When we blow up a balloon normally, they can stay inflated for a couple of days. But we all know that helium balloons normally go flat within one day.

Why?

The answer comes when we consider what pressure is. Put simply, pressure is caused by the gas molecules whizzing around inside the balloon and banging against the sides. Each time they collide with the side of the balloon, they bounce off, but in doing so push the rubber out a little bit.

Now it makes sense to say that the heavier the molecule, the greater the impact when it hits the sides of the balloon, and therefore the more pressure it exerts.

Well, helium is a very small molecule, and air is much heavier, being composed mostly of nitrogen and oxygen. In fact, the ratio of the weights of helium to air is about 7 to 1. So the air molecules are about seven times as heavy as the helium.

But here’s the funny thing – according to the ideal gas law, a certain number of molecules of air under the same conditions will exert exactly the same amount of pressure as the same number of molecules of helium.

But how can that be? If we have just seen that the air molecules are heavier, and exert more force, shouldn’t they cause more pressure if there is the same number of them?

Well, as it happens, there is one other factor that we must take into account – speed. How fast are the molecules travelling? Well, it turns out that the heavier air molecules are not moving as quickly as the helium molecules so although each impact exerts more force there aren’t as many impacts as there are fewer molecules.

On the other hand, the much lighter helium is moving far more quickly, and therefore colliding with the walls of the balloon more often.

And this is the simple reason why the balloons go flat – there are defects in the structure of rubber that allow the occasional gas molecule to escape. Put simply, on a statistical basis alone, since the helium molecules are colliding more often with the walls of the balloon, they will escape more quickly. So it’s purely a function of the frequency of their collisions with the sides of the balloon. A statistical factor you might say.

Hydrogen, the other common lighter than air gas, is also a very small molecule, and would do the same as the helium (and probably quicker).