Charles's Law
Summary
Charles's law says that for a fixed amount of gas at constant pressure, volume is directly proportional to absolute (Kelvin) temperature, so V₁/T₁ = V₂/T₂. The most testable point is that T must be in kelvins: doubling the Kelvin temperature doubles the volume.
Detail
Meaning: For a fixed amount of gas at constant pressure, volume is directly proportional to absolute temperature. Heating raises the molecules' average kinetic energy, so they hit the walls harder; if the container can expand (a balloon, or a piston at constant external pressure), the volume grows until the pressure is restored. Real gases deviate from the law near condensation.
Equation: V₁/T₁ = V₂/T₂, where V is volume (m³; any consistent unit works in a ratio) and T is absolute temperature (K), with pressure and moles of gas held constant.
On the MCAT: A balloon, syringe, or piston is heated or cooled, and you predict the new volume or the direction of change. A graph of V versus T in kelvins is a straight line through the origin. Plotted against °C, it is a straight line that extrapolates to zero volume at −273.15 °C.
Common confusion: Using Celsius in the ratio. Going from 20 °C to 40 °C does not double the volume; going from 293 K to 313 K raises it only about 7%. Also keep the gas laws apart: Boyle relates P and V inversely (T constant), and Gay-Lussac relates P and T (V constant).
Sources
- Chemistry: The Central Science
- Chemistry: A Molecular Approach
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.