Hot air expands demonstration for schools : Fizzics Education


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Hot air expands demonstration

Hot air expands demonstration

Follow FizzicsEd 150 Science Experiments:

You will need:

  • Hot water from a kettle or thermos

    CAUTION: adults to use this only!

  • A glass bottle
  • A balloon
  • A tall plastic container


A thermos, yellow balloon, plastic cup and glass bottle on a desk
1 A yellow balloon on a glass bottle

Attach your balloon onto your glass bottle.


Place your glass bottle into the large plastic cup.


Carefully pour the hot water over the neck of the glass bottle. You should start to see the balloon start to inflate.

CAUTION: adults to use this only!


Keep going until the balloon is inflated as per the picture!

Try placing the bottle into cold water… what happens to the inflated balloon now?

6 Teacher showing how to do an experiment outside to a group of kids.

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– Help students learn how science really works

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7 A man holding a blow torch onto a white tile whilst wearing safety glasses

Get the Unit of Work on Heat Energy here!

  • What actually is heat?
  • How does heat move through different materials?
  • How does heat change the properties of materials and more!

Includes cross-curricular teaching ideas, student quizzes, a sample marking rubric, scope & sequences & more

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What is going on?

Hot gases expand, cold gases contract!

All gases have freely moving molecules. As you add more heat to a gas, the molecules move faster and push harder against the sides of a vessel that holds them. The more the gas molecules collide against the sides of the vessel, the greater the air pressure.

By covering the glass bottle in hot water, the heat energy moved into the contained air which then increased the air molecules movement inside the bottle and therefore the air pressure as well. This increased air pressure expanded the balloon!

Variable testing

More about variable testing here

  • Try different size balloons. Can you inflate each one?
  • With an adult, try different water temperatures
  • Try different shaped bottles

Going further

What you have observed is an example of how the pressure of a gas is proportional to the temperature of the gas. In simple terms, this means that if you increase the pressure, you increase the temperature. This works both ways, if you increase the temperature, you increase the pressure!

This is Gay Lussac’s Law, stated more precisely as below:

The pressure of a given mass of gas varies directly with the absolute temperature of the gas, when the volume is kept constan

which can be written mathematically as follows:

Gay Lussac equation p1/t1 = p2/t2


P1 = Initial pressure

T1 = Initial temperature

P2 = Final pressure

T1 = Final temperature

You can explore the effect of changing temperature, volume & pressure in the interactive simulation by the University of Colarado Boulder PhET Interactive Simulations project

Just click on “Ideal” which refers to what happens to an Ideal Gas, which is a pretend gas which has molecules that don’t chemically react with each other. This means that you can see what happens to these molecules as you change the temperature, pressure and volume of the container much more easily.

A man with a glove above a liquid nitrogen vapour cloud

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