Soap and surface tension

I've seen variants of this experiment done by a few different people, and it seems to have become a popular way to illustrate why children should wash their hands.  As the small boy isn't always great at washing his hands, and sometimes refuses soap, I thought a time when he's got yet another cold and there's a threat of Covid-19 becoming more widespread in the UK would be a good time to try it.

Ready to experiment with black pepper, cinnamon and mixed herbs

The idea is that you sprinkle something on the surface of water which is held in place by surface tension.  Surface tension is due to the cohesive interactions between adjacent water molecules (just like capillary action in our colour walking experiment).  Soap molecules disrupt the surface tension, and you see this because the sprinkled substance moves rapidly away from the soap.

Pouring a layer of water in a plate

I've seen this done recently with ground up coloured chalk.  I didn't fancy grinding up chalk, and instead we tried it with ground black pepper, ground cinnamon and dried mixed herbs!  All three worked, so I suspect an array of ground spices or herbs would work too - they just need to be light enough not to sink.

The small boy's thumb has no effect on cinnamon

I put out a light-coloured plate (so it was easy to see what was happening), a beaker of water, a little pot of diluted liquid hand soap (about a 1:10 dilution) and the pepper, cinnamon and herb tubs/jars.  I invited my son to put some water in the plate and sprinkle whichever substance he chose on the surface; he wasn't sure which one to pick, so we started with black pepper.  I helped him sprinkle a small amount across the plate.  I then asked him to stick his finger in the middle of the plate to see what happened.  He observed that he got black pepper stuck to his finger, but didn't notice anything else.  I asked him to dip his finger in the soap and then do the same thing, and he watched the pepper move quickly away from his finger towards the edge of the plate.

After dipping in soap, the thumb makes the cinnamon move rapidly away

We had a quick chat about the fact the soap does something to the water which makes the pepper or dirt wash away more easily.  I didn't go into how surface tension works, but I asked him if he thought washing his hands with just water or with soap and water would be better for cleaning them, and he thought - correctly - that soap and water would be best.

He wanted to do it again, and chose cinnamon the second time and I asked him what he thought would happen.  He predicted that nothing would happen if he touched it, but if he put soap on his finger the cinnamon would move.  He was right.

Herbs moved away from his thumb which still had some soap left

The third time he chose the mixed herbs, but when he touched them the first time, they moved away from his thumb!  He wasn't expecting this, and I asked him why it might have happened.  He wasn't sure, but I explained that he'd used the same thumb that he dipped in the soap for the cinnamon (when he'd randomly chosen a different finger from the pepper), so it must have still had some soap left.  To check that I was right, we set it up again and he used a finger he'd not tried before.  The sprinkle of herbs didn't move until he dipped the finger in soap solution and tried again.  So there was nothing different about the herbs, it was just that the soap hadn't all washed off when he touched the plate with the cinnamon.

It's a fun demonstration of what soap does to surface tension, but I prefer the activity we did with paint and hand washing to show how germs can be spread and how well hands need to be washed to remove them.  It's also not necessary to offer a choice of what to sprinkle on the surface, but this did give a nice opportunity to encourage him to predict what would happen, and to explore why his prediction was wrong on one occasion.  It also gave me something to refer to later in the day when he wasn't keen to use soap when washing his hands, and I just reminded him of how the soap had made the pepper, cinnamon and herbs move and how it helps get rid of dirt and germs too.