Science Experiments: Magic Gas

This fun science experiment is one of six Christmas science experiments that are now available to download from the National Science Learning Centre’s iTunes U website

Properties of Gas as demonstrated by this Magic Gas experiment

Equipment used: Cup, candle, alka seltzer


This Christmas science activity demonstrates the properties of gases, in particular that CO2 is heavier than air.

Dissolve the alka seltzer in the cup with a small amount of water in it. Let it fizz for a minute (What is the reaction happening here?) The outcome of this reaction is that CO2 is given off and collects in the cup.

Lift the cup and then tip some of the gas in the cup over the candle flame. Do not pour the water over the flame. The candle should go out.

The alka seltzer gives off CO2 gas which is heavier than air so it collects within the cup. When you pour the gas out it sinks down over the flame which displaces the oxygen around it, putting the flame out. Flames need three ‘ingredients’ to burn; oxygen, fuel and ignition. Removing one of these will prevent or extinguish the flame.

Science Curriculum use:

The Magic Gas science experiment is an excellent way to demonstrate properties of gas, in this instance that some gases are heavier than air.

It can also be used in conjunction with the Jumping Flame Science Experiment (Also available in iTunes U.

Primary science ideas:

– Examples of gases that are lighter or heavier than air
– That gases can flow
– Principles of Fire Extinguishers

Secondary science ideas:

– What is the reaction? Investigate
– What factors influence the density of gas — use particle theory to model this


1. Research different types of fire extinguishers and how they work
2. Rates of reaction experiments — how long until the Alka Seltzer stops fizzing? What factors affect this?
3. Take two wine bottles — fill with water until you get the same ‘note’ when blown across the top. Then add Alka Seltzer to one bottle and observe what happens to the note — an example of the difference in how fast sound travels in different gases.


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