Experiment ~ Measuring Oxygen

Warning! This experiment uses fire. Be very careful and be sure an adult is around to help.

For this experiment, you will need:

- a pie pan or shallow bowl

- a candle

- a glass jar large enough to hold the lit candle

- water

Light the candle and let a few drops of melted wax fall on the middle of the pan. Place the bottom of the candle into this wax to secure it in place. Carefully add about an inch of water to the pan. Relight the candle if it has gone out, and place the jar over it. Watch carefully. After a minute or so, the candle will go out, and the water will rise up into the jar.

This shows that the candle has burned up the oxygen, and the water has risen into the jar to take its place, right? WRONG!!!!! If you watch carefully, you will see why is it wrong. When you first place the jar over the candle, air bubbles OUT of the jar. If you are slow about placing the jar over the candle, you might not notice this, but if you cover the candle in one quick motion, you will see the air bubbling out. Once the candle goes out, the water begins to rise in jar.

Now, lets think about that. If the water was rising because the oxygen was burned up, it would rise while the candle was burning and stop as soon as the flame went out. Is that what you saw? No. Then what really did happen?

As the candle burns, it is heating the air in the jar, causing it to expand. This causes the bubbles that leave the jar. The candle is burning oxygen, but the oxygen does not vanish. It combines with carbon from the burning wax to form carbon dioxide, another gas that also takes up space.

When the candle goes out, the air begins to cool, which causes it to contract. As the air gets smaller, the water rises into the jar.

Taken From here