Heat

The form of energy that passes between two samples owing to the difference in their temperatures.

Temperature

The property of matter which determines whether heat energy will flow to or from another object of a different temperature.

Heat energy will flow from an object of a high temperature to an object of a lower temperature. An object with a high temperature does not necessarily contain more heat energy than one with a lower temperature as the temperature change per unit of heat energy supplied will depend upon the specific heat capacity of the object in question.

Triple point

The temperature at which all three phases of water – solid, liquid and gas – are in equilibrium at 611.73 Pa. It occurs at 0.01 °C.

Kelvin

One kelvin is equal to 1/273.16 of the thermodynamic triple point of water. A change in temperature of 1 K is equal in magnitude to that of 1 °C.

Kelvin must be used when performing calculations with temperature. For example, the volume of gas at 20 °C is not double that at 10 °C: 10 °C is 283.15 K so the temperature must rise to 566.30 K (293.15 °C) before the volume of gas will double.

Celsius/centigrade

Celsius (formerly called the degree centigrade) is a common measure of temperature in which a change of 1 °C is equal in magnitude to a change of 1 K. To convert absolute temperatures given in degrees celsius to kelvin, you must add 273.15. For example 20 °C = 293.15 K.