Science Trick - Coloured Water Density Trick

This activity is a bit tricky and possibly very messy, but also a fun way to learn about density.

Adding salt to water increases its density, so the less dense plain water sits on top. The two don’t mix.

coloured water science density trick - density trick for kids

What you need

Two glasses of the same size
A piece of card
Food colouring
Salt
Water

Salt Water Density Trick Instructions

Two glasses filled with liquid. The bottom one contains blue food colouring, water and salt. The top glass is plain water. The water glass is on top of the blue water glass with a piece of card in between them.

Fill both glasses with water right up to the brim.
Add food colouring and salt to one glass and stir well. I didn’t measure the salt but added a generous amount.
Put the card over the glass containing just water and carefully place it over the glass of coloured water and salt.
Even more carefully remove the cardboard.

Why doesn’t the water mix?

The salty water has a higher density than the plain water, and so stays at the bottom.

You can see here what happens if you don’t add the salt. The plain water mixes with the coloured water.

If you put the salt water on top, it all mixes, this is because the dense salt water tries to move down to the bottom.

I would recommend doing this science trick over a sink, or somewhere you don’t mind getting wet.

Mom to Posh Little Divas did an activity last week with water and a card where you can lift a glass of water up and the card stays stuck to it. This is a brilliant demonstration of air pressure and would be great to do at the same time as this activity.