**The volume, or the space that a particular gas occupies, decreases when it is cooled.** The direct relationship between the volume and temperature of a gas is known as Charles' law, which states that for all gases at constant temperature, an increase in temperature results to an increase in volume.

Gases can be defined using four physical quantities: volume (V), pressure (P), temperature (T) and the number of gas atoms (n). The general correlation between these values is mathematically represented by the equation PV = nRT, otherwise known as the ideal gas law, where "R" denotes the proportionality constant. The standard units include liters for V, atmospheres for P, Kelvin for T and moles for n.

The Kelvin scale is used for the ideal gas law because changes in the temperature of a gas does not result to a direct variation in its volume when using the Celsius scale. Absolute temperature, which is 0 Kelvin, is equivalent to -273 degrees Celsius. When using the Fahrenheit scale, it is necessary to add 460 to obtain the absolute temperature in the Rankine scale.

The ideal gas equation indicates that decreasing the temperature causes a reduction in its volume. When a gas is cooled down, the atoms comprising the gas have less energy to move around. The atoms tend to condense and move closer together, causing them to occupy a less amount of space.