Surface Tension - Pennies and Water (Dave Farina)

Author

Carol Cao

Principle(s) Illustrated

1. Surface Tension

2. Water Properties

3. Hydrogen Bonding

Standards

• Chemistry Standard-

  • Students know how to identify solids and liquids held together by van der Waals forces or hydrogen bonding and relate these forces to volatility and boiling/ melting point

Questioning Script

Prior knowledge & experience:

For this lesson, I think it's best that students don't have too much prior knowledge. Because this is a discrepant event, students should guess. The only prior knowledge is that water is a liquid and if you fill a cup with too much water, it will overflow.

Root question:

How many pennies can you put into the cup before the water spills over the sides? What is causing the water to not flow over the side?

Target response:

The correct answer varies depending of the size of the cup and the size of the brim. However, the number of pennies is substantially more than students predicted. In this demo, we got 68 pennies in the cup before the water flowed over. Students are to explain the idea of surface tension and hydrogen bonding.

Common Misconceptions:

Many students will see that the cup is filled to the brim and will predict a small number of pennies (somewhere between 1-10) can go into the cup.

Photographs and Movies

References

Surface Tension (Wiki)

Hydrogen Bonding