⋅kg −1⋅K −1. The SI unit of specific heat capacity is joule per kelvin per kilogram, J⋅kg −1⋅K −1. Informally, it is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. In thermodynamics, the specific heat capacity (symbol c) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. For the specific heat capacities of particular substances, see Table of specific heat capacities.
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