Which gas law describes volume being directly proportional to temperature at constant pressure?

• A. Charles' Law
• B. Boyle's Law
• C. STP
• D. PV = nRT

Answer: (A)

Volume varies directly with temperature when the pressure is kept constant. This is Charles' Law. When the temperature rises, the gas molecules move more rapidly, and to maintain the same pressure, the gas must expand, so its volume increases in proportion to temperature (V ∝ T) with T measured in Kelvin. If you warm a flexible container like a balloon at fixed pressure, you’ll see its volume increase linearly with temperature. The temperatures must be in Kelvin for a true proportional relationship. The other options don’t describe this direct V–T relationship: Boyle’s Law describes P and V at constant temperature, STP is just a set of standard conditions, and the full ideal gas law PV = nRT relates all variables but doesn’t specify that V scales with T at constant P.