You might have seen this viral (ahem) video doing the rounds – a preschool teacher is using the ‘pouncing pepper’ demonstration to show her students how soaps keep germs away.
Myth: The ‘pouncing pepper’ demonstration shows how soap repels germs from our hands.
Truth: Of course, anything that gets people washing their hands more often is definitely a winner (whether or not there’s a global pandemic). And pouncing pepper is a great demonstration of a scientific phenomenon, but perhaps not the one you might think…
You can do this demonstration at home using just a few materials. A cotton tip or finger is dipped into some water with pepper sprinkled on top, and gets covered with pepper. But when the finger-dip is repeated with detergent, the pepper instantly jumps away!
So does soap repel germs in the same way it appears to repel the pepper in this demonstration? No. In truth, pouncing pepper doesn’t actually demonstrate the effectiveness of soap in removing germs.
This experiment works because of water’s surface tension. Water likes to stick to itself, and surface tension is a bit like a skin formed by the water molecules at the surface. The pepper is small and light enough that the surface tension can support it. But something bigger or heavier, like a person, can break through. You’ll know all about this if you’ve ever bellyflopped into a pool!
So how does soap affect all this?
Soaps and detergents reduce the surface tension of water – this is part of what helps them clean away oils from our hands and dishes. But the germs don’t exactly leap away.
Bacteria and viruses are partially made up of fats, which are broken down by detergents – the detergent reduces the water’s surface tension, allowing it to get between the bits of oil. Detergent is also a long molecule with one end that attracts water, and the other attracts oils. This allows the oil to mix with the water, and be washed away as you rinse.
So back to our pepper experiment. When you touch the detergent to the surface, the surface tension is reduced in that one spot. It’s a similar effect to popping a balloon – if the tension is reduced in one spot, the higher tension everywhere else pulls back from that spot, making the ‘hole’ bigger. And the pepper just helps us to see how those water molecules at the surface are moving.
So this awesome experiment is a great demonstration of how soap changes the surface tension of water, but unfortunately, germs don’t leap away from soap like the pepper does. Which means you need to keep washing your hands! Properly! Go and do it now!