Which statement expresses Gay-Lussac's Law?

• A. At a constant volume, the pressure and temperature of a gas are directly related.
• B. At a constant pressure, the volume and temperature are inversely related.
• C. At a constant temperature, the pressure and volume of a gas are directly related.
• D. At a constant volume, the pressure and temperature of a gas are directly related.

Answer: (D)

Gay-Lussac's Law is about how the pressure of a gas changes with temperature when the volume is kept fixed. When the temperature rises, the gas molecules move faster, colliding with the container walls more often and with greater force. Those more energetic collisions increase the pressure. So, at a constant volume, pressure and temperature are directly related—P increases as T increases (and this relationship is most accurate when temperature is measured in Kelvin). The temperature must be in Kelvin for the proportionality to hold across all temperatures.

The other ideas mix up different gas laws: with constant pressure, volume and temperature are not inversely related; that scenario is described by Charles's Law (volume increases with temperature at constant pressure). And with constant temperature, pressure and volume are not directly related; they are inversely related (Boyle's Law). Thus the statement describing a direct, proportional link between pressure and temperature at fixed volume best reflects Gay-Lussac's Law.