Boyle's Law — Squeeze a Syringe of Air

In 1662 Robert Boyle trapped air and squeezed it, and found that halving its space doubles its pressure. He called air a 'spring' — the more you compress it, the harder it pushes back. You will feel and measure that spring for yourself.

Pull the plunger of a syringe back to draw in air, then seal the tip firmly (a blob of modelling putty or a finger held tight will do). Now the air is trapped. Note the starting volume marked on the barrel.

Push the plunger in to smaller and smaller volumes and feel how much harder you must push each time — that push is the pressure. To put numbers on it, stand the sealed syringe upright and pile known weights on the plunger, or press it against a force meter, and record the volume at each pressure. Notice you can never quite push it to zero: the air fights back ever harder.

What the constant product tells you. (1) Pressure and volume are INVERSELY proportional only while the temperature is held fixed — squeeze fast and the air also heats up, which is why a bicycle pump gets warm. (2) Plotting pressure against 1/volume gives a straight line, the tidy proof of the law. (3) Boyle's law is the first of the gas laws; joined with Charles's law it becomes the ideal gas law that runs engines, refrigerators and weather. (4) It explains why a scuba diver must never hold their breath while surfacing — the lung air expands as the pressure drops — why your ears pop on a plane, and how your own chest lowers its pressure to pull air in with every breath.