Surface Tension of Water

Author: Krista Botton
Discrepant Event - Teacher's Guide
SED 695B
Activity 1
Activity 2
 


Detailed Explanation of Discrepant Event

Principles illustrated

  • Cohesion and adhesion of water molecules
  • Hydrogen bonding

Activity #1: How many drops of water fit on a penny???

1. Have the student predict how many drops of water will fit on a penny and record their prediction.

2. Allow the student to test their prediction using a pipette to add water in a dropwise fashion and count the number of drops will pile on until the penny "runneth over."

3. Have the student analyze their prediction and consider WHY the number was different than anticipated.

Activity #2: Metal floats???

1. Have the student predict if several objects will sink or float if put in a tub or beaker of water. (Objects can include: wood blocks, straw, penny, paperclip, straightpin, toothpick, needle)

2. As a class demonstration, sink or float each object (be sure to carefully place the objects so that paperclips or even needles will not break the surface tension and will float!).

3. Have students analyze their predictions. Which were correct and which were not? Have them make predictions why.

Video of floating needle

Standards

Chemistry
  • Standard 2h: 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 temperatures.


 

Questioning Script for Activity #2

Prior knowledge & experience:

  • What materials usually float? Give some examples.
  • What materials usually sink? Give some examples.
  • What causes some objects to sink and others to float?

Root question:

  • Will each of these items sink or float?

Target response:

  • Which items floated?
  • Which items sank?
  • Why were paperclips and staightpins able to float although made of metal, an object that usually sinks?

Common Misconceptions:

  • Water runs and will not pool on top of the penny
  • Metal objects sink
  • Surface tension of water is not strong enough to float objects on top of the water surface

 

References & Links:

Wikipedia - Surface tension

Surface tension - description and application