Charles's Law
Volume is proportional to temperature at constant P.
Formula
Variables
Example
Warm a balloon and it expands; cool it in liquid nitrogen and it shrinks.
How it works
Charles's law says that if the pressure stays the same, a gas expands when heated and contracts when cooled because its molecules move faster or slower and take up more or less space. The key idea is that temperature must be measured on an absolute scale like kelvin, since zero kelvin corresponds to the theoretical point where gas particle motion would be minimal. This relationship is only accurate for a fixed amount of gas at constant pressure, not when gas is added, removed, or compressed.
When to use it
Use this law when a gas sample is heated or cooled without changing the pressure, such as in a piston, a balloon, or a gas trapped under atmospheric pressure. It is useful for finding a new volume from a temperature change, or a temperature from a volume change.
Worked example, step by step
A gas occupies 2.5 L at 300 K. What volume will it have at 360 K if pressure stays constant? Use Charles's law: V1/T1 = V2/T2 Substitute the known values: 2.5/300 = V2/360 Solve for V2: V2 = (2.5 × 360) / 300 V2 = 900 / 300 V2 = 3.0 L Final answer: The gas expands to 3.0 L.
Common mistakes to avoid
- Using degrees Celsius instead of kelvin
- Forgetting that the pressure must stay constant
- Mixing up the variables and solving for the wrong quantity
Did You Know?
Heat a gas and it expands — the principle that lifts hot-air balloons.
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More in Thermodynamics & Heat
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BasicHeat needed to change a substance’s temperature.
Latent Heat
IntermediateHeat for a phase change at constant temperature.
Ideal Gas Law
IntermediateRelates pressure, volume, moles and temperature.
Boyle's Law
BasicPressure and volume are inversely related at constant T.
Gay-Lussac's Law
IntermediatePressure is proportional to temperature at constant V.
Combined Gas Law
IntermediateCombines Boyle, Charles and Gay-Lussac.