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Salinity and the density of water

You will need:

  • 1 clear 5 litre plastic container with a lid, cheap is best
  • 2 x 1 litre measuring jugs filled with water
  • Red and blue food colouring
  • Modelling clay
  • Salt and a measuring spoon


1. Add 4 drops of blue food colouring to one bowl and 4 drops of red food colouring to the other bowl. Be careful, if the colours are too strong you may not be able to see the result clearly..

2. Add at least 12 tablespoons of salt to the blue solution. Stir well to dissolve all the crystals.

3. Cut your plastic lid in half so that it can be inserted into your container vertically. Try to make the the shape of the cut lid match the inner contour of your container as much as possible. If you dont want to use the lid, cardboard will work however it will not be as re-usable.

4. Roll out some moulding clay into a 'snake'. Place the 'snake' around the edges of the lid, making sure that the clay is adhered to bothe sides of the lid. Insert the lid into the container.

5. Carefully seal all gaps between the plastic surfaces with the moulding clay

6. Gently pour both liquids at the same time into the container, one side red and the other blue. If too much is added on one side the uneven water pressure may cause your lid seals to break.
You should now have a container with two different coloured liquids separated by a plastic lid

7. In one movement, pull the lid vertically out of the container. The two different solutions will now mix together. If all has gone well you should find that the blue saline solution will clearly be below the red freshwater as two distinct regions.
salt density

Why Does This Happen?

Put simply, an objects density is related to the amount of 'stuff' in a given space. In better terms, density is the ratio of an objects mass to its volume. Adding more salt to the water made it more dense, making it sink below the freshwater above it. On average, sea water has around 35 grams of salt for every litre of water.

Density gradients drive currents to and from the polar regions of the world. When dense water masses are first formed they move towards the ocean floor. This moving mass of dense water is seeking to find a stable position along the ocean floor. As the cold salty water falls to the ocean floor, water must fill the space above it, setting up a situation for mass movement of water. As equatorial seawater warms up it expands, and is driven by wind and the differing density gradient toward the polar regions. This movement is known as Thermohaline circulation.

The movement of cold polar water towards equatorial regions provides an important source of nutrients for equatorial waters. Australian southern fisheries thrive on the upwelling of these deep ocean currents, as fish populations expand when their natural food sources have more nutrients to grow.

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Reference: Brady, J. E. & Holum, J. R. (1993). Chemistry. The Study of Matter and Its Changes.
John Wiley & Sons, New York

Williams, W.D. (1980). Australian Freshwater Life. Macmillan Education Australia, Melbourne.

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