Make a Fire Extinguisher

The three amigos

Just like any other fire extinguisher, this one works by acting on the fire triangle. For fire to happen, we need three things: oxygen, fuel and heat. If just one of these ingredients is missing, there will be no fire.

When you lit the candle, you provided heat. The candle provides fuel and oxygen comes from the air. But what was the mysterious invisible force that came from the jug to put out the fire?

It was carbon dioxide, a gas that is more dense than air, so you can actually pour it!

In the chemical reaction between the bicarbonate of soda and vinegar, carbon dioxide was produced. Did you see the bubbles of gas and hear the fizzing as they popped? This was the carbon dioxide being released. Since it’s more dense than air, it collects inside the jug. You just don’t necessarily know it because you can’t see it.

Relight the fire

When you tipped your jug, the carbon dioxide poured out like an invisible river. It filled the jam jar from the bottom upwards, replacing the oxygen so that the fire went out. If you try to relight your candle, you’ll find you can’t. You haven’t disturbed the carbon dioxide in the jar. Allow the hot wax to set and then pour the carbon dioxide out of the jar and try re-lighting the tealight again.

Interestingly, if you blow directly into the jar and replace the carbon dioxide with your breath, the candle still might not light. Your exhaled breath contains less oxygen than the air, because your body has removed some of the oxygen to keep you alive. Normal air contains about 20 per cent oxygen, but exhaled air contains only about 16 per cent, which is right on the boundary where fire can exist.

Now try blowing into the jar from a distance. The flow of your breath will drag oxygen-rich air in with it and it will be possible to light the candle again. Fascinating!

Important bit

Carbon dioxide is used in some fire extinguishers for good reason. You should never pour water on an electrical fire. Why? Because electricity could conduct through the water stream and give you an electric shock.

If you have a fire involving a flammable liquid, like petrol or oil, you must never add water. Doing so could cause it to spit and spread the fire.

In both these situations, a carbon dioxide fire extinguisher will exclude the oxygen in relative safety. It should not be used in small spaces though, because fire reduces oxygen in the air. Adding extra carbon dioxide to a room is dangerous to anyone without breathing equipment.