Charles's Law
Charles's law V₁/T₁ = V₂/T₂ shows how gas volume changes with temperature at constant pressure.
Step-by-step examples included.
The Formula
At constant pressure, the volume of a gas is directly proportional to its absolute temperature. When you heat a gas, it expands. When you cool it, it contracts.
Variables
| Symbol | Meaning |
|---|---|
| V₁ | Initial volume |
| T₁ | Initial temperature (must be in kelvin, K) |
| V₂ | Final volume |
| T₂ | Final temperature (must be in kelvin, K) |
Example 1
A balloon has a volume of 2.5 L at 20°C. What is its volume at 80°C (pressure constant)?
Convert to kelvin: T₁ = 20 + 273.15 = 293.15 K, T₂ = 80 + 273.15 = 353.15 K
Rearrange: V₂ = V₁ × T₂ / T₁ = 2.5 × 353.15 / 293.15
V₂ ≈ 3.01 L
Example 2
A gas occupies 500 mL at 300 K. To what temperature must it be heated to expand to 750 mL?
Rearrange: T₂ = T₁ × V₂ / V₁ = 300 × 750 / 500
T₂ = 450 K (which is 176.85°C)
When to Use It
Use Charles's law when pressure is constant and temperature or volume changes.
- Predicting how a gas volume changes with heating or cooling
- Hot air balloon calculations
- Industrial gas storage and transport
- Always use kelvin for temperature — never Celsius or Fahrenheit