Boyle's law
Summary
Boyle's law says that for a fixed amount of gas at constant temperature, pressure and volume are inversely proportional, so PV stays constant. Halving the volume doubles the pressure. It is an ideal gas result that real gases follow closely at low pressure and high temperature.
Detail
Meaning: At constant temperature, the average kinetic energy of gas molecules does not change. If the container volume shrinks, the molecules strike the walls more often, so pressure rises. Pressure and volume are therefore inversely proportional for a fixed amount of gas. The law is an ideal gas result and fits real gases well at low pressure and high temperature.
Equation: P₁V₁ = P₂V₂ (n and T constant), where P is pressure in pascals (Pa), V is volume in cubic meters (m³), and subscripts 1 and 2 mark the initial and final states. Any matching units work in the ratio.
On the MCAT: Expect a syringe, piston, or breathing setup. Inhalation enlarges the chest cavity, lung pressure falls below atmospheric pressure, and air flows in. A graph of P versus V is a hyperbola, while P versus 1/V is a straight line through the origin. A typical calculation: volume drops to one third, so pressure triples.
Common confusion: Students mix it up with Charles's law, where volume is directly proportional to temperature in kelvin. Boyle's law applies only when temperature and moles are held constant. If temperature also changes, use the combined gas law, P₁V₁/T₁ = P₂V₂/T₂.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.