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Thirsty Candle

Flat out like a lizard drinking! Watch as a candle appears to suck water into a glass!

Australian Curriculum links

  • Science > Chemical Sciences > Year 5 > ACSSU077

You'll need

  • water
  • shallow bowl
  • food dye
  • tea candle
  • lighter or match
  • drinking glass

Try this

Safety: This experiment requires adult supervision. Perform this experiment away from curtains or other flammable materials.

  1. Place a small amount of water (to about 1 cm in depth) in a bowl.
  2. Add two drops of food dye to the water.
  3. Place the tea candle in the centre of the water, making sure the wick remains dry.
  4. Use a lighter or match to light the candle.
  5. Turn the glass upside-down and place the glass over the candle.
  6. Watch what happens to the candle flame and the water.

Further investigation

Mark the water level on the side of the glass and repeat the experiment with extra candles to see how high you can get the water level to rise. Or, see what happens if you; use different sized glasses; use different shaped glasses; make the glass hot before putting it over the candle or make the glass cold before putting it over the candle.

What's happening?

Air is all around us and is pushing on us from all directions. Air pushes on other objects too and this pushing is called air pressure. Even though we don’t notice air pressure, it’s always there.

When the candle burns inside the glass, the air inside the glass warms. Warm air takes up more space than cool air. As the air inside the glass gets warmer, it expands and pushes against the sides, creating higher air pressure inside the glass than outside the glass. To restore the balance, some of the high pressure air inside tries to move towards the lower pressure air outside. You may have observed this in the form of little bubbles escaping from the bottom of the glass while the candle was still lit.

The candle burns in a combustion reaction. For the candle to stay alight, it needs a constant supply of oxygen. However, when the candle is trapped in the glass, it quickly uses the available oxygen. When there isn’t enough oxygen inside the glass, the candle stops burning.

After the candle goes out, the air inside the glass cools down. The colder air inside the glass has a lower pressure than the air outside the glass. To restore the balance, some of the higher pressure air outside the glass tries to move towards the lower pressure air inside the glass. This pushes on the water, forcing the water into the glass. The water level keeps rising until the air pressure inside the glass is the same as the air pressure outside the glass.

Note: Even though oxygen is used during the combustion reaction, this does not greatly affect the pressure inside the glass. This is because a similar amount of carbon dioxide is also produced from the reaction, resulting in very little change in air pressure.

Real world links

This activity is similar to what happens when you suck water through a straw. When you suck on the straw, the air pressure inside your mouth decreases. This means that the pressure inside your mouth is lower than the pressure outside your mouth. The high pressure outside your mouth forces the water up the straw.