Candle Science Trick - Extinguish Fire With Carbon Dioxide

Blow Out a Candle Using Science

Blow out a candle by pouring a glass of what appears to be air onto the flame. This easy science trick demonstrates what happens when air is replaced with carbon dioxide.
Blow out a candle by pouring a glass of what appears to be air onto the flame. This easy science trick demonstrates what happens when air is replaced with carbon dioxide. Trish Gant / Getty Images

You know you can put out a candle flame by pouring water on it. In this science magic trick or demonstration, the candle will go out when you pour 'air' onto it.

Candle Science Magic Trick Materials

  • a lit candle
  • a transparent glass (so people can see what is inside the glass)
  • baking soda (sodium bicarbonate)
  • vinegar (weak acetic acid)

Set up the Magic Trick

  1. In the glass, mix together a little baking soda and vinegar. You want roughly equal amounts of the chemicals, like 2 tablespoons each.
  1. Put your hand over the glass to keep the carbon dioxide from mixing too much with the outside air.
  2. You're ready to blow out a candle. If you don't have a candle handy, you can cover the glass with plastic wrap to store the carbon dioxide.

How to Blow Out the Candle with Chemistry

Simply pour the gas from the glass onto the candle. Try to avoid splashing liquid on the flame, since it's not exactly amazing when water puts out a fire. The flame will be extinguished by the invisible gas. Another way to perform this trick is to pour the gas that you just made into an empty glass and then pour the apparently empty glass over the candle flame.

How the Candle Trick Works

When you mix baking soda and vinegar together, you produce carbon dioxide. The carbon dioxide is heavier than air, so it will sit in the bottom of the glass. When you pour the gas from glass onto the candle, you are pouring out the carbon dioxide, which will sink and displace the (oxygen-containing) air surrounding the candle with carbon dioxide.

This suffocates the flame and it goes out.

Carbon dioxide gas from other sources works the same way, so you could also perform this candle trick using gas collected from the sublimation of dry ice (solid carbon dioxide).

How Blowing Out a Candle Works

When you blow out a candle, your breath contains more carbon dioxide than it did when you inhaled the air, but there's still oxygen that can support wax combustion.

So, you may be wondering why the flame is extinguished. It's because a candle needs three things to sustain a flame: fuel, oxygen, and heat. The heat overcomes the energy needed for the combustion reaction reaction. If you take it away, the flame can't sustain itself. When you blow on a candle, you force the heat away from the wick. The wax drops below the temperature needed to support combustion and the flame goes out.

However, there is still wax vapor around the wick. If you bring a lit match close to a recently extinguished candle, the flame will re-light itself.