Heat energy is most intense in substances whose molecules are moving rapidly in a very disorderly way. Such a substance will give up some of its heat to another substance whose molecules are less agitated. When this happens, the heat is said to “flow” from one substance to another (or from one body to another). The energy transfer is indicated by a change in temperature.

Temperature, therefore, is not the same thing as heat—although the two words are often used interchangeably. Temperature can be defined as the degree of intensity of hotness or coldness. “Hotness” and “coldness,” however, are comparative terms. A flame, for example, is hot when compared with ice but cold when compared with the sun. This definition of temperature, therefore, is vague and unscientific, although it does convey the correct impression that temperature is a measure of relative intensity rather than of quantity.

A more specific definition is: temperature is the ability of one body to give up heat energy to another body. A hot body becomes cooler, and a cold body becomes warmer, as long as heat is flowing from one to the other. The hot body has a greater ability to give up heat and therefore has a higher temperature. After a time the two bodies may reach a condition of heat equilibrium, or balance of heat intensity. Then, heat flow ceases. At the point of equilibrium both bodies can be said to be at the same temperature.

Temperature is measured by means of instruments called thermometers. Several temperature scales have been devised for relating the hotness and coldness of bodies to fixed temperatures, such as the freezing point and boiling point of water. On most temperature scales, the unit of temperature is called a degree. The Kelvin scale is an exception; its unit of temperature is the kelvin.

The Fahrenheit, Celsius (or centigrade), and Reaumur scales are used in the range of temperatures important for human comfort, laboratory experiments, and industrial processes.

The Rankine scale and the Kelvin scale are based on the concept of absolute zero; all temperature readings on these scales are positive numbers. The Kelvin scale is widely used in scientific work. The Rankine scale is used primarily by British and American engineers.

Experiments have shown that every 1° C. increase or decrease in temperature causes the pressure exerted by a gas to increase or decrease at the constant rate of 1/273.15 of its pressure at 0° C. This means that at -273.15° C. an ideal (theoretical) gas would exert no pressure at all. Since experiments with real gases have shown a clear relation between pressure and temperature, zero pressure would indicate that the ideal gas had lost all its ability to give up heat. Its molecules would be absolutely motionless. This is impossible—molecules are always agitated, to some extent—and therefore the absolute zero of temperature remains a theoretical concept. The concept is, however, a useful one, for it gives a base point to which all temperature measurements may be referred, in positive numbers.

The idea that absolute zero can never be reached is sometimes considered important enough to be called the third law of thermodynamics. Scientists have succeeded in cooling substances to within a small fraction of a degree above absolute zero. The study of the behavior of substances at very low temperatures is called cryogenics.

Absolute zero is the lower limit for temperature, but there is no upper limit. The hottest substances known are ionized gases in certain stars, with temperatures of a billion degrees or more.

The heat released or absorbed in a physical or chemical process can be measured with an instrument called a calorimeter. Commonly used units for measuring heat are the calorie and the British thermal unit, or Btu. Heat is also measured in such other units as the joule (the unit of energy in the SI, or metric system).