Roy fixed a candle at the bottom of a trough containing water. After lighting the candle, he inverted a gas jar over it. What did he observe? *
1 point
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The flame continued to burn and water rose in the jar.
The flame continued to burn and water became milky.
The flame was put off and black smoke was deposited on the sides of the jar.
The flame was put off and water rose in the jar.
Answers
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.
The flame was put off and black smoke was deposited on the sides of the jar.
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