Why Heat Inflates Air

The rule is simple: hot gases expand, and cold gases contract!

All gases are made of freely moving molecules. When you add heat energy to a gas, those molecules move faster and collide more forcefully against the sides of the vessel holding them. The more frequently and powerfully these molecules strike the walls, the higher the internal air pressure becomes.

By placing the glass bottle in hot water, heat energy moved into the trapped air. This increased the kinetic energy of the air molecules inside, raising the pressure. Because the balloon is flexible, this increased pressure pushed the rubber outwards, causing the balloon to inflate!

Variable testing

More about variable testing here.

• Balloon Volume
  Try different size balloons. Does a larger balloon require more heat to show a visible change in inflation?
• Temperature Range
  With an adult, try different water temperatures. Is there a “threshold” temperature where the balloon suddenly starts to stand up?
• Container Geometry
  Try different shaped bottles. Does the volume of air inside the bottle change how quickly the balloon reacts to the heat?

Going further: Gay-Lussac’s Law

What you have observed is a perfect example of how the pressure of a gas is directly proportional to its temperature. In simple terms: if you increase the temperature, you increase the pressure (as long as the volume stays the same).

This is known as Gay-Lussac’s Law, which states: The pressure of a given mass of gas varies directly with the absolute temperature of the gas, when the volume is kept constant.

P1 / T1 = P2 / T2

Whereby:

• P1 = Initial pressure
• T1 = Initial temperature
• P2 = Final pressure
• T2 = Final temperature

Interactive Simulation

Explore the effect of changing temperature, volume, and pressure in this interactive simulation by the University of Colorado Boulder PhET project. Click on “Ideal” to see how molecules react in a perfect environment!