Extinguishing fire with CO₂

People and animals need a gas called oxygen, which we breathe in through our lungs. The cells in our body use oxygen and produce a gas called carbon dioxide, which our lungs breathe out. If there was no oxygen in our environment, we would suffocate. (Since there is no oxygen on the Moon, astronauts must take it with them in cylinders.)

Likewise, a fire cannot burn if there is no oxygen. As the flame burns, oxygen is consumed.

Let’s conduct an experiment.
We will need a candle, a match and a glass. Ask an adult helper to assist you with the experiment since fire can be dangerous. Let the adult helper light the candle and cover it with the glass. After burning for a while, the flame will gradually die down and finally go out. This happens because the oxygen trapped in the glass was used up.

Is there another way to put out a candle? Yes, there are several ways to do that. Let’s take a look at an unusual but very interesting method of extinguishing candles. For this experiment, you will need candles, matches, a glass, a balloon and an effervescent tablet (calcium, vitamin or aspirin).

Push the effervescent tablet into the balloon. Pour some water into it and quickly squeeze the balloon’s neck to prevent the gas leaking out. The tablet will slowly dissolve in the water. The balloon will gradually become filled with gas, i.e. carbon dioxide, which is denser than air (i.e. it sinks in the air – like a rock in water).

Ask an adult helper to light the candle. Once the tablet has dissolved, slowly pour the carbon dioxide into the glass. Just think of the carbon dioxide as an invisible fluid and you will be able to do it.
The gas is now in the glass. It will not escape because it is heavier than air. Take the glass and pour the invisible liquid (carbon dioxide) over the candle. The candle will go out because the flame is no longer surrounded by oxygen, only carbon dioxide. A fire extinguisher functions in a similar way. It contains carbon dioxide stored under pressure. When we activate the fire extinguisher, we pour the carbon dioxide over the flames. The fire is thus cut off from the oxygen supply and is extinguished.