Candle and Beaker Experiment

The 1st grade students observed a demonstration experiment last week in science.  They were asked the question; "What happens when you light a candle in a plate full of water and then place a beaker over it?"

Students gave all kinds of predictions such as, "The candle will melt.", "The water will get hot," and my favorite, "It will explode!"  They drew their predictions on their lab sheets first.  Next it was time to do the demonstration.

Imagine their surprise to see what really happened!  So what's the science behind this?

The candle flame heats the air in the beaker, and this hot air expands.  When the flame goes out, the air in the beaker cools down and the cooler air contracts. The cooling air inside of the beaker creates a vacuum.  This imperfect vacuum is created due to the low pressure inside the beaker and the high pressure outside of the beaker.  Most people think the vacuum is sucking the water into the beaker.  The more accurate explanation is that the air is exerting pressure from an area of high pressure to an area of low pressure. The water therefore is pushed into the beaker from the higher air pressure outside!  Students then drew what really happened on their lab sheets, using words and arrows to highlight what happened.