To find out what happens to the air used up during burning
Light a candle and cover it with a gas jar. Remove the gas jar, place some lime water in it, cover it with the gas jar cover and shake. (Lime water is the clear liquid obtained if quicklime is shaken with water and allowed to settle). Note what happens. Repeat the experiment with burning paper, wood shavings, and taper and shake the gas left in each case with lime water.
You will observe that mist collects on the sides of the gas jar in each case, and the gas left turns lime water milky.
We find that the gas left in this experiment turns lime water milky. If you take a glass tube and blow through it into lime water, you will notice that the gas you breathe out also turns lime water milky. You will learn later on that this gas is carbon dioxide. We can conclude therefore that when a candle burns in air, it uses the air to form a gas called carbon dioxide, and water which appears as mist.