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Student Activity - Saline currents

Activity idea

When you add salt to a glass of water and stir the water, the salt disappears. We all know the salt has not just vanished – if you taste it, you know it is still there. Have you ever thought whether the salt in the water has any effect on the water? Freshwater that has been collected on land flows back into the oceans of our world. Do the freshwater and saltwater just mix, and if so, would this happen in any particular way? Let’s investigate.

You will need

  • 4 translucent (see-through) bottles of equal size with lids – 1.5 litre plastic bottles are good
  • piece of thin cardboard, preferably waxy
  • blue food dye
  • water – enough to full 4 bottles
  • salt
  • marker pen
  • 2 trays or a workbench set up to allow for accidental water spillage


  1. On 2 of the bottles write FRESH and on the other 2 bottles write SALT.
  2. Fill all 4 bottles with water.
  3. Add a few tablespoons of salt to both of the SALT bottles.
  4. To 1 of the FRESH bottles, add a few drops of food colouring, enough to turn the water obviously blue.
  5. To 1 of the SALT bottles, add a few drops of blue food colouring, enough to turn the water obviously blue.
  6. Put the lids on all bottles and shake them, making sure the dye is evenly mixed and the salt is dissolved.
  7. Remove the lids.
  8. Set up the bottles as shown, making sure that the blue dyed bottles with the blue water are on the bottom. (Please make sure you do this experiment on a waterproof surface that accommodates for any unplanned water spillage.)
  9. Press the waxy cardboard on top of the water bottle without the dye before inverting it.
  10. To invert the top bottles without spilling them, try placing the cardboard over the top of the bottle, lining them up and then carefully removing the card.
  11. What do you think is going to happen? Take a note of your predictions and try to be as specific as possible.
  12. Observe what happens once the card has been removed.
  13. Check whether your predictions were correct.
  14. Try the same experiment with the other bottle set-up.
  15. Think and talk about your observations. What factors were the same in the two experiments and what was different? Was your predication correct?


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