Boyle's law is a basic law in chemistry describing the behavior of an ideal gas under a constant temperature. An ideal gas is a perfectly pure gas undergoing perfect elastic collisions with its container. No such gas actually exists, but real gases behave closely enough to ideal gases that we can use theories applying to the latter to describe the former.

Boyle's law states that the volume of a gas is inversely proportional to the pressure applied to it. In other words, the pressure of a gas times its volume is a constant. It is not necessary to know the exact value of this constant to meaningfully understand the behavior of gases. The law was discovered by Robert Boyle in 1662. The standard measurements for volume and pressure are cubic meters and pascals (or atmospheres).

One obvious example of Boyle's law in action is in a syringe. In a syringe, the volume of a fixed amount of gas is increased by drawing the handle back, thereby lessening the pressure. The blood in a vein has higher pressure than the gas in the syringe, so it flows into the syringe, equalizing the pressure differential.

Boyle's law is one of three gas laws which thoroughly describe the behavior of gases under varying temperatures, pressures and volumes. The other two laws are Gay-Lussac's law and Graham's law. These gas laws are striking, early examples of how the behavior of physical matter can be approximated in a rigorous mathematical way.

Another way of describing Boyle's law is that when you push a gas, it tends to push back. Without the massive amount of gravity holding them together, the solar system's gas planets would rapidly diffuse in all directions, quickly depressurizing. There is a limit to the amount we can compress any given gas sample, because eventually the pressure becomes so great that it bursts out of any container we can create for it.