Heat capacity (C) is the input of heat energy (Q) required to raise the temperature (T) of a substance by 1°C. It varies for different substances and quantities. Molar heat capacity is per mole, specific heat capacity is per gram. Heat capacity is used in engineering and meteorology. It does not account for phase changes, which have their own properties, such as heat of fusion and heat of vaporization.
The input of heat energy (Q) required to raise the temperature (T) of a substance by one degree Celsius (1°C), is defined as its heat capacity (C). Being an “extensive” property, the value of C varies not only from substance to substance, but also for different quantities of the same substance. To accommodate for this, heat capacities can be expressed in terms that incorporate quantities or quantities. If we refer to the heat capacity per mole of material, we are talking about molar heat capacity; if instead it is the heat capacity per gram of material, it is the specific heat capacity or, more simply, the “specific heat”. These terms are of great value when speaking of pure substances.
Engineering problems often give C as “given”, while Q is “unknown”. The equation is Q=smΔT, where m is the mass in grams and ΔT is the temperature increase in degrees Celsius. Heat capacity can be a key parameter for a variety of reasons. To illustrate, materials with higher heat capacities are sometimes used as heat sinks, as they absorb heat like a sponge. Water is notable in this regard, as having the highest known C value of common substances, making it eminently suitable for use as a radiator coolant.
In meteorology, heat capacity plays a role in several phenomena, including why wind along the coast blows in a different direction during the day than it does at night. Land has a lower heat capacity than water, so the land warms up faster than the sea during the day, while it cools down faster at night. The air is cooler over the ocean during the day, but on land at night. Warm air is light and rises, allowing cooler, heavier breezes to replace it. During the day, these breezes blow from land to sea, while during the night the opposite is true, affecting both shorebirds and glider pilots.
Heat capacity does not account for phase changes, as in the melting of ice to form water. A separate consideration is given to this phenomenon: this property is called the “heat of fusion”. Similarly, the conversion of liquid to gas is called the “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 domestic refrigeration systems, has an even higher heat capacity and heat of fusion.
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