It's time for another great experiment with the Let's Find Out crew - Amy, Mark and Captain Zoom.

Tune in to the show weekdays on RTÉjr at 7am and 2.30pm or watch it any time on RTÉ Player!

Here, Zoom wonders why some things float and others things sink. She discovers that it's all about whether the object is more dense or less dense than the liquid it is in.

Adding salt to water makes it more dense, but can Darragh and Ronan make water float on water with their saltwater density column?

Always have adult supervision when trying any of these experiments...

You will need:

  • A graduated cylinder
  • Some card or stiff paper, torn into small pieces
  • Salt
  • A tablespoon measure
  • Liquid (not gel) food colouring (at least three different colours)
  • A dropper
  • A jug of warm water (about 2 litres)
  • A measuring jug
  • A funnel
  • A straw
  • Tape
  • Large cups or glasses or containers that can hold more than 600mls (at least three)
  • Something to stir the water eg a spoon.

What to do:

  1. Place the straw onto the end of the funnel and use the tape to secure it in place.
  2. Measure out 600mls of warm water and add it to the first glass.
  3. Repeat for the other two glasses (so you now have 600mls of warm water in each glass)
  4. Using the dropper add six drops of food colouring to each glass; choose a different colour for each one so, for example, blue in the first, yellow in the second and red in the third.
  5. Mix the food colouring into each glass of water with the stirrer.
  6. Add 8 tablespoons of salt into the glass of blue water and stir until all the salt has dissolved (or at least until there are only a few grains left).
  7. Add 4 tablespoons of salt into the glass of yellow water and stir until all the salt has dissolved.
  8. Do not add any salt to the glass of red water.
  9. Carefully pour the blue salty water into the graduated cylinder.
  10. Now place a layer of the torn up card on top of this layer (you can check out the video link above to get an idea of how to do this, he used Styrofoam instead of card but the idea is the same).
  11. Now take the funnel with the straw and hold it in the graduated cylinder so that the open end of the straw is very close to the layer of paper.
  12. Ask a helper to hold the funnel and straw in place while you SLOWLY and CAREFULLY pour the yellow salty water into the funnel.
  13. You should see that the yellow salty water sits as a layer on top of the blue salty water and the layer of paper sits on top of the yellow salty water.
  14. Now it’s time to add the red water, place the funnel and straw into the graduated cylinder so that the open end of the straw is very close to the layer of paper and SLOWLY and CAREFULLY pour the red water into the funnel.
  15. Carefully remove the pieces of paper.

What’s happening?/The science bit:

When we dissolve salt in water we change something called density. Water with salt in it is more dense than water with no salt in it. And things that are less dense float on things that are more dense. You should have a layer of blue salty water at the bottom of the graduated cylinder with a layer of yellow salty water in the middle and a layer of red water on top. Which layer do you think is the least dense and which layer is the most dense?

You can see more great experiments HERE!