Salt gradient rainbow

Last year we played with sugar density gradients, making a 'traffic light' coloured one and a rainbow in a rocket-shaped bottle.  The idea is that you use different concentrations of sugar solution with food colouring and carefully layer them so they don't fully mix.  I wanted to do something similar, but didn't want to use the small amount of sugar I was able to buy for anything other than baking!  I realised that we had an alternative available as we have a large tub of table salt (sodium chloride).

Ready to make a rainbow salt gradient

We already have a rainbow suncatcher in our window, but I thought we could make a rainbow coloured salt gradient to put in our window so that it would be visible to other children as my sons both enjoy spotting them in other people's windows.  The morning sun would also shine through it and make a nice rainbow effect to cheer up my boys.

Making coloured salt solutions

When I first suggested it, my son was in a bit of a grump and didn't want to.  However, he was more enthusiastic later on so we gave it a go.  We used:
- Red, blue and yellow food colouring
- Salt (sodium chloride)
- Some tubes to mix the different concentrations in - you could use cups or yoghurt pots
- A bottle to make the rainbow in - we used a 250ml bottle, if you had lots of salt you could make a bigger one.  Taller and narrower shapes will work better.
- Warm water
- A teaspoon

We made a small amount (about 20ml) of a very concentrated red salt solution too put at the bottom of the tube to make purple, then equal volumes (about 80ml) of blue, yellow and red salt solutions.  In the small volume of red, we put a teaspoon and a half of salt.  In the blue we put three teaspoons.  In the yellow we put a teaspoon and a half.  The larger volume of red had no salt, it was just red coloured water.

The more concentrated solutions with most salt are more dense, so they would sink if layered on top of the less dense solutions.  The order of adding them to the bottle is therefore the small volume of very concentrated red solution, the blue, then yellow and then the red which is just a food colouring and water mix.  It's really important to add them slowly and carefully so they don't mix.  It doesn't matter for the first one, so my son poured that in.  We added the rest with a pipette together, so it ran down the side of the bottle.  If you don't have a pipette, tilt the bottle and pour very slowly - it's better suited to older children than mine, but he enjoyed the process and the end result, he just couldn't do everything for himself.

Completed rainbow (backlit)

The end result is a rainbow because some of each of the primary colours mix between the layers of each colour/concentration even if you're really careful adding them, so you get purple at the bottom, green between the blue and yellow and orange between the red and yellow.  It looks most impressive when backlit, with light shining through and making the colours more vibrant.


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