Charles's Law
Definition and meaning of Charles's Law in chemistry.
Charles's Law states that at constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute (Kelvin) temperature. Mathematically, V/T = constant, or equivalently V1/T1 = V2/T2 for two states of the same gas sample.
In more detail
As a gas is heated at constant pressure, its molecules move faster and collide with the container walls more forcefully and frequently, so the volume must expand to keep the pressure unchanged. The relationship only holds when temperature is expressed in kelvin, since volume extrapolates toward zero at 0 K (absolute zero) for an ideal gas. Charles's Law is a limiting case of the ideal gas law (PV = nRT) and was formulated from experiments by Jacques Charles in the 1780s, later published by Joseph Louis Gay-Lussac in 1802.
Key facts
| Field | Physical Chemistry |
|---|---|
| Formula | V/T = k (constant P, n) |
| Also known as | Law of volumes |
| Discovered by | Jacques Charles (c. 1787); published by Gay-Lussac (1802) |
A balloon holds 2.0 L of air at 300 K. If heated at constant pressure to 600 K, its new volume is found from V1/T1 = V2/T2: (2.0 L)/(300 K) = V2/(600 K), giving V2 = 4.0 L.
Frequently asked questions
Why must temperature be in kelvin for Charles's Law?
Because the direct proportionality V ∝ T only holds on an absolute scale where zero volume corresponds to zero temperature; using Celsius would give negative or undefined ratios and break the proportionality.
How does Charles's Law relate to the ideal gas law?
Charles's Law is a special case of PV = nRT in which pressure (P) and moles (n) are held constant, isolating the direct relationship between volume and temperature.