Charles' law
Summary
Charles' law says that for a fixed amount of an ideal gas at constant pressure, volume is directly proportional to absolute temperature: V₁/T₁ = V₂/T₂. Heating makes the gas expand. The testable trap is that T must be in kelvins, not °C.
Detail
Meaning: For a fixed amount of gas at constant pressure, volume is directly proportional to absolute temperature. Higher temperature means greater average kinetic energy, so molecules hit the walls harder; for pressure to stay constant, the container must expand. A plot of V versus T in kelvins is a straight line through the origin.
Equation: V₁/T₁ = V₂/T₂, where V is volume (m³, or any unit used on both sides) and T is absolute temperature (K), with n and P constant.
On the MCAT: Expect a balloon or piston heated or cooled at constant pressure: find the new volume, or pick the correct V–T graph. Doubling the Kelvin temperature doubles the volume. The law follows from PV = nRT with n and P held fixed.
Common confusion: Using Celsius. Going from 25 °C to 50 °C does not double the volume; 298 K to 323 K raises it by only about 8%. Convert with K = °C + 273 first. Do not mix it up with Gay-Lussac's law (P ∝ T at constant V and n) or Boyle's law (P ∝ 1/V at constant T and n).
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.