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Classroom activity:
Concepts about Heat and Density

There are several ways to demonstrate or explore the concepts of heat and density. These include:

Feeling the heat:

A. Light a candle and hold your hand about 5 inches from the side of the flame. Now move your hand about 5 inches above the flame. What do you notice? What is the explanation?

If air were a good conductor, there would be no difference. However, we can also use this demonstration to show that warm rises. As the air molecules are warmed by the flame, they move more quickly and take up more space. They become less dense and therefore rise above the cooler air molecules.

B. Repeat the demonstration for a more dramatic result by holding the head of a wooden match about inch to the side of the flame for about ten seconds. (The match will not burst into flame.) Then move the match to the same distance above the flame. (The match will burst into flame.)


Comparing the liquids:

Fill one film canister about full with ice water and a second one about full with very hot water. Hold a piece of white paper behind the canisters so that you can see what happens. Add 2-3 medicine droppers of food coloring to each of the canisters. (Slightly submerge the pipette) What happens? Why?

Heat causes molecules of water to move more quickly. Therefore the food coloring is more quickly dispersed throughout the warmer liquid.

Marble molecules:

Use a small container filled about 2/3 with marbles as illustrated. Explain to students that the marbles represent molecules. Hold the container and shake it minimally, so that the marbles only vibrate, but do not move away from one another.

This represents molecules in a solid state - barely vibrating and taking up a minimal amount of space. Now add energy by shaking them a bit more, so that the molecules flow within the container. This represents molecules in a liquid state. They are moving more quickly and taking up a greater amount of space. Now add even more energy by shaking the container so that the marbles bounce off one another and out of the container. This represents a change to the gaseous state. The molecules are moving more rapidly and bounce off one another to take up greater space still.

Discuss with students how this can be a model of changes in molecular density and movement as energy is added.


  • Adding heat energy causes molecules to move more quickly.

  • Adding heat energy to most liquids and gases causes them to expand (become less dense).

  • Adding heat energy can change the state of a gas, liquid, or solid.

Additional demonstrations of density:

  • Using a density column

  • Stuffing a box with crumpled paper

  • Different sodas poured into water

  • Water displacement

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