You’ll need

  • Water
  • A shallow tray, with a bottom wide enough to fit an upside-down glass
  • Food colouring
  • A tea candle (also known as a tea light)
  • A lighter or box of matches
  • A drinking glass, small enough to fit upside down in the bowl
  • Non-permanent marker

You’ll need

Warning!

Adult supervision required. Do this experiment away from curtains or other flammable materials. Be careful not to drop and break the glass.

  1. Gather your materials on a flat surface.
  2. Pour 1 cm of water into the tray.
  3. Add 2 drops of food colouring to the water.
  4. Place the tea candle in the centre of the water, making sure the wick stays dry.
  5. Use a lighter or match to light the candle.
  6. Turn the glass upside down and place it over the candle.
  7. Mark the water level on the side of the glass with a non-permanent marker.
  8. Watch what happens to the candle flame and the water.
  9. Mark the water level again after it rises.

Questions to ask

How high can you get the water level to rise? Repeat the experiment and see what happens if you:

  • use 2 tea candles
  • use a larger or smaller glass
  • use a different shape glass
  • heat up the glass by running it under hot water before putting it over the candle (be sure to use oven mitts)
  • cool the glass in the fridge for about 30 minutes before putting it over the candle.

Mark the height of the water each time you change a part of the experiment.

What's happening

Air is all around us and is pushing on us from all directions. This pushing is called air pressure. Even though you don’t notice air pressure, it’s always there.

When the candle burns inside the glass, the air inside the glass warms up. Warm air takes up more space than cool air. As the air inside the glass gets warmer, it expands and pushes against the sides of the glass. This creates higher air pressure inside the glass than outside the glass. If the air pressure is high enough, it will push the water down until some air leaks past the edge of the glass. You may have seen little bubbles escaping from the bottom of the glass while the candle was still lit.

For the candle to stay lit, it needs a constant supply of oxygen. Oxygen is one of the gases that makes up air. The candle is trapped in the glass, and it quickly uses all the oxygen around it. When the oxygen in the glass runs out, the candle stops burning.

After the candle goes out, the air inside the glass cools down. Soon it reaches the same temperature as the air in the room. Now the air pressure inside the glass is lower than in the room because there is less air in the glass than before. (Remember – some air bubbled out.) The outside pressure pushes down on the water harder than the inside pressure. This forces some water up into the glass. The air inside now has less room, so is at higher pressure. The water stops rising when this extra pressure plus the weight of the rising water balance out the higher pressure outside the glass.

Did you know

This activity is similar to what happens when you drink through a straw. When you suck on the straw, the air pressure inside your mouth goes down. This means that the pressure inside your mouth is lower than the pressure outside your mouth. The higher pressure outside your mouth (pressing down on the top of your drink) forces the water up the straw.