As temperature increases, the density of liquids and gases decreases; as temperature decreases, the density increases. Density is the amount of mass per unit of volume.
As temperature increases, the molecules in liquids and gases move more quickly, which makes them collide with one another more often. This causes the molecules to spread apart, which means that there are fewer molecules are in a given volume and the substance's density decreases. As temperature decreases, the molecules move more slowly, so they collide less and take up less space and the substance's density increases. Water has a density of 1,000 kilograms per cubic meter at 4 degrees Celsius and a density of 958.4 kilograms per cubic meter at 100 degrees Celsius.