You’ll need

  • One-third of a cup of bicarbonate of soda (also called bicarb soda and sodium bicarbonate)
  • A large, clear jug
  • One-third of a cup of vinegar
  • A lighter or box of matches
  • A candle

What to do

Warning!

Candles, lighters and matches can be dangerous and burn. Ask an adult before using them.

  1. Gather your materials on a flat surface.
  2. Put the bicarbonate of soda in the jug, then add the vinegar. The mixture will bubble and fizz.
  3. Light the candle.
  4. Once the mixture in the jug has stopped fizzing, carefully tilt the jug over the candle without pouring out the liquid. The candle should go out.

Questions to ask

What was formed during the reaction between the vinegar and bicarbonate of soda that caused the candle to go out?

Try tilting the jug into an empty jug, and then pouring the ‘empty’ jug over the candle flame. Did the candle go out?

Instead of tilting the jug over the candle, try blowing soap bubbles into the jug. What happens to the bubbles?

What's happening

When bicarbonate of soda and vinegar are mixed together, a chemical reaction makes carbon dioxide gas (also written by its chemical formula CO2). This gas can be poured from the jug like water. When poured on the candle, the carbon dioxide separates the flame from oxygen. Because flames need oxygen to burn, the candle goes out.

Mixing bicarbonate of soda and vinegar causes a chemical reaction that makes carbon dioxide gas, water and a salt. We can write out what’s happening as a chemical reaction:

bicarbonate of soda + vinegar → a salt + gas + water

or, using chemical names:

sodium bicarbonate + acetic acid → sodium acetate + carbon dioxide + water

or, using chemical formulas:

CH3COOH + NaHCO3 → NaCH3COO + CO2(g) + H2O

(C means carbon, H means hydrogen, O means oxygen and Na means sodium. The g means gas.)

Carbon dioxide gas is denser than air. This is why it just sits in the jug and does not float away. When the jug is tipped over the candle, carbon dioxide pours out and falls down, pushing air out of the way. Flames need oxygen to burn. Without it, they go out quickly. Because carbon dioxide sinks under air, and air includes oxygen, it separates the oxygen from the flame. This is why the candle goes out.

Did you know

Fire extinguishers filled with carbon dioxide are often used to put out fires that can’t be put out with water (such as electrical fires). In these cases, the extinguishers are filled with pure carbon dioxide gas, which is stored under pressure. A 5 kg carbon dioxide extinguisher can release almost 3,000 l of carbon dioxide gas in less than 1 minute!

Take care with carbon dioxide, you will get sick if you breathe it in!