That Sinking Feeling

Helium-filled balloons are lots of fun because the balloon lifts up into the air. But they don't last long. Within a day or two, they begin to sink lower and lower, and to get smaller and smaller. Why does this happen?

This is an example of a process known as effusion. Effusion occurs when gas molecules are able to escape their container through a little hole. All gas molecules, whether they're heliums in the balloon, or nitrogens, oxygens, or something else in the air, are moving around really fast (around 1000 miles per hour!!). Every now and then, one of them finds a hole and gets out. Understand that these are very, very tiny holes - too small for you to see. Helium atoms are much smaller than molecules like nitrogen, oxygen, or carbon dioxide, so it's a lot easier for them to get out the hole. They also move faster, so they find the holes more quickly.

You might try some experiments with effusion. Get two helium-filled balloons - one rubber and one Mylar (the shiny kind). Do they both lose helium at the same rate? Compare a helium-filled balloon to one inflated with air or your breath (which has more carbon dioxide than the air you breathed in). Can you see the difference in gas effusion?

Hee-Hee... Helium!

What's in those balloons that float up into the sky? Helium! What's so special about helium, that it makes balloons lighter than air?

It's all about density. Density is defined as the weight of a substance divided by its volume. For example, the density of iron is 491 pounds per cubic foot (a cube 12 inches on each side), or 7870 grams per liter. It's pretty obvious that, if you had a piece of iron smaller than a cubic foot, it would weigh less. But the ratio of the weight of the smaller piece to its volume would be the same as the larger piece.

If you mix substances with two different densities, the more dense one will sink, or looking at the the other way around, the less dense substance rises. If you dropped a chunk of iron (whether large or small) into water, it would sink. But if you put a piece of wood (most kinds, at least) into water, it floats because its density is less than that of water. Helium gas is less dense than air (0.011 pounds per cubic foot or 0.18 grams per liter for helium versus 0.078 pounds per cubic foot or 0.078 grams per liter for air), so balloons filled with helium float!

Fun Facts 3

And the element trivia just keeps coming!

• There is a lot more europium (Eu, #63) on the moon than there is on earth. So what? Scientists believe this means that the earth and the moon did not form in the same way.
• Phosphorus (P, #15) comes in two forms. Red phosphorus doesn't react; it is stable. White phosphorus, on the other hand, bursts into flame in air. Match heads contain red phosphorus. When you light a match, you provide enough energy to turn the red form into the white form, which catches fire immediately.
• Helium (He, #2) was first detected on the sun. It was some time before scientists realized that it existed on earth, too. Helium is usually found trapped under the ground along with natural gas.
• The red color on your television or computer monitor comes from compounds of yttrium (Y, #39).
• Wilhelm Roentgen (1845-1923) discovered X-rays. Roentgenium (Rg, #111) was named in his honor, even though he didn't have anything to do with its discovery.
• The metal tantalum (Ta, #73) is used to make artificial joints, such as hips and knees.
• Protactinium (Pa, #91) has been used to study the way in which oceans and other bodies of water changed after the last Ice Age.
• Astatine (At, #85) is the rarest element that occurs in nature. There is only about one ounce of it on the entire earth!
• A compound of cerium (Ce, #58) is used to treat people who have been badly burned.
• Poor Element #105! It took a long time for scientists to decide what to call it. Usually, the discoverers of an element name it. But in this case, Russian and American scientists had created it at almost the same time in the 1960's, and it was hard to say who was actually first. Scientists argued about names for #105 and several other elements for over 30 years before deciding that #105 would be called dubnium (Db), after the Russian city where the scientists who discovered it worked.