Your challenge is to demonstrate that salty water is denser than freshwater.

Be creative and design and make saltwater jars. You will need:

• two cups of water
• two same sized (identical) glass jars
• water
• half a cup of salt
• red and blue food colouring
• a spoon

What to do

• Make predictions about which would be heavier (more dense), freshwater or saltwater.
• Test ideas by adding some blue food colouring to one of the cups of water. Stir it with the spoon until the water is completely blue.
• Repeat this by adding the red food colouring to the other cup of water and use the spoon to stir it.
• Add the salt to the red water and stir it to help the salt dissolve.
• Pour half of the blue coloured water into one of the glass jars and half of the red salty water into the other jar.
• Slowly top up the jar containing the blue water with the remaining red coloured water.
• Then, carefully and slowly pour the rest of the blue water into the jar containing the red salty water.
• Without disturbing either jar, place lids on the jars and let them stand for a while. Observe what happens.

How it works

Adding salt to the water makes it denser. In this activity, the ‘red’ water contains the salt, and when it is poured on top of the ‘blue’ water, it sinks through and the colours mix.

When the solutions are poured the other way around (the ‘red’ before the ‘blue’), the non- salty water floats on top because it has been placed on a liquid with a higher density.

[This video on YouTube (2:16 mins) shows another way of representing the ocean in a bottle, complete with waves.]

(Copyright: Australian Science Teachers Association 2020, except where indicated otherwise. This work is licensed under a Creative Commons Attribution NonCommercial 4.0 International licence.)