Table of Contents
What Is Charles's Law?
Charles's Law states that the volume of an ideal gas at constant pressure is directly proportional to its absolute temperature. Discovered by Jacques Charles in the 1780s and later refined by Joseph Louis Gay-Lussac, this law explains why gases expand when heated and contract when cooled, forming the basis for hot air balloons, thermometers, and many industrial processes.
The law requires temperatures in absolute units (Kelvin) because the proportionality constant includes an origin at absolute zero. At 0 K, an ideal gas would theoretically have zero volume. In practice, all real gases liquify before reaching absolute zero, but the linear relationship between volume and temperature holds well for gases above their condensation point and at moderate pressures.
Formula
Example Calculations
| V1 (L) | T1 (K) | T2 (K) | V2 (L) | Change |
|---|---|---|---|---|
| 1.0 | 273 | 373 | 1.37 | +37% |
| 5.0 | 300 | 600 | 10.0 | +100% |
| 3.0 | 400 | 200 | 1.5 | -50% |
| 10.0 | 500 | 250 | 5.0 | -50% |
Applications
- Hot air balloons work because heating air inside the balloon increases its volume, reducing its density below ambient air.
- Weather balloons expand as they rise because atmospheric pressure decreases (combined with Charles's and Boyle's laws).
- Car tire pressure increases in hot weather because the air volume tries to expand in a fixed container, raising pressure instead.
- Gas thermometers use the volume change of a gas to measure temperature with high precision.
Frequently Asked Questions
Why must temperature be in Kelvin?
Charles's Law describes a direct proportionality: V = kT. This only works with an absolute temperature scale where zero means zero molecular motion. Using Celsius would give incorrect results because 0 degrees C is not zero temperature. For example, doubling from 10 to 20 degrees C does not double the volume, but doubling from 283 K to 566 K does.
Does Charles's Law apply to real gases?
Charles's Law applies well to real gases at moderate temperatures and low pressures. Deviations occur near a gas's condensation point or at very high pressures where intermolecular forces become significant. The van der Waals equation provides corrections for real gas behavior in these conditions.
What is the relationship between Charles's Law and the ideal gas law?
Charles's Law is a special case of the ideal gas law (PV = nRT) where pressure and amount of gas are held constant. The ideal gas law combines Charles's Law, Boyle's Law (pressure-volume at constant temperature), and Avogadro's Law (volume-amount at constant temperature and pressure) into a single comprehensive equation.