Out, Out, Brief Candle!

In the last post, you learned how important the wax is to burning a candle. In this experiment, you will discover another requirement for burning - air. Burning wax is an example of a combustion reaction, in which carbon and hydrogen in the fuel (wax) combines with the oxygen in the air to form carbon dioxide and water. The more oxygen, the longer the candle will burn.

What you'll need:

Votive candle

Lighter
Several drinking glasses of different sizes

Watch or other timer

Light one of the candles and cover it by inverting the smallest glass over the flame so that it is covered. See how long it takes for the candle to go out. Re-light the candle, and cover it with the next largest glass. Does the candle stay lit longer? Do the same thing for each size of glass.

You will generally find that, that larger the glass, the longer the candle stays lit. The explanation for this is pretty simple - larger glasses trap larger amounts of air (which contains oxygen), and it takes longer for the oxygen to get used up!

Fix a Flat with CO2?

You can buy repair kits for bicycle tires that include a small cylinder of CO₂ gas, which reinflates the tire. It's meant just for short emergency repairs, but some people leave the CO₂ in the tire longer than intended. Based on what you learned about CO₂ in the last two posts, what do you think happens to the tires if the CO₂ is not replaced with air? (Hint: think about what the tire is made of!)

Where Did the Gas Go?

After a couple of hours, you may observe that the CO₂-filled balloon is noticeably smaller than the air-filled balloon. By the next day, there won't be any doubt. Where is the gas going?

"Aaahhh," you might say, "I remember talking about effusion, which explained why helium-filled balloons deflate faster than air-filled ones. Tiny helium atoms can escape faster through itty-bitty holes in the balloon than the molecules in air can. It must be the same effect!" That's a very good connection to draw, so let's see if it fits with what we know about the sizes of gas molecules.

Here are some gas molecules, drawn more or less to scale:

Most of air is nitrogen, which exists as N₂ (shown in blue). Oxygen is O₂ (red) and carbon dioxide is CO₂ (gray and red). Other gases shown for comparison are argon (aqua), water (red and black), and methane or natural gas (gray and black). Helium (He) is the little orange circle!
Does it still make sense to say that He and CO₂ effuse quickly because they are so much smaller than the other molecules in air? Of course not! CO₂ is actually the biggest molecule shown! What is going on?

I have to admit that I was puzzled by this at first, because I was thinking just like you are. But it turns out that the speed with which gas molecules escape from a thin-walled container like a balloon isn't just determined by the size. And carbon dioxide is known to move through many kinds of rubber very quickly indeed (rubber is said to be very permeable to CO₂).

But why? It turns out that CO₂ is very comfortable mixing with the rubber. So these molecules get absorbed into the balloon wall, and work their way through to the outside (a process called diffusion). Other molecules, such as nitrogen and oxygen, don't mix so easily, and they stay in the balloon longer, although they eventually diffuse out as well.

So, the moral of this story is, you can't always assume that one scientific principle applies in every case - sometimes you have to look deeper. And frankly, that's one of the things that makes science so interesting!

The Amazing Self-Inflating Balloon!

Are you getting tired of blowing up balloons for these experiments? Here's a way to get chemistry to do the work for you!

What you'll need:
1 empty plastic bottle (about 20 ounces in volume)
Measuring spoon (1 tablespoon)
Funnel
Balloon
Vinegar
Baking soda

Pour about one inch of vinegar into the bottle. Using the funnel, put one tablespoon of baking soda into the balloon. Carefully stretch the balloon opening over the mouth of the bottle, letting the balloon hang down over the side, as shown in the photo above. When you're ready to go, lift the balloon up so that the baking soda falls down into the vinegar. You'll immediately see lots of bubbling, and will observe the balloon grow larger! What's going on?

Baking soda (chemical known as sodium hydrogen carbonate) reacts with vinegar (or acetic acid) to give off lots and lots of carbon dioxide gas, or CO₂. The gas has nowhere to go but into the balloon! When the balloon is at its largest, remove it from the bottle and tie the end off to seal the CO₂ in. Blow up another balloon of the same type using your breath to about the same size and tie it off. Can you tell the difference between the two balloons? Put them aside for a few hours, and we'll come back to them later.

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?

Acid Reactions

One thing that acids do really well is to react with other chemicals. If you did the dirty-penny-in-cola experiment, you've already seen one such reaction, when the phosphoric acid in the cola removed the copper oxide coating from the penny. But the reaction itsefl wasn't very interesting while it was happening. Here are some acid reactions that are more exciting. They're kind of messy, but safe, as long as you follow the instructions carefully.

Acids really like to react with chemicals known as bases, which you can think of as "anti-acids" - they neutralize each other. You can collect some acids and some bases that you might have around your home, and observe their reactions.

What you'll need:
Small plastic cups
Eye dropper
Teaspoon
Mortar and pestle
Water
Marker
Newspaper or plastic sheeting
Household acids, such as vinegar, lemon juice, cola, aspirin, Vitamin C tablets
Baking soda (sodium hydrogen carbonate, which is a base)

First spread the newspaper or plastic sheeting around the area where you will be working because things may get messy! If you are using any acids in solid form, such as aspirin, grind them with the mortar and pestle and dissolve in water. Place a small amount of each acid into a plastic cup and label it. Add about half a teaspoon of baking soda to each acid, and stand back! What happens?

You should see a very fast and exciting reaction, with lots of foamy bubbles forming. These bubbles are filled with carbon dioxide, which is released by the reaction of the acid with baking soda. You might want to test other household acids to see if they have the same reaction.

Make A "Rubber" Egg!

What happens when you drop an egg? It breaks, right? Well, not if you remove the shell first! Here's how to do this...

What you'll need:
1 egg
Vinegar
1 jar with lid
Spoon

This is very simple - put the egg into the jar, then pour vinegar over it to cover. Place the lid on the jar loosely, just to trap the vinegar smell. Look at the egg - what's happening? Soon after adding the vinegar, you'll see small bubbles forming on the eggshell. These are bubbles of carbon dioxide (CO₂) gas. They come from the chemical reaction of the vinegar with the calcium carbonate in the shell. Remember that the formula for calcium carbonate is CaCO3. When you remove CO₂ from CaCO₃, you're left with CaO, or calcium oxide.

But wait - this gets better! Let the egg stay in the vinegar for several days. Eventually, the whole shell will dissolve! The egg will get larger because it absorbs some vinegar, and it will be held together by the proteins that were originally in the shell. The vinegar makes these proteins tougher, and the egg will now feel like a water balloon! Using a spoon, remove it from the jar and hold it in your hand. Hold the egg about an inch above a counter and drop it - you will actually see it bounce! But be very careful when doing this. This "rubber" egg is very delicate, and if it breaks, you will have a small "eggsplosion"!!

Goal!

So how do those scent molecules get through the air to your nose? They do this by a process called diffusion. The nitrogen, oxygen, carbon dioxide, and other molecules in the air are moving around very fast (about 1000 miles per hour, even faster than a jet airplane!) When a scent molecules gets into the air, the molecules in air begin to hit the scent molecules, kicking it around like a soccer ball. Sooner or later, they kick it into your nose! Now, your nose probably can't smell just one scent molecule, but there are gazillions of air soccer players kicking around billions of scent molecules, so eventually a lot of them get to your nose. Not surprisingly, the farther away your nose is, the longer it takes to smell something. Also, the colder it is, the slower all the molecules move. And the bigger the scent molecule, the longer it takes to smell (usually).

Here's a way you can observe diffusion in action. Line up about 5 or 6 family members or friends in a row, each about 2-3 feet apart. You can do this in a large room, down a hall, or outside. Have them all face away from you and close their eyes. Tell each person to raise a hand when he or she smells something. Open a bottle of something smelly, like mouthwash, perfume, or rubbing alcohol. Turn around and look for raised hands. You will probably see that the people closest to you raise their hands quickly, but that it takes longer for the people farther away. Some may never smell it at all (especially if you're outside on a windy day). Try the same experiment with different kinds of smells and with more people!

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.