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Title:  Which Grape is Heavier?

Materials

By:  Tessa Sudholt

1.  Fresh grapes

2.  7-Up Soda

3.  Clear drinking glass

Context

The concept involved in this lesson relates adhesive and cohesive forces (CO2 bubbles adhering to the grape skin) and the effects of hydrophobic and hydrophilic properties.  This activity could be used as the introduction to a unit about water and hydrogen bonding.

Procedure

1.  Pour and fill a glass with 7-Up Soda

2.  Peel one grape

3.  Take an unpeeled grape on the other hand and ask, "Which of the two grapes is heavier?"

4.  Drop both grapes in and observe.

Teaching Model

Experience, Expect, Explain.  The students will observe and then have an opportunity to compare their expectations with the results.

Questioning Script

1.  Why does the peeled grape sink to the bottom?

2.  Which of the two grapes is lighter in weight?

3.  What makes the unpeeled grape float?

4.  Would an unpeeled grape also float in water?

5.  What property does the grape peel have?

Explanation

This demonstration is suitable for teaching about bonding interactions, i.e. hydrogen bonds.  The unpeeled grape has water-repelling--hydrophobic--properties and thus the CO2 bubbles from the 7-Up can adhere to this surface.

The peeled grape does not have hydrophobic skin, thus it is hydrophilic--water-attracting.  The bubbles of the 7-Up have no way of adhering to the grape.  This causes the grape to stay at the bottom of the glass.  The unpeeled grape becomes lighter in weight, because of the adhering bubbles, and rises to the surface of the liquid.  There is loses come of the bubbles to become heavier again.  It may sink for a while, picking up more bubbles, to bob up to the surface again.

References

This discrepant event can be found in the Discovery Inquiries book on page 125.