Quick Activity – Surface Tension

by Deb on April 16, 2010

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Surface tension is the ‘skin’ on top of water.  It’s not that important to us, but if you were very small it would be very important.  It helps insects walk on the ceiling and on water, like this water strider.  It also helps drops form.  Here’s a quick and easy activity to do when you’re trying to cook dinner, although be prepared for water everywhere!  Set the cups up in a large, flat container on a cloth or nappy then it can all be tipped straight in the sink.

Normally if you look at water in a glass it is slightly higher at the edges and lower in the middle.  This is called the meniscus (m-NIS-cus).  But when a container is filled right to the top, surface tension lets you slightly overfill it.

This means there is a very cool trick you can do.  Almost fill a cup and sprinkle a bit of pepper on the top.  Pepper floats and is easy to see.

Carefully put a bit more water in to fill it, …

and, …

ta-daaah!  That black line around the edge is the pepper, it all scooted over to the side.

What’s happening? It looks like magic but it’s quite simple, the pepper is relatively heavy  and is rolling down to the lowest part of the water.  At first, the meniscus means the middle of the water is lower.  When you carefully fill it the surface tension means it makes a small mound and the edges are lower, so that’s where the pepper goes.  Great fun to watch!

Surface tension can be disturbed by things like soap and detergent.  Float a bit more pepper and then add a drop of detergent, the pepper starts sinking.

You can see here where it’s falling down.

After that, I just let them go.  Eye droppers, syringes, water, spoons and containers are always a big hit in our house, just keep an eye on the pepper because I got the sneezes really badly 😀

Do you play around with water?

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{ 3 comments… read them below or add one }

loco April 16, 2010 at 11:42 pm

man this thing is crazy…..


Annette April 17, 2010 at 7:56 am

What a great project to do with the kids!


Marita April 18, 2010 at 11:18 am

ooooh cool idea.

My daughter and I recently did this experiment http://leechbabe.wordpress.com/2010/03/29/homeschool-science-water-glue/

I think adding the pepper would make it much more interesting. Think we will be revisiting it soon.


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