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Surface Tension

Surface tension, in a glassSurface tension is a fascinating phenomenon.  Make a simple observation by pouring a glass of water all the way to the top.  Gently place a small paperclip on top of the water.  If you are careful, the paper clip should float on top of the water, despite the fact that it is almost 8 times denser than water.  Don’t believe me?  Give the paperclip a little tap to push it below the surface and it will sink to the bottom.

But before you do that, look carefully at the water around your floating paperclip (or the photo above).  Notice how the top of the water acts a bit like a sheet or skin, sinking down near the edges of the clip and rounded up in the middle.

spider-317986_640In general,  water molecules stick to each other.   Water is “stickier” with other water than it is with air.  There is only so much “stickiness” or attractive force to go around.  Since the water molecules on top are touching less water than water in the middle of the cup, it has a stronger “stick” or force with the other water molecules on top.  This makes it harder for objects to push through and easier for things like paperclips and water spiders to stay on top of the water.  If you want to learn more about surface tension check out this page from the USGS.

So now that you understand surface tension you are ready to have some fun!

Fill a bowl with water (at least an inch or two deep).  Sprinkle ground pepper over the top of the water.  Use as much as you like, but not so much that the pepper breaks the surface tension and starts to sink – just cover the surface liberally.

Touch the tip of your finger to the center of the bowl.  Does anything happen?

Of course not!  Why should it?

Now put a drop of liquid soap on the tip of your finger and then touch the tip of your finger to the center of the bowl.  Does anything happen?

You bet!  The pepper should all rush to the outside edges of the bowl.

So what happened?  Soap lowers the surface tension or “stickiness” of the water molecules.  If they are not sticking together, the water molecules start to spread out and carry the pepper with them.

By now you should be overflowing with ideas to try!  Does this just work with pepper or will other floating items work as well like cereal or even Legos?  What about other liquids?  Do they have surface tension like water?  Which liquid has the greatest surface tension?  How can you measure this?  What affect does adding sugar or salt or anything else have on the surface tension of water?

Do you have any other surface tension questions to investigate?  Share them by clicking the “Leave a Reply” button at the top of this post.