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?

Ready, Set, ... Jet!

If you looked at the history of flight links in the last entry, you may have noticed that many of the earliest "flying machines" were actually gliders, which flew without power. Like a modern glider, they needed help getting going. The importance of the Wright Brothers' invention was that their airplane had an engine, and could take off from the ground.

Modern airplanes usually use either propellers or jets to move. Let's talk about jet engines here. Jet engines work by blowing gas out through a small hole called a nozzle. Air is sucked into the front of the engine by a fan. The air is then mixed with fuel and ignited. The hot gas, which is at high pressure, comes rushing out the back, pushing the jet engine (and whatever it is attached to) forward.

You can make a simple jet engine with things you may have around the house.

What you'll need:

Balloon

Tape

Drinking straw

Long piece of string

Blow the balloon up and just hold the empty end closed. Tape the drinking straw to the balloon as shown in the picture. Let the air out of the balloon.

Now find a place to tie the string that lets you stretch it out to its full length. Try a doorknob in a fairly large room. Tie one end of the string, then thread the string through the drinking straw. Stretch the string out, keeping the balloon near your end. Blow the balloon up and hold the end closed again. Make sure that the string is straight, then let go of the balloon. As the air rushes out, the balloon will take off along the string. How far can you make it go?

A Little Brittle

In the last experiment/recipe, you learned how to stop crystals from forming by cooling very fast. In this activity, you'll learn another way to prevent crystals. In the kitchen, "table sugar" usually means a molecule called sucrose, which is usually made from sugar cane or sugar beets. Corn syrup, on the other hand, contains other sugar molecules, called glucose and fructose. If you add these types of sugar to the sucrose, the glucose and fructose lead to "mistakes" when crystals try to form, and the result is - no crystals (or at least, not very many)!

This is a recipe for nut brittle, which uses corn syrup mixed with table sugar to keep crystals from forming. The presence of crystals would not change the taste of the candy, but would make it feel gritty in your mouth. Nut brittle is most commonly made with peanuts, but you can also make it with other nuts, if you don't like peanuts or can't eat them.

What you'll need:
1/2 cup light corn syrup
1 cup sugar
1 cup dry-roasted peanuts or other nuts
2 teaspoon butter
1 teaspoon vanilla extract
1 teaspoon baking soda
Salt (if you want)
Microwave oven
Large microwave-safe bowl
Spoon
Baking pan (10" x 10")
Oven mitts

Rub the baking pan with about 1 teaspoon worth of butter and put it aside. Mix the corn syrup and sugar in the bowl and microwave on high for 4 minutes. The sugar should have started to melt, but still be clear. Remove from the oven and stir in the nuts. Be careful - the bowl will be VERY hot. Microwave on high about 3 more minutes or until the mixture turns light brown. Take the bowl out of the microwave again and stir in the other teaspoon of butter and the vanilla extract. Microwave on high again for 1 minute. Remove the bowl again, and stir in the baking soda (what do you see happening?). Quickly pour the mixture into the baking pan and let it cool. When the brittle is safe to handle, slide it out of the pan, and break it into small pieces. If you're going to store it, make sure it's in an airtight container.

This recipe includes another chemical reaction that has nothing to do with sugars or crystals. When you added the baking soda, the brittle got cloudy because thousands of tiny gas bubbles formed. Where did they come from?? When baking soda is heated to 160^oF, it gives off carbon dioxide gas. The sugar/nut mixture is much hotter than this, so the gas is given off very quickly. The bubbles don't change the taste of the nut brittle, but do make the candy lighter and easier to chew.