Re: Re: Why does a helium balloon sink

Hi Michelle, thanks for asking your question.

The simple answer to your question is that the balloon leaks helium over time until it finally sinks to the ground.

Here's a picture of what happens:

Before you fill the balloon with helium (or even air), the pressure on the inside of the balloon is the same as the pressure outside, as shown by the arrows. As you fill the balloon with helium, the pressure inside becomes far greater than the pressure outside (more arrows). That's also why you tie a balloon off at the bottom, to keep the added air inside. Over time helium molecules manage to seep through the wall of the balloon, escaping into the atmosphere.

Helium balloons float because helium is lighter than the other gasses in the earth's atmosphere. What we call air actually consists of many different gases, namely nitrogen, oxygen and carbon dioxide. All of these gasses 'weigh' more than helium. You can think of it like blowing bubbles in water.. the bubbles rise to the surface because they are lighter than the water. The same principle applies in this case, the helium rises within our atmosphere because the rest of the air molecules are heavier and prefer to stay near the bottom. In fact, helium is so light that it rises and escapes from our atmosphere into outer space. Most of the helium at circuses and other festivals comes from pockets of the gas trapped in the earth's crust. These pockets date back to the time the earth formed in the solar system. Since helium escapes from our atmosphere once it has become free, it is conceivable that we may someday run out of helium, though certainly not in the near future..

-Lynn Bry