Magic of Science

Rising water trick

For this trick you will need a candle, matches, some food dye, a bowl and a long slim glass.

Place the the candle in the bowl with the coloured water as shown.

Now light the candle and place the glass over it.

Watch what happens.

Explain what you see. Form a hypothesis to explain how the water is forced up the glass

You may wish to consider the following questions before you attempt an explanation.

Roughly what percentage of the air is due to oxygen? 20% 80% 10% 1%

What is produced when the candle burns? oxygen and water carbon dioxide and water nitrogen and oxygen

What happens when gas is heated ? its volume shrinks it becomes denser its volume increases

What is the best explanation for your answer above? gas molecules decrease in speed gas molecules become denser the gas molecules increase in speed gas molecules become less dense

Why does the flame become extinguished? oxygen runs out water vapour increases inside the glass nitrogen runs out carbon dioxide runs out

Wax is composed of hydrocarbon molecules that are, on average,25 carbons long. The general formula for wax can be described simply, for our purposes, as C₂₅H₅₂ So we can write the equation below

2C₂₅H₅₂(s) +76O₂(g) => 52H₂O(g) + 50CO₂(g)

One suggestions to explain how the water rises up the glass is that there are less molecules in the air inside the glass after the candle goes out than before. This creates a partial vacuum that sucks the water up the glass. However, according to the equation above there appears to be more gas molecules present after the reaction than before.

How many gas molecules are used to burn 2 molecules of wax? 1 2 76 52 102 78

How many gas molecules are produced when two wax molecules burn? 1 2 76 52 102 78

This reaction will produce more molecules at the end of the reaction than before the reaction less molecules at the end of the reaction than before the reaction the same number of molecules at the end of the reaction than before the reaction

Suggest what might happen to the water vapour molecules after the candle goes out. decrease in size condense into liquid increase in size speed up

Suggest what might happen to the carbon dioxide gas molecules. Design an investigation to prove this. Hint You may use universal indicator.

What is your best explanation for the rising water trick?