Which law expresses that pressure is directly proportional to temperature at constant volume?

• A. Boyle's Law
• B. Gay-Lussac's Law
• C. Charles' Law
• D. Avogadro's Law

Answer: (B)

Pressure at a fixed volume rises directly with temperature because the gas molecules move faster as temperature increases, leading to more and stronger collisions with the container walls. Since the volume can’t change, those collisions push harder and more often, raising the pressure in proportion to the temperature. This is Gay-Lussac’s law: with volume and amount fixed, pressure is proportional to temperature (P ∝ T). In practice, using Kelvin for temperature makes the relationship linear, so P1/T1 = P2/T2 when T is in kelvin. This ties directly to the ideal gas law, since P = nRT/V and keeping V and n constant makes P scale with T. Using Celsius would distort the proportionality, underscoring why absolute temperature matters. The other laws describe what happens when different variables are held constant: pressure with volume (Boyle), volume with temperature (Charles), or volume with number of particles (Avogadro), not pressure with temperature at fixed volume.