Week 3 at Science Club: Surface Tension

Last week we used water as part of our investigations into density. This week we were taking a closer look at water and finding out about one of the properties that makes it special; surface tension. The molecules in water tend to stick together and this force is strong enough at the surface of the water to form a type of “skin”. Some animals, like pond skaters, can use this skin to move around on.

Water droplets on pennies

To visualise surface tension, at the beginning of the session the children added drops of water to the surface of a penny. The water beaded up and created a tall dome shape as the water molecules at the surface clung to each other. The children counted how many drops they could add before the water spilled over the edge. The record was over 40 drops!

Floating paperclips Surface tension allows some things that are heavier than water to float on top of it. We can test this with paperclips; if you drop one in a cup of water it will sink, but if you lay it carefully on top, the surface tension will allow it to float. What happens if we put a drop of washing up liquid where the paper clip is floating? It was difficult to make the paperclips float, but for those who managed it we found that adding washing

up liquid made the paperclip suddenly sink. This is because washing up liquid lowers the surface tension, meaning the paperclip can no longer sit on top of the water. Magic Boats

The children choose one of the foam ‘boats’ I had prepared earlier and carefully floated the boat at one end of a foil tray filled with plain tap water. Next they touched a drop of washing up liquid with a cocktail stick to the back of the boat and watched what happened. You can see in the picture below the boat just shooting out of sight!

Foamboatsreadytogo!

Pondskaterusingsurfacetension

Paperclipfloatingonwater

Paperclipabouttotakeadive!

Addingdropletsofwatertoapenny

You can try and carry out your own experiments with these at home; do boats made of different materials travel different distances, or do they go faster or slower? How about boats of different sizes, or different designs? You could use a ruler to measure how far the boats have travelled. NB. You will need to rinse out your tray and refill it with clean water after each boat ‘launch’ so that you can recreate the surface tension ready for your next attempt. What is happening? Washing up liquid lowers the surface tension allowing the paperclips to sink and the ‘boats’ to start moving across the surface of the water. When the boat is sitting on top of the water, the surface tension of the water holds it on the surface and is the same on all sides, so your boat doesn't move. Washing up liquid lowers the water’s surface tension (making water molecules stick together less is what helps soaps clean dishes and clothes more easily). When you touch the middle incision of your boat with washing up liquid the surface tension pulling the boat back is less than the water pulling it forward. It is this difference in surface tension that makes the boat move forwards. The ‘Magic Finger’ Trick To finish the session I showed the children a little trick that involves filling a small bowl or saucer with water and covering the surface with pepper. We discussed whether I could use my finger to move the pepper, but when I put my finger in the water nothing much happened. However, after a few ‘Abracadabras’ (while I secretly put a bit of washing up liquid on the tip of my finger) the next time I put my finger into the water the pepper suddenly rushed outwards to the edge of the bowl! This was of course because the washing up liquid had lowered the surface tension, causing the pepper grains to move quickly away. If you haven’t already seen this little trick in action, give it a go at home!

Foam'boat'floatingonwater Travellingawayfromthewashing-upliquid