V₁/T₁ = V₂/T₂ (constant pressure)
Jacques Charles discovered in 1787 that at constant pressure, gas volume is directly proportional to absolute temperature (in Kelvin): V₁/T₁ = V₂/T₂. This means heating a gas from 273 K (0°C) to 373 K (100°C) increases its volume by 37%. You must use Kelvin — Celsius or Fahrenheit give wrong results because they have arbitrary zero points. Absolute zero (0 K = -273.15°C) is where gas volume would theoretically reach zero. Practical applications: hot air balloons (heated air expands and becomes less dense), thermal expansion of gases in engines, and understanding why tire pressure increases on hot days. Charles's Law is a special case of the ideal gas law PV = nRT.