Charles’s law
At constant pressure, the volume of a fixed amount of gas is directly proportional to its temperature in kelvins.
V1 / T1 = V2 / T2
Heating a gas makes its particles move faster and hit the walls harder. If the container can expand, it does, until the pressure inside matches the pressure outside again.
Worked example: warming a balloon
A balloon has a volume of 2.00 L at 300 K. What is its volume at 450 K, at the same pressure?
V1 / T1 = V2 / T2
2.00 L / 300 K = V2 / 450 K
V2 = 2.00 L × 450 K ÷ 300 K = 3.00 L
Check: the temperature went up, so the volume should go up. It did.
When pressure, volume and temperature all change, use the combined gas law:
P1 × V1 / T1 = P2 × V2 / T2
Common mistake
Using degrees Celsius. Going from 20 °C to 40 °C does not double the volume. In kelvins that is 293 K to 313 K, an increase of about 7%.