Wikijunior:Big Book of Fun Science Experiments/Rainbow columns

Ingredients

 * Water
 * Salt
 * Food colouring

WHAT TO DO

 * Take 2 large spoonfuls of salt and dissolve them in 100ml of water.
 * Add a few drops of green food colouring.[[Image:Beakergreen.png|link=]]
 * Then you take another large spoonful of salt and dissolve it in 100ml of water.
 * After, add a few drops of yellow food colouring, (If you don't have yellow, use any other colour.)[[Image:Beakeryellow.png|link=]]
 * Add a small spoonful of salt and dissolve it in 100ml of water.


 * Add a few drops of blue food colouring.


 * Add some red food colouring to 100ml of water, without adding any salt.
 * Gently add a bit of the green water to a test tube.
 * Add a bit of the yellow water very gently. [[Image:Rainbowtesttube.png|right|link=]]
 * Repeat this two more times with the blue and red water.
 * If it mixes together, start again and keep on trying until you get a rainbow.

Principle
The principle of this experiment is density. Density is the ratio between volume and mass. When different substances of different densities are added to a container, one frequently floats on top of the other because it has lower density. The perfect example of this is oil on top of water.

The green water has the most salt in it, so it is the most dense. It remains at the bottom of the test tube.

The red water has the least salt in it, so it is the least dense. It remains at the top of the test tube.