Archimedes and a Rainbow density experiment!

Density is a really tough concept to grasp.

We confuse ourselves by referring to our weight all the time when we really mean our mass. (We should not say weighing scales, we mean mass scales!)

Mass is effectively ‘how much stuff’ is there!

Density is how much mass is in a volume (or space).

Think of a sock drawer (imagine your bedside table drawer). In it you can have 10 pairs of socks. That drawer will have a certain density. If you add another 10 pairs of socks (20 pairs) the density will change because the mass has changed (increased) but volume has stayed the same. Like wise if you put your 10 pairs of socks in a bigger drawer the density will change as you have now put the same mass in a bigger volume!

I shall tell you a little story!

Archimedes and The Eureka! 

Once there lived a scientist called Archimedes who worked for the King.

The King summoned him to his chamber one day with a problem.

‘I have had this beautiful crown made out of pure gold’ he said ‘But it does not look pure to me…look at the colour! The problem is the crown has the same mass as the gold I gave to the craftsmen to make it with. I know that they stole some but I cannot prove it. I want to chop off their heads!”

‘Oh that is a problem’ replied Archimedes

‘Well, now it’s your problem’ said the King.’Becasue if you don’t find out how they did it I will chop off your head too, I am in that much of a bad mood!”

Poor Archimedes. He went away and could not stop thinking. Morning, noon and night he wracked his brains. He started to accept that his head would be chopped off and decided to have one last hot bath!

As he stepped in he relalised that some of the water spilt over the edge!

‘Eureka!’  He cried “I’ve got it!. I’m a blooming genius I’ve really got it!”

He went back to explain to the king. ‘Metals of the same mass have different volumes which changes their density’ he said. ‘We can test this by the process of dispersion. If we place a metal of one mass into a full beaker of water, the amount of that spills over the side (the dispersed water) is equal to the volume of the metal. If we put in another metal of the same mass, the amount of water it will disperse will be different. We can now test the crown. We shall measure the amount of water dispered by the mass of the pure gold you gave and see if it is the same as the crown. If the volume is different, you can chop of their heads!’

‘Oh goody’ said the king. ‘I like a good head chop”

And that is how they discovered density!


So now I am going to show you a very simple experiment but it is very pretty!

You will need:




a glass

Food colourings

Wahing up liquid

Vegetable oil


1. Measure out the same volume of each of the liquids. Colour the water and the milk if you wish

2. Starting from the bottom…pour in the honey. Make sure none touches the side, pour striaght into the middle

3. Slowly pur the washing up liquid on top

4. Then add the milk

5. Then add the water

6. Finally top with vegetable oil!

Tadah! Your rainbow in a glass! Or in my case not so rainbow like as I coloured the milk and it slightly mix with the coloured water and I ended up with a sort of disgusting purple. On retrospect I would skip either the water or the milk and just use one or the other. Remember that the vegetable oil won’t hold a colour. But the liquids did separate which is the main thing. (When I wrote pretty I had not conducted the experiment. I would have done it again, but I need the last of the washing up liquid for the dishes!)

Not quite as rainbow like as I would have hoped, although my fingers are from the food die are!

The science bit! 

Each of the liquids have the same volume but because their is a different mass of molecules or parts squashed into those volumes it makes them have different densities and therefore one can sit on top of the other – the more dense the heavier it is.

Go on have a go!

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