The Southern Ocean and global climate

Activity 1

Thermal expansion of water

Global warming is likely to cause the sea level to rise for a number of reasons, the most important of which is the thermal expansion of water. This means that warmer water occupies a greater volume of space. Use the following method to show that when water is heated it expands.

Materials (for each small group)

  • round-bottomed flask
  • clamp and stand
  • 2-holed bung
  • water coloured with food colouring
  • thermometer
  • graduated capillary tube, or capillary tube with a paper scale taped to it
  • Bunsen burner
  • matches
flask


Procedure

  1. Fill the round-bottomed flask with coloured water.
  2. Close it with the 2-holed bung.
  3. In one hole place the thermometer, in the other the graduated capillary tube (see diagram).
  4. To show thermal expansion, heat the bottom of the flask.

Questions

  1. Why is cooler water more dense than warmer water?
  2. Thermal expansion of water is one reason why a rise in sea level could be a consequence of global warming. Suggest another reason.

Teachers notes

Remind students that sea temperature increases due to global warming would only be in the order of 1-2°C and would take place primarily in the surface layers. In this activity the effect of thermal expansion is greatly exaggerated because the volume of the capillary tube is very small compared to the volume of the flask

  1. Water doesn't behave in quite the same way as other liquids. When a substance is heated it usually expands and becomes less dense, and cooling has the opposite effect, causing it to contract.

    As water cools, it contracts and its density increases, as expected, until it reaches about 4°C. But then, further cooling causes it to become less dense and to actually expand. At 0°C, pure water turns into ice. Ice takes up more space and so is less dense than water at 4°C. The consequence of this anomaly is that ice floats on top of water. If, instead of floating, ice sank, much more of the water in the cold regions of the world would be frozen. This is because water, cooled to 0°C by the air above it, would turn to ice and sink, and more ice would then be formed on top and sink. But a layer of floating ice acts to protect the water beneath it from being frozen by the air.

    For more information about water density, its relation to Antarctic waters and diagrams of the water masses around Antarctica:

  2. The melting of snow and ice on Antarctica and Greenland (as well as non-polar glaciers) is usually mentioned as a reason for the sea level rising as a result of global warming.

    For more information about research projects to measure changes in sea level:

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Posted February 1998.