Friday, December 11, 2015

What a Drop of Water Can Teach Kids about Cohesion

The boys and I have been doing a lot of simple science lately and we're loving it. This experiment takes five minutes and four supplies. Don't let that convince you otherwise, it's surprisingly fun.

What You Need
Waxed paper
Cup of water
Plastic drinking straw

What to Do
Put the straw in the water and cover the top of the straw with your finger. Lift the straw out of the water and trap some water inside at the bottom of the straw. Move the straw over your sheet of waxed paper and release your finger from the end of the straw. You'll deposit a drop of water. Repeat these steps until you have a few water droplets on the paper.

Now grab the toothpick and dip one end in the water. Move that end of the toothpick near (but not touching) a water droplet. Watch as the drop of water moves toward your toothpick. 

Now drag the toothpick around. Your droplet moves too.

What's Happening
What you're seeing is cohesion, an attraction between molecules of the same type. The water molecules in the droplet are attracted to the water molecules on the toothpick and thus, move with a pull of force called cohesion.

This great experiment came from Q. L. Pearce's book Kitchen Science Experiments. Check it out!

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