Why do ballons fly away when you let go of them? We've got the answer.

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Introduction to How Helium Balloons Work

There is something incredibly neat about helium balloons! If you buy one at the circus or fair, you can hold its string and it will ride along above you. If you let go of the string, it will fly away until you can't see it anymore.

If you hav­e ever wondered why it flies away, then read on. In this edition of HowStuffWorks, you'll find out all about helium!

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As long as the helium plus the balloon is lighter than the air it displaces, the balloon will float in the air.

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Floating in General

Most of us feel comfortable with the idea of something floating in water. We see that happen every day. In fact, people themselves float in water, so we have a way of directly experiencing water flotation. The reason why things float in water applies to air as well, so let's start by understanding water flotation.

Let's say that y­ou take a plastic 1-liter soda bottle, empty out the soft drink it contains, put the cap back on it (so you have a sealed bottle full of air), tie a string around it like you would a balloon, and dive down to the bottom of the deep end of a swimming pool with it. Since the bottle is full of air, you can imagine it will have a strong desire to rise to the surface. You can sit on the bottom of the pool with it, holding the string, and it will act just like a helium balloon does in air. If you let go of the string the bottle will quickly rise to the surface of the water.

The reason that this soda bottle "balloon" wants to rise in the water is because water is a fluid and the 1-liter bottle is displacing one liter of that fluid. The bottle and the air in it weigh perhaps an ounce at most (1 liter of air weighs about a gram, and the bottle is very light as well). The liter of water it displaces, however, weights about 1,000 grams (2.2 pounds or so). Because the weight of the bottle and its air is less than the weight of the water it displaces, the bottle floats. This is the law of buoyancy.

Helium Flotation

Helium balloons work by the same law of buoyancy. In this case, the helium balloon that you hold by a string is floating in a "pool" of air (when you stand underwater at the bottom of a swimming pool, you are standing in a "pool of water" maybe 10 feet deep -- when you stand in an open field you are standing at the bottom of a "pool of air" that is many miles deep). The helium balloon displaces an amount of air (just like the empty bottle displaces an amount of water). As long as the helium plus the balloon is lighter than the air it displaces, the balloon will float in the air.

It turns out that helium is a lot lighter than air. The difference is not as great as it is between water and air (a liter of water weighs about 1,000 grams, while a liter of air weighs about 1 gram), but it is significant. Helium weighs 0.1785 grams per liter. Nitrogen weighs 1.2506 grams per liter, and since nitrogen makes up about 80 percent of the air we breathe, 1.25 grams is a good approximation for the weight of a liter of air.

Therefore, if you were to fill a 1-liter soda bottle full of helium, the bottle would weigh about 1 gram less than the same bottle filled with air. That doesn't sound like much -- the bottle itself weighs more than a gram, so it won't float. However, in large volumes, the 1-gram-per-liter difference between air and helium can really add up. This explains why blimps and balloons are generally quite large -- they have to displace a lot of air to float. The following diagram shows the different lifting capacities of different volumes of helium:

A 100-foot-diameter balloon can lift 33,000 pounds! Here is how you can figure out the lifting capacity of the helium in a spherical helium balloon:

  1. Determine the volume of the balloon. The volume of a sphere is 4/3 * pi * r3, where r is the radius of the balloon. So first determine the radius of the sphere (the radius is half the diameter). Cube the radius (multiply it by itself twice: r*r*r), multiply by 4/3 and then multiply by Pi. If you are measuring your balloon in feet, that gives you the volume of the balloon in cubic feet.
  2. One cubic foot of helium will lift about 28.2 grams, so multiply the volume of the balloon by 28.2.
  3. Divide by 448 -- the number of grams in a pound -- to determine the number of pounds it can lift.

So, for example, a 20-foot balloon has a radius of 10 feet. 10* 10 * 10 * 3.14 * 4/3 = 4,186 cubic feet of volume. 4,186 cubic feet * 28.2 grams/cubic feet = 118,064 grams. 118,064 grams / 448 grams per pound = 263 pounds of lifting force.

Although not used much anymore, hydrogen balloons were once quite popular. Hydrogen weighs just 0.08988 grams per liter. However, it is highly flammable, so the slightest spark can cause a huge explosion.

So why are helium and hydrogen so much lighter than air? It's because the hydrogen and helium atoms are lighter than a nitrogen atom. They have fewer electrons, protons and neutrons than nitrogen atoms do, and that makes them lighter (the approximate atomic weight of hydrogen is 1, helium is 4 and nitrogen is 14). Approximately the same number of atoms of each of these elements fills approximately the same amount of space. Therefore, the gases made of lighter atoms are lighter.

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Hot Air

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What about hot air balloons? They work by similar principles. If you heat up a gas it expands. In the case of a hot air balloon, when the gas inside the balloon expands the extra gas is pushed out the bottom of the balloon, meaning that there are fewer atoms inside the balloon, meaning that the air in the balloon is lighter than the air outside the balloon.

The amount of lifting power is controlled by how hot the air is. If you heat the air inside the balloon 100 degrees F hotter than the outside air temperature, then the air inside the balloon will be about 25 percent lighter than the air outside the balloon. So a cubic foot of air weighs about 35 grams at 32 degrees F. A cubic foot of hot air at 132 degrees F will weigh 25 percent less, or about 26.5 grams. The difference is 8.5 grams or so. So a hot air balloon has to be much bigger to support the same weight, but it will float because hotter air is lighter than cooler air.

You can get a sense for how much air contracts and expands as its temperature changes by performing the following experiment:

  1. Take two Ziplock bags (1-gallon size) and blow them up (you can do this by zipping the bag closed, then unzipping a small hole at one end of the zipper). Blow each bag up like a balloon and seal it while holding pressure on the last breath. You want these bags to be full -- you want the plastic on both inflated bags to be tense.
  2. Now let the bags sit on the counter for a couple of minutes and cool off. You pumped 98.6 degree F air into them, and you want the temperature to drop to room temperature. The bags will probably become a little less tense in the process of cooling (makes sense...) so add one more puff of air to make them tense again.
  3. Now stick one of the bags into your freezer for about three minutes, while leaving the other one on the counter. When you take the bag in the freezer out it will have deflated some. How much? By about 10 percent to 15 percent. It has deflated because cooler air is denser than warmer air. Compare the cold bag to the bag on the counter -- the cold bag will not be tense at all. Then a funny thing will happen as the cold bag warms up -- it will get tense again and return to its original size!

You can clearly see that warmer air takes up more space than cooler air. Therefore, warmer air is lighter than cooler air, and that is what makes a hot air balloon float!

For more information on hot air ballooning, click here for a collection of links and check out How Hot Air Balloons Work!

Where Helium Comes From

If you put helium in a balloon and let go of the balloon, the balloon rises until it pops. When it pops, the helium that escapes has no reason to stop -- it just keeps going and leaks out into space.

Therefore, in the atmosphere there is very little helium at any given time. The helium that is there comes from alpha particles emitted by radioactive decay (see How Nuclear Radiation Works for details on alpha decay). In places that have a lot of uranium ore, natural gas tends to contain high concentrations of helium (up to 7 percent). This makes sense, since the decay of uranium emits lots of alpha particles and a natural gas pocket tends to be a sealed container underground. Helium is cryogenically distilled out of natural gas to produce the helium we put in balloons.

For more on helium and related topics, check out the links on the next page!

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