You are sitting on a bicycle pump. The handle is up, the cylinder is full of air. Now push the handle down. The air inside gets squashed into a smaller space — its volume decreases. At the same time, you feel the resistance building: the air is pushing back harder. That is pressure increasing.
That is the raw physical feeling of Boyle's Law. Squeeze a gas, it fights back more. Let it expand, it relaxes.
Now the precise idea: if you take a fixed amount of gas (no air leaking in or out) and keep its temperature absolutely constant, then the volume it occupies and the pressure it exerts are inversely proportional. Double the pressure, the volume halves. Triple the pressure, the volume shrinks to one-third. Halve the pressure, the volume doubles.
Mathematically, this is written as:
P∝V1(at constant T and fixed mass)
Or, more usefully, as a product that stays constant:
where k is some constant number for that particular sample of gas at that temperature.
P1V1=P2V2
This is the working form. If you know three of the four values (initial pressure, initial volume, final pressure, final volume), you can find the fourth.
Why does this happen? Imagine the gas as billions of tiny balls (molecules) zipping around in a box. Pressure comes from them banging into the walls. If you shrink the box, each molecule has less distance to travel between walls, so it hits the walls more often. More collisions per second means higher pressure. The molecules themselves haven't changed speed (temperature is constant), but they are now crowded into a smaller space, so the wall gets hit more frequently.
A very common mistake is to think pressure and volume are directly proportional — that increasing one increases the other. That is wrong for a fixed mass at constant temperature. They move in opposite directions. When one goes up, the other goes down.
A graph of pressure against volume is a curve that swoops downward — a rectangular hyperbola. A graph of pressure against 1/V is a straight line through the origin. That straight line is the signature of a perfect inverse proportion.
Boyle's Law: For a fixed mass of gas at constant temperature, the volume is inversely proportional to the pressure.
Boyle's law is one of the earliest gas laws taught in the NCERT/CBSE Class 11 Chemistry chapter on states of matter, and "Boyle's law: definition, formula & real-world examples" is a very frequently searched query for this topic. It's also a building block for ideal gas equation questions that appear consistently in JEE Main and NEET important-question lists.