Ideal Gases

This activity will demonstrate that there is a relationship between the temperature, pressure and volume of gases.

Time requirement: 50 minutes as an activity, 30 minutes as a demonstration.

Materials

Procedures

  1. Inflate the balloon and secure the end so that the air does not escape.
  2. Measure and record the circumference of the balloon at room temperature.
  3. Add heat to the balloon by placing the balloon in a container of hot/warm water. Cover the container and allow the balloon to heat up.
  4. Measure and record the circumference of the hot balloon.
  5. Let the balloon sit and return to room temperature.
  6. Now allow the balloon to lose heat energy. Place the balloon in a container of ice water. Cover the container.
  7. Measure and record the circumference of the cold balloon.

Observation questions

  1. How much did the circumference of the balloon increase in size when heat energy was added and the temperature increased?
  2. How much did the circumference of the balloon decrease in size when heat energy was removed?
  3. Why did the balloon increase in size when heat energy was added?
  4. Why did the balloon decrease in size when heat energy was removed?
  5. The density of a gas (or liquid or solid) is equal to its mass divided by its volume. When the balloon was heated, did its volume increase or decrease? Did the density increase or decrease?

To think about

This is actually a complicated demonstration! If the gas inside a rigid container is heated, then the pressure inside will increase because the gas molecules will move more quickly and exert a greater force on the walls of the container. The volume of the enclosed gas will not change, however, because the gas is not free to expand. The density will not change because the volume does not change and because the amount of gas inside the container does not change.

The balloon is not a rigid, container, however. As the gas inside the balloon is heated, the pressure inside the balloon begins to increase which causes the balloon to expand. The expansion of the balloon allows the volume of the gas to increase and the density to decrease. The increase in volume reduces the pressure inside the balloon. Ultimately, the pressure inside the heated balloon is probably somewhat greater than inside the balloon at room temperature, but it certainly is not as great as it would be inside a rigid container heated to the same temperature.

The atmosphere, of course, is not like a rigid container. As air is heated it does indeed expand and its density does decrease. Likewise, air masses contract when they are cooled. Warm air rises and dense, cool air sinks.

Last updated: August 26, 1997
Joe Twicken / joe@nova.stanford.edu
Rob Wigand