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Exploding Milk Colours

Ages 4+ with an adult10 minutesMess: Low (food colouring can stain)
Grown-ups: please supervise this experiment the whole time. Read the safety notes first.

Add drops of food colouring to milk, touch it with a dab of washing-up liquid and watch the colours burst outwards like a potion.

WOW FACT

Washing-up liquid molecules have one end that is attracted to water and one end that is attracted to grease. That's how they lift grease off your dinner plates.

🧰 What you'll need

A plate or shallow dishOne with a rim, so the milk stays put.
✔ You probably have this
Full-fat milkJust enough to cover the plate.
✔ You probably have this
Food colouringTwo or three colours.
✔ You probably have this
Washing-up liquidOnly a dab.
✔ You probably have this
Cotton budsFrom the bathroom cupboard.
✔ You probably have this

✨ Want to go further? (optional)

Plastic pipettesDrop colours exactly where you want them.
View on Amazon (amazon.co.uk)
Safety goggles for kidsGets children into good scientist habits, and keeps splashes out of eyes.
View on Amazon (amazon.co.uk)
Children's science experiment bookDozens of easy experiments for rainy days.
View on Amazon (amazon.co.uk)

💛 We may earn a small commission if you buy, at no extra cost to you. We only suggest things we'd use ourselves. How this works.

What to do

  1. Pour enough milk onto the plate to cover the bottom. Let it settle.
  2. Add a few drops of each food colouring near the middle, without stirring.
  3. Dip a clean cotton bud in the milk and touch the centre. Not much happens.
  4. Now put a small dab of washing-up liquid on the end of a cotton bud.
  5. Touch the soapy bud to the milk in the middle and hold it still. Watch the colours burst away!
  6. Touch other spots and watch the swirls. When it stops, try again with semi-skimmed milk. Which works better?

What's going on?

The surface of milk is held together by surface tension, a bit like a thin stretchy skin. Washing-up liquid weakens that pull where it touches, so the surface around it pulls away and carries the colours with it. Washing-up liquid molecules are also attracted to the tiny drops of fat in milk, and as they move around and grab the fat the milk keeps swirling. Once the washing-up liquid has spread through the milk, the movement stops. Full-fat milk usually gives the biggest show.

Safety notes