I also got a chance to spend some time chatting with the education staff at the museum. While we were swapping ideas for fun science, we took a close look at an old, and often misunderstood experiment.

Materials:

Plan:

Now comes the fun, and the time when you need to observe very carefully. Turn the glass bottle upside down, so that the opening is at the bottom. Place the bottle over the candle and set it down into the water.

Watch what happens. After a short time, the candle goes out and the water rises into the jar. As we have seen in the past, the water does not rise into the jar because we are burning up the oxygen. As the oxygen burns, it combines with carbon from the candle wax to form carbon dioxide, which takes up the same amount of space as the oxygen. To see what is really happening, we need to repeat the experiment and watch very closely. Also, by using a very large bottle, things are slowed down a bit, making it easier to see what is happening.

As soon as the bottle is in the water, you should see air bubbling out. Very quickly, the bubbles will stop, and the water will start to rise into the jar. About the same time, the candle flame will start to get smaller, and after a few seconds, it will go out. The water will continue to rise for another few seconds.

If the old explanation were true and burning up the oxygen was causing the water to rise, we would expect the water to start rising as soon as the bottle was in the water, and it would stop rising as soon as the candle went out. Instead, the first thing we see is air coming out of the bottle. The candle flame is heating the air, causing it to expand. As the air around the flame gets bigger, it pushes some of the air out of the bottle. That is where the bubbles are coming from.

The next part is tricky, and it took using the large bottle and some thought to figure it out. The candle is burning, and the water begins to rise into the bottle. If the candle is still heating the air, shouldn't the air still be expanding and bubbling out of the bottle?

Think about what is happening inside the bottle. The candle is heating the air around the flame. That hot air expands, but it also rises. As the heated air rises to the top, it comes in contact with the glass bottle. The glass absorbs some of the heat, causing the air to cool, and get smaller again. As the hot air spreads out, it comes in contact with more and more of the glass bottle. Quickly you reach a point where the glass is cooling more air than the candle is heating. At that point, the water will begin to climb into the bottle.

Then the candle starts to run low on oxygen. As the flame gets smaller, it is heating even less air, so the water rises faster. When the oxygen level is low enough, the flame will go out. At that point, if "burning up the oxygen was the reason for the rising water, it should stop rising at this point. Instead, it rises faster, as the air in the bottle continues to cool.

If you could capture all the air that bubbled out of the bottle at the start, and put it back into the bottle, you would have the same amount of gas that you had when you started. Heating made it expand and cooling made it contract, but the amount of gas is still pretty much the same.

From Robert Krampf's Science Education Company

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