Teaching Sequence

Work through this resource material in the following sequence:

20 minutes

Tune in: What will change?

20 minutes

Find out: Experiment time!

20 minutes

Reflection: What did we learn?

Tune in: What will change?

Step 1.

Explain to students that you will demonstrating that hot air rises.

Step 2.

Students write their prediction in the first section of the Student Worksheet.

Find out: Experiment time!

Step 1.

Fill the small cup or beaker with very hot (almost boiling) water.

Step 2.

Add several drops of food colouring.

Step 3.

Stretch the plastic wrap smoothly over the top of the cup and seal it with the rubber band.

Step 4.

Fill the large jar or beaker with cold water from the tap, so that the jar is almost full.

Step 5.

Use a pair of tongs to pick up the cup of hot water and place it into the large jar. The top of the small cup should be well below the level of water in the large jar.

Step 6.

Using your knife, slice open the plastic wrap (with one long slice) and watch what happens!

Step 7.

Pay particularly close attention to what happens as the coloured water gets to the top. Does it stay there? Why or why not?

How does it work?

As air heats up, its molecules expand and spread out, making the air less dense than it was before.

In this experiment, the warmer air floats up through the denser cooler air, which is heavier and so wants to sink down close to the ground.

The warm air uses up all its heat energy by rising, and starts to cool off. Its molecules move closer together, causing it to become denser and sink again. We can see this when the coloured water doesn't stay at the top but starts to sink towards the bottom of the larger container.

Convection is the transfer of heat energy through the movement of a fluid, such as air or water. One common example of convection is boiling water. When you heat water in a pot on the stove, the heat causes the water at the bottom of the pot to become hotter and less dense, so it rises to the top. As it rises, it cools down and becomes more dense, so it sinks back down to be reheated. This creates a convection current that transfers heat energy from the stove to the water.

This clip demonstrates a more complex version of this experiment. Consider adding blue coloured ice-cubes to your beaker to replicate this experiment.

Warm Air Rises - Cold Water Sinks, Warm Water Rises (https://youtu.be/bN7E6FCuMbY)

Why Does Hot Air Rise?

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