Charles's Law
At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature.
Formula / equation
V₁ / T₁ = V₂ / T₂
Unit
m³/K
In depth
Heating increases molecular speed, so the gas must expand to keep the collision rate — and hence the pressure — unchanged. Extrapolating the straight-line V against T plot to zero volume gives −273.15 °C, which is how absolute zero was first identified.
Examples in the real world
A balloon shrinking visibly when dipped into liquid nitrogen and re-inflating as it warms.