Question

Activity 6.2
Glass jar
To prove that air/oxygen is the
supporter of combustion,
Light a candle and cover it with a glass
jar or beaker as shown in figure.
What do you observe?
Why does the candle stop burning
after some time?
The candle burns till oxygen is present
in the air inside the inverted glass jar.
Fig 6.2: Oxygen is the supporter of combustion, tell the observation and conclusion of this activity​

Answer 1

Answer:

Experiment: Cover a burning candle with a pitcher so that the candle is in an air-tight room sealed by the water at the ground. Observations: After some time, the candle dims and goes out. Just before the candle dies, the water level rises to almost 1/10 th of pitcher height. No air bubbles are seen. The water level stays up for many few minutes more.

The chemical aspect: oxygen O2 and paraffin Cn H2n+2 react. The burning produces water H 2O and carbon dioxide C O 2. For n=1 we balance the equation as follows:

2 O2 + C H 4 = C O 2 + 2 H 2 O

Because twice as much oxygen is burned than carbon dioxide released, the air volume decreases. The physical aspect: the candle heats the air and expands it. This cancels the depletion of the oxygen temporarily and the water level stays down. When the oxygen is depleted, the candle goes out and the air cools. The volume of the air decreases and the water rises. The temporary temperature change delays the rise of the water. As several readers have pointed out, also the water condensation should be mentioned. While water is initially gas, it condenses and helps to delay the effect.

Summary: There are two different effects. Both a chemical and a physical reasoning are needed to explain what we can see. Both physics and chemistry matter. The initial cancellation effect can confuse the observer. Mathematics plays a role when the chemical equations are balanced.