Harnessing direct solar energy – a progress report

Activity 5

Showing the movement of water as it heats

Hot water weighs less than cold water. This is because a substance occupies more volume and is therefore less dense when it is heated. In gases and liquids, a light substance will rise to the top – hot water rises above cold water. This principle is made use of in most water heaters.

Materials (for small group)

wide test tube (about 4 centimetres in diameter)

glass tube about the same width as the test tube (or a second test tube with the bottom cut off)

2 lengths of glass tubing

2 corks (or bungs) with two holes in each

cold water

food colouring

thermometer

p

Set up the apparatus (as shown in the diagram) and place it in direct sunlight.

Record the temperature of the water in the tube every two minutes until there is no further evidence of the movement of water through the system.

Questions

  1. Do you think the water has now stopped getting hotter?

  2. What can you do to test your opinion?

  3. Describe what happened to bring about the changes in the appearance of the water.

Teachers notes

  1. The temperature of the water will continue to rise until the rate at which heat is gained from the sun equals the rate at which heat is lost from the water to the surrounding air.

  2. Students could test their opinion by continuing to note the temperature for another 5 minutes.

  3. As water heats it will expand, forcing the coloured water in the lower tube up through the pieces of glass tubing. Water movement may not be visible the entire time if the differential between the colour in the bottom tube and top tube is lost due to mixing.
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Posted February 1997.