Myth-busting Monday: Pouncing Pepper

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!

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Pouncing Pepper

Watch how detergent changes the surface tension of water in this amazing demonstration!

Materials

  • White plate
  • Finely ground pepper
  • Detergent or liquid soap
  • Water
  • Cotton tip (or finger)

Instructions

  1. Pour some water into the plate, then gently add a light sprinkling of pepper.

2. Touch your cotton tip (or finger) onto the surface of the peppery water. Observe what happens.

3. Put a small amount of detergent or liquid soap onto your cotton tip, then touch it to the surface of the water again. Observe what happens.

Further investigation...

  • What happens if you touch the surface of the water with the detergent a second time?
  • Repeat the experiment (clean the plate in between) with different types of soaps or detergents. What differences do you observe in the results?
  • Clean and refill the plate with water. What other small, light objects can you find that can be supported by the water’s surface tension?

What's happening?

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. You can feel the resistance of surface tension if you slap your hand onto the water in a bucket or bath – or belly-flop into a pool!

Soaps and detergents are both examples of surfactants, which are chemicals that reduce the surface tension of water. This is part of what helps them clean away oils and dirt from our hands and dishes.

When we touch the surface of our pepper water with the detergent, it reduces the surface tension in that one spot. This means that the surface tension is higher everywhere else, so the rest of the molecules at the surface pull back from that spot, stretching the ‘hole’ until the whole surface is affected. It’s a similar effect to popping a balloon. This movement between areas with different surface tensions is called the Marangoni effect.

The pepper in this demonstration helps us to see how those water molecules at the surface are moving. After the pepper has ‘pounced’, the lower surface tension let some of those bits of pepper sink to the bottom.

You may have noticed that you can only make the pepper ‘pounce’ once. If you want to repeat the experiment, you’ll need to rinse all the detergent off the plate and refill it with clean water.

Check your understanding

  1. What happened when you touched the surface of the water without the detergent?
  2. What happened when you touched the surface of the water with the detergent?
  3. Could you make the pepper ‘pounce’ more than once? Why/why not?
  4. Explain your understanding of these scientific terms: surface tension, molecule, surfactant

Curriculum Links

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Make a Rainbow

A rainbow forms in the fine spray from the hose

A rainbow is often a welcome sight when the sun comes out after a rain storm. Find out how to make your own rainbow when there’s not a cloud in sight!

Materials

  • A sunny day
  • A hose with a mist attachment
  • An open sunny space, such as a back yard

Safety first!

  • Always be sun smart when outside during the day – wear a hat, sunscreen, and clothing that covers as much of your skin as possible. 
  • It’s safest to stay in the shade in the hottest part of the day – fortunately, this experiment works best when done in the morning or afternoon.
  • Be aware that the wet ground might be slippery. Consider doing this experiment on a grassy area or near a garden so the water you use won’t go to waste!

Instructions

  1. Stand in your sunny space with the sun behind you. You should be able to see your shadow in front of you.

2. Turn on your hose. If your hose attachment has a choice of nozzles, choose the one that makes the water drops the smallest – for best results it should be a fine mist.

3. Move the spray around in front of you until you see a rainbow form in the droplets!

A rainbow forms in the fine spray from the hose
Get the angle just right, and you'll see a rainbow form in the fine mist from the hose!

Further investigation

  • While looking at your rainbow, try moving to a different spot in your sunny space. Does the rainbow appear in the same place it did before?
  • If you are doing this experiment with a friend, get them to stand a short distance from you. Can they see your rainbow too? What if they have a turn with the hose and make their own rainbow – can you see it from where you are? 

What's happening?

A rainbow forms when sunlight hits small drops of water in the air. Water is denser than air, so the light slows down and bends (refracts) a tiny amount when it enters the water drop. The light bounces around inside the raindrop, then exits again at a different angle.

White light is actually made up of lots of different colours mixed together, but our eyes see them as six distinct colours – red, orange, yellow, green, blue, and violet. Each of the different colours that make up white light bends a slightly different amount inside the water drop. When the light exits the water drop, each of these colours shows up as a distinct band.

The location of the rainbow that you see depends on the angle between your eyes, the sun, and the water drops. When you moved, your eyes (hopefully) moved along with you – and therefore, so did your rainbow. Someone standing in a different spot in your back yard won’t see the rainbow in the same place you do – they might not even see it at all!

When you see a rainbow in the sky, there are usually many more water drops than you can make with your hose, so lots of people can see it at the same time. However, everyone will see it in a slightly different place depending on where they are standing. And unfortunately, this means that it’s impossible to visit the end of a rainbow. (Sorry.)

More on this topic

  • Rainbows are very mathematical – visit this site to learn more about the maths behind how they are formed.
  • You now know that viral images such as this one, claiming to show ‘a rainbow viewed from above’, don’t show anything of the sort! This cool rainbow-coloured phenomenon isn’t necessarily fake though…it could be due to polarisation of the window glass. But that’s another story…

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