Friday 1 June 2012

Rising water experiment

Another experiment that teaches us about air pressure and is super fun to try is this one.  You should be careful as this one involves matches so it is best to have a responsible adult present when trying this out. 

You will need: 
  • A shallow bowl 
  • A glass
  • A tea light
  • A lighter or matches
  • Water (with added food colouring is optional)


fill your shallow bowl until it is about 1cm deep in water.


Carefully place the tea light in the centre of the dish. 

Light the tealight then hover the glass (upside down) over the flame.  This allows the glass to fill with hot air

After a minute or so you should feel the glass is warmer, at this point drop the glass  (without smashing the glass) over the candle. 

What happens? 
The water should be sucked (vacuumed) up into the glass with the vacuum we created!

The candle flame heats the air in the glass, the hot air inside the glass then expands. Some of the expanding air escapes out from under the glass; you may have noticed that there were bubbles around the glass when the glass was lowered over the water - when the flame goes out, the air in the glass cools down and the cooler air contracts.

As the air cools inside the glass it creates a vacuum. This vacuum is made because of the low pressure inside the glass and the high pressure outside of the glass.  So you could say the vacuum is sucking the water into the glass, which is kind of right.  Scientists use the term vacuum instead of suck, so if you want to be like a real scientist always say vacuum not suck!

A scientist would explain what happens in this experiment as gases forcing pressure from an area of high pressure to an area of low pressure.

You may think that because the flame is burning oxygen out of the air in the glass that that is the reason for the water rising.  There is a possibility that it would have some effect on the water rising but only a fraction as what makes the water rise so rapidly is the expansion and contraction of the gases within the glass. 

This was what happened when Honi, age 4 had a go...

This is a favourite experiment that I do with my girls and like to bring to my science groups where possible. 

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