Thirsty candle experiment

You will need…

• A transparent shallow Gratnells tray filled ~1cm deep with water
• A pint glass
• Green food colouring
• Two tealight candles
• A box of matches or a lighter
• A spoon
• Optional: different sized glasses or glass vessels e.g. conical flask, measuring cylinder
• Optional: tall, thin candles or birthday candles
• Optional: other colours of food colouring
• Optional: stopwatch

What to do…

• Fill the tray with water to approximately 1cm deep and add 2-3 drops of green food colouring to colour the water, mix well with the spoon to evenly disperse the colour. The water depth should only come about half way up the sides of your tealight candle.
• Place a tealight candle in to the middle of the tray being careful not to let it get wet.
• Light the candle.
• Place an upturned pint glass over the candle.
• Observe what happens.

What is happening?

A common misconception is that the consumption of oxygen by the flame in the glass is a factor in the water rising. There may be a slight possibility that there would be a tiny rise in the water from the flame using up oxygen, but, as a similar amount of carbon dioxide is produced during the combustion reaction there is very little change in the overall air pressure. What you actually see is the water level rise almost all at once and continue to do so even after the flame goes out.

This is because, at first, the flame heats the air inside the glass and the hot air expands quickly. Some of the expanding air is forced out from under the glass, watch the video closely and you will see some bubbles escaping from the side of the glass. When the flame fades and goes out, the air in the glass cools and cooler air contracts, taking up less space. The contraction creates a weak vacuum – or lower pressure – in the glass. But where is the higher pressure? It’s outside the glass pressing down on the water in the tray, this is known as atmospheric pressure. The higher atmospheric pressure pushes water from the tray into the glass, taking up space in the glass until the air pressure is equalised between the inside and outside of the glass. The water stops rising when the air pressure is re-balanced and the equilibrium is restored.

The candle goes out because it requires oxygen to burn, the oxygen in the glass is quickly used up.

But why does the water in the glass look darker green than the water in the tray? This is simply down to the depth of the water. When initially the water and air are forced out of the glass, the water in the glass gets shallower and appears to turn paler in colour, then as the glass fills with water it gets deeper and looks darker. At the same time, the water in the surrounding tray becomes shallower and appears to get lighter. The water gets darker because the deeper the water the more distance light has to travel through the water and the more light is absorbed by the water before it has chance to bounce off the bottom of the tray and back up to the surface and our eyes.

Other things to try…..

Start a stopwatch when the glass touches down over the candle, time how long it takes for the flame to go out. Mark on the side of the glass how far up the liquid has travelled. Record the time elapsed and the volume of liquid collected. Does the size/volume of the glass used effect your results? Does the number of candles used effect the result? Does the temperature of the glass you use effect the result? Does the colour of food dye used effect the result? How many repeats should you carry out? What other variables would you have to control to ensure a fair test?

You may also like to look into Charles’s Law and carry out further investigations.

Health & Safety

As with all Gratnells Learning Rooms What’s in my tray activities you should carry out your own risk assessment prior to undertaking any of the activities or demonstrations. Please take particular care when using naked flames and candles.