Charles' law describes the relationship between which two properties of a gas?

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Volume of a gas changes in direct proportion to its absolute temperature when the pressure is held constant. This is Charles' law: V ∝ T with T measured in Kelvin. As you heat the gas at fixed pressure, its molecules move more rapidly and push the container walls outward, so the volume expands; cooling slows them down and the volume contracts. Using Kelvin is essential because it keeps the relationship linear and avoids issues with negative temperatures. Other gas laws describe different scenarios—for example, keeping T constant links pressure and volume (Boyle’s law), or keeping V constant links pressure and temperature (Gay-Lussac’s law). So the relationship described by Charles' law is between volume and temperature.