What is Specific Heat?

Specific heat is a measurement used in thermodynamics and calorimetry that states the amount of heat energy necessary to increase the temperature of a given mass of a particular substance by some amount. While different scales of measurement are sometimes used, this term usually specifically refers to the amount required to raise 1 gram of some substance by 1.8°F (1° Celsius). It follows that if twice as much energy is added to a substance, its temperature should increase by twice as much. Specific heat is usually expressed in joules, the unit typically used in chemistry and physics to describe energy. It is an important factor in science, engineering, and in understanding the Earth’s climate.

Heat energy and temperature are two different concepts, and understanding the difference is important. The first is a quantity in thermodynamics that describes the amount of change that a system can cause to its environment. The transfer of this energy to an object causes its molecules to move around more rapidly; this increase in kinetic energy is what is measured, or experienced, as an increase in temperature.

These two properties are often confused. The first is the number of joules required to increase the temperature of a given mass of a substance by some unit. It is always given “per unit mass,” for example, 0.45 j/g°C, which is the specific heat of iron, or the number of joules of heat energy to raise the temperature of one gram of iron by one degree Celsius. This value is therefore independent of the amount of iron.

Heat capacity — sometimes called “thermal mass” — is the number of joules required to raise the temperature of a particular mass of material by 1.8°F (1°Celsius), and is simply the specific heat of the material multiplied by its mass. It is measured in joules per °C. The heat capacity of an object made of iron, and weighing 100g, would be 0.45 X 100, giving 45j/°C. This property can be regarded as the capacity of an object to store heat.

The specific heat of a substance holds more or less true over a wide range of temperatures, that is, the energy required to produce a one degree rise in a given substance varies only slightly with its initial value. It does not apply, however, when the substance undergoes a change of state. For example, if heat is continually applied to a quantity of water, it will produce a rise in temperature in accordance with water’s specific heat. When the boiling point is reached, however, there will be no further increase; instead, the energy will go into producing water vapor. The same applies to solids when the melting point is reached.

A now outdated measure of energy, the calorie, is based on the specific heat of water. One calorie is the amount of energy required to raise the temperature of one gram of water by 1.8°F (1°C) at normal air pressure. It is equivalent to 4.184 joules. Slightly different values may be given for the specific heat of water, as it varies a little with temperature and pressure.

Different substances can have very different specific heats. Metals, for example, tend to have very low values. This means that they heat up quickly and cool down quickly; they also tend to expand significantly as they get hotter. This has implications for engineering and design: allowance often has to be made for the expansion of metal parts in structures and machinery.

Water, in contrast, has a very high specific heat — nine times that of iron, and 32 times that of gold. Due to the molecular structure of water, a great deal of energy is needed to increase its temperature by even a small amount. It also means that warm water takes a long time to cool down.

This property is essential to life on Earth, as water has a significant stabilizing effect on the global climate. During the winter, oceans cool down slowly and release a significant amount of heat into the environment, which helps keep the global temperature reasonably stable. Conversely, in summer, it takes a great deal of heat to significantly increase ocean temperatures. This has a moderating effect on climate. Continental interiors, far from the ocean, experience far greater extremes of temperature than coastal regions.