Definition · Plain-language
Charles’s law
Charles’s law states that, for a fixed amount of gas at constant pressure, the volume is directly proportional to its absolute (kelvin) temperature.
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Volume rises with temperature
Charles’s law, attributed to Jacques Charles in the 1780s, describes how a gas at constant pressure changes volume as its temperature changes. The volume is directly proportional to the absolute temperature: heat the gas and it expands, cool it and it contracts, in exact proportion. Dividing volume by absolute temperature always gives the same value, written V/T = constant, or V₁/T₁ = V₂/T₂ when comparing two states. Crucially, the temperature must be measured on the absolute scale in kelvin, not in degrees Celsius, because only the kelvin scale starts at true zero.
Why kelvin matters
The proportionality only works with an absolute temperature scale. On the Celsius scale, zero is an arbitrary point (the freezing of water), so a gas at 0 °C plainly has a real, non-zero volume. The kelvin scale instead begins at absolute zero — the coldest possible temperature, about −273.15 °C — where the volume of an ideal gas would extrapolate to zero. Using kelvin makes the relationship cleanly proportional: doubling the absolute temperature doubles the volume. This is why every gas-law calculation must convert Celsius to kelvin first, by adding 273.15.
Everyday examples
Charles’s law explains many warming-and-cooling effects. A hot-air balloon rises because heating the air inside makes it expand, lowering its density so it floats on the cooler, denser air around it. A balloon left in a cold car shrinks, then swells again as it warms. Bread and cakes rise partly because trapped gases expand in the oven’s heat. The molecular explanation is that heating speeds up the gas molecules; to keep the pressure constant as they strike the walls harder and faster, the gas must expand into a larger volume.
Key facts
At a glance
- Definition: at constant pressure, gas volume is proportional to absolute temperature
- Equation: V/T = constant, or V₁/T₁ = V₂/T₂
- Condition: fixed amount of gas, constant pressure
- Temperature: must be in kelvin (absolute scale), not Celsius
- Effect: doubling the absolute temperature doubles the volume
- Example: a hot-air balloon rises because heated air expands and becomes less dense
Common misconceptions
What people often get wrong
Often heard: You can use Celsius temperatures in Charles’s law.
Actually: You must use the absolute (kelvin) scale. Celsius has an arbitrary zero, so the proportionality only works once temperatures are converted to kelvin by adding 273.15.
Often heard: Charles’s law works at any pressure during the change.
Actually: It holds only at constant pressure. If pressure also changes, you need the combined gas law, since pressure, volume and temperature are all linked.
Often heard: Gases expand when heated because new gas is created.
Actually: No gas is added. Heating makes the existing molecules move faster, so to keep pressure constant the gas occupies a larger volume.
Going deeper
