Boyle's Law states that the pressure and volume of a fixed amount of gas are inversely proportional when temperature remains constant. In equation form, PV = constant: compressing the gas raises its pressure, while allowing it to expand lowers the pressure.
Robert Boyle published the relationship in 1662 after experiments using air trapped in a J-shaped glass tube and mercury. Earlier observations by Richard Towneley and Henry Power helped establish the pattern, but Boyle systematically tested and documented it.
The law makes sense through the kinetic theory of gases. Gas molecules constantly collide with the container walls. When the same molecules are squeezed into a smaller volume, collisions with each unit of wall area become more frequent, increasing pressure. Doubling the volume ideally halves the pressure.
Boyle's Law applies most accurately to ideal gases and requires a fixed amount of gas and constant temperature. It is often confused with Charles's Law, which connects volume and temperature at constant pressure, or Avogadro's Law, which connects volume and amount of gas.