Drops on a penny and the surface tension of water.

We can practice counting and comparing numbers 0 - 20, We’re measuring and recording. We can compare numbers, recognize patterns and make connections to decide.

Materials

  • Water

  • Liquid soap, a few drops

  • eye droppers for pairs or for each student

  • pennies

  • cups marked ‘A’ and ‘B’

This lesson is effective as a repeated activity that builds number sense 11-20+ . The focus isn’t on the actual number of drops, since they seem to splatter sometimes. The real focus is that the student counts dependably, without skipping numbers. Students use the droppers and count how many drops will bulge up on a penny before it spills. Usually the range is from 10 to 25 or so. Show students how to use the droppers slowly, and hold them close but not too close. The obvious way to do this is with ordinary water and that is case ‘A’.

It looks like a magnifying lens, and if you look carefully at the back of an old penny you can see Lincoln seated in the Lincoln Memorial. By looking along the top of the table, from the side of the penny, you can see the water bulge is bigger than the penny itself.

Here’s a shiny example with a bulging top.

Students can try several times if they record the count every time in their journal.

When all have had a chance, we open the journals. Ask students to tell us what their lowest and highest drop count was. The numbers can be written on the board and the class should look for patterns. Suggest a range that shows a typical amount of drops we were able to put on a penny.

Show the students a cup of water and add a few big drops of liquid soap. Mix it a little. Ask the students if they expect anything will change. Let the students count how many drops they can get on a penny.

What’s the only difference between case ‘A’ and case ‘B’ ? The soap ruined it !

Surface tension lets insects stand on the surface of the water. When you add the soap, it breaks up the surface tension and the water can’t bulge up on top like it did with ordinary water. That’s why it spills much sooner. First graders don’t ask and I never have tried to explain that. The point is that you can’t guess about a fact. You have to try it to see for yourself.