British science week: Home science experiment – exploding colour

We’d heard about a pretty cool effect when soap is added to milk and food colouring, so we decided to give it a try and work out exactly what was happening. Suffice to say, the results were colourfully explosive.

Our task was to use two variables, water and milk, and see the difference when we added soap to the mix. Our hypothesis predicts that there should be little to no effect in water, but repeating the process in milk should produce a colour explosion.

You can read through our method, results and conclusions here, but it would be great if you guys have a try and send in some photos of what you created. Maybe you could change some of the variables, using oil and other liquids, or see if something other than soap will produce the same effect.

If you’re planning a science project, educating students or just looking to amaze your friends, this fun experiment produces some amazing results. You’ll probably find all the things you need in your kitchen cupboard, so there’s no excuse not to try it!

What you’ll need:

Washing-up liquid
Food colouring
Cotton swabs
Water
Plate
Milk

Method 1

Exploding colour experiment - Method 1
Exploding colour experiment – Method 1

With all our apparatus together, we set about testing our experiment with our first variable: water. After pouring water into a shallow plate, we then added food colouring, which settled as a blob in the middle, before adding a bit more for science (and definitely not just to make it look nicer).

Result

Exploding colour experiment - Results 1
Exploding colour experiment – Results 1

We left the food colouring to settle for a minute or so, and then collected some of our washing-up liquid on the end of a cotton swab. We dipped the swab into the centre of the pool of colouring and, as our hypothesis correctly predicted, nothing happened. Would the same be true when we used milk?

Method 2

Exploding colour experiment - Method 2
Exploding colour experiment – Method 2

We poured milk (it’s best to use whole milk, we’ll explain why later) into the same plate, with the water removed of course. Again, we added food colouring, which immediately looked different to our water method. The colouring was clumping together, rather than spreading out uniformly.

Result

Exploding colour experiment - Results 2
Exploding colour experiment – Results 2

Again, we dipped a cotton swab into our washing-up liquid and introduced it into the centre of the food colouring. This time, the food colouring receded away from the soap in an ever-expanding swirling circle, with the colour running in lines to the edge of the circle until it hit the edge of the plate. The colour continued to swirl and move when we removed the swab. Interesting, but what was happening?

Conclusion

Exploding colour experiment - Conclusion
Exploding colour experiment – Conclusion

The key to this experiment is the fat and protein in the milk, which is why we used whole milk. When the soap is added, it reduces the surface tension where it is applied by changing the bonds in the molecules of the protein and fat in the milk. Outside the influence of the soap the surface tension remains high, so the outer milk pulls the central milk near the soap away. The food colouring follows these currents and produces this neat effect. Once the surface tension is uniform in the liquid, the process stops, as you can see in our separate example using just green colouring.

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