What Is Heat Capacity?

The input of heat energy (Q) required to raise the temperature (T) of a substance one degree Celsius (1° C), is defined as its heat capacity (C). Since it is an "extensive" property, the value of C varies not only from substance-to-substance, but also for different amounts of the same substance. To adjust for this, heat capacities may be stated in terms incorporating quantity or amount. If reference is made to heat capacity per mole of material, it is called molar heat capacity; if it is to heat capacity per gram of material instead, it is the specific heat capacity (s) — or more simply, the "specific heat." These terms are of greatest value when referring to pure substances.

Engineering problems often provide C as a "given," while Q is "unknown." The equation is Q=smΔT, where m is the mass in grams and ΔT is the rise in temperature in degrees Celsius. Heat capacity can be a key parameter for a host of reasons. To illustrate, materials of larger heat capacities are sometimes used as heat sinks, because they absorb heat like a sponge. Water is noteworthy in this regard, as it exhibits the greatest C value known among common substances, making it eminently suitable for use as radiator coolant.

In meteorology, heat capacity plays a role in several phenomena, including why wind, along the coast, blows in a different direction in day than it does at night. Land has a lower heat capacity than water, so land heats up quicker than the sea by day, whereas it cools more rapidly at night. Air is cooler over the ocean by day, but over the land at night. Warm air is light and rises, allowing cooler and heavier breezes to replace it. During the day, these breezes blow from land-to-sea, while during the night, the opposite is true, which facts influence shore birds and glider pilots alike.

Heat capacity is not intended to take into consideration phase changes, as in the melting of ice to form water. Separate consideration is given to this phenomenon — this property is called "heat of fusion." Similarly, the conversion of liquid to gas is called "heat of vaporization." Ice has an exceptionally high heat of fusion, imparting stability to earth’s weather systems and making home refrigeration practical. Curiously, ammonia gas, once used in industrial and home refrigeration systems, has an even higher heat capacity and heat of fusion.