Here's why:
* Charles's Law states that the volume of an ideal gas is directly proportional to its absolute temperature, keeping pressure constant.
* Absolute temperature is measured in Kelvin. This scale starts at absolute zero (-273.15°C), where theoretically all molecular motion stops.
* Celsius is a relative scale, meaning its zero point is arbitrary (the freezing point of water). Using Celsius in Charles's Law would lead to incorrect results.
In summary, you must use Kelvin when applying Charles's Law.
