Gay-Lussac's law

Meaning: For a fixed amount of an ideal gas held at constant volume, pressure and absolute temperature rise and fall together. Heating raises the average kinetic energy of the molecules, so they hit the walls harder and more often, and pressure increases. A plot of P versus T (in kelvin) is a straight line through the origin.

Equation: P₁/T₁ = P₂/T₂, where P is pressure (Pa) and T is absolute temperature (K). Amount of gas (mol) and volume (m³) are held constant. This follows from PV = nRT.

On the MCAT: Typical setups are a sealed rigid container, such as an aerosol can, a tank or a closed flask, that is heated or cooled. You predict the direction of the pressure change or calculate a new pressure. A graph of P versus T may be given, and extrapolating the line to P = 0 gives absolute zero (0 K, about −273 °C). Questions may also ask you to compare it with Boyle's law (P and V at constant T) and Charles's law (V and T at constant P).

Common confusion: Students plug in Celsius. Going from 20 °C to 40 °C does not double the pressure. Convert first: 293 K to 313 K is only about a 7% increase. Also, the law needs constant volume. If the container can expand, such as a piston or balloon, volume changes and this law does not apply directly.