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

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

Answer: (A)

Volume changes directly with temperature when pressure is kept constant because as temperature rises, gas molecules move faster and tend to push against the container walls more, so the gas expands to maintain the same pressure. This direct proportionality is described by Charles's Law, which relies on Kelvin temperatures to hold the relationship true (V ∝ T in Kelvin). Using Kelvin matters because it starts at absolute zero, ensuring proportionality holds without offsets that Celsius would introduce. In practical terms, heating a gas at fixed pressure makes it occupy more space, as seen in hot air balloons and piston expansions. Other gas laws involve different fixed conditions: at constant volume, temperature changes affect pressure; at constant temperature, volume and pressure trade off; and at constant temperature and pressure, adding more gas increases volume.