Exploding colours
It's an explosion of colour! Some very unusual things happen when you mix a little milk, food colouring, and a drop of washing up liquid. Just like the firework rockets exploding in the sky!
You will need:
- Milk (whole or semi-skimmed)
- Dinner plate
- Food colouring (red, yellow, green, blue)
- Washing up liquid
- Cotton buds
What to do:
- Pour enough milk in the dinner plate to completely cover the bottom and allow it to settle.
- Add one drop of each of the four colours of food colouring - red, yellow, blue, and green - to the milk. Keep the drops close together in the centre of the plate of milk.
- Place a drop of washing up liquid on the tip of the cotton bud.
- Place the soapy end of the cotton bud in the middle of the milk and hold it there for 10 to 15 seconds. Look at that burst of colour!
- Add another drop of washing up liquid to the tip to the cotton bud and try it again. Experiment with placing the cotton bud at different places in the milk. Notice that the colours in the milk continue to move even when the cotton bud is removed. What makes the food colouring in the milk move?
What’s going on?
Milk is mostly water but it also contains vitamins, minerals, proteins, and tiny droplets of fat suspended in solution. Detergents (such as soaps and washing up liquids) contain molecules with a fat-loving end and a water-loving end. The soap moves quickly through the milk and the rapidly mixing fat and soap causes swirling and churning. The food colour molecules are bumped and shoved everywhere. When there are clusters of soap molecules and fat droplets everywhere the motion stops.
There's another reason the colours explode the way they do. Since milk is mostly water, it has surface tension like water. The drops of food colouring float on the surface and stay there. Detergent wrecks the surface tension by breaking the bonds between water molecules and allowing the colours to whizz throughout the milk. Wow!
