 What happens when you release a helium balloon and it floats up high into the atmosphere? Does it expand, shrink or stay the same? This can be answered using Boyle's Law. Boyle's Law is named after chemist and physicist Robert Boyle who first published results of his experiments with gases in the 1600s. It states that for a fixed mass of an ideal gas at a fixed temperature, the pressure and volume are inversely proportional. In other words, at same temperature, the product of pressure and volume of an ideal gas is constant. This can also be written as P1V1 equals P2V2 to describe what happens if the pressure or volume change. As the volume decreases, the air becomes compressed and the pressure increases. And vice versa, if the pressure decreases, the volume must increase. And it's a direct inverse relationship. So for example, if the volume is halved, then the pressure is doubled. This relationship can also be written as pressure times volume equals k, where k is a constant specific to the gas involved. An interesting observation is if we graph one over pressure and volume, we have a linear relationship. This can be written as volume equals k times one over pressure, where the gradient is k. Back to the balloon example. For the most part, the pressure in the atmosphere decreases with altitude as the balloon goes up. For simplicity, let's assume that the temperature stays constant. This means that there's less atmospheric pressure in the balloon. Boyle's law tells us that if the pressure decreases, the volume must increase. The balloon expands to increase its volume until it pops.