Penny Drops

Essential Question
How does water keep its shape as a water drop? How do other water drops interact with one another?

Objectives

 * Distinguish the difference between nonpolar and polar substances.
 * Predict what will happen when different substances are used instead of water

Introduction & Student Background
How do water droplets hold their shape? What liquids do you think are the least and most similar to water, and why?

In this activity, students will first learn about surface tension and how water molecules interact with each other by placing water droplets onto a penny and explaining how hydrogen bonds and intermolecular forces work. They will then try the same process with different liquids, and compare the different intermolecular processes.

Vocabulary
hydrogen bonds - a chemical bond between an electronegative atom, like fluorine, oxygen, or nitrogen, and a hydrogen atom bound to another electronegative atom.

intermolecular bonds - weak chemical bonds between molecules.

Materials

 * pennies
 * corn syrup
 * tap water
 * rubbing alcohol
 * vegetable oil
 * pipettes/droppers
 * paper towels

PREP

 * 1) Set up paper towels on different tables and put different liquids in clearly labelled plastic cups.
 * 2) Put 2-4 pennies on each paper towel/station.Pennydrop.jpg

ACTIVITY

 * 1) Have students place drops of water on the penny and count how many drops of water can fit on the penny.
 * 2) After the water spills over, ask them what the shape of the water was before it spilled.
 * 3) Explain about water's polarity and how this causes the water molecules to stick together, causing surface tension.
 * 4) Point out the difference in polarity from the other substances (rubbing alcohol, corn syrup, and/or vegetable oil). Have students predict how many drops the other liquids will hold.
 * 5) Have students place drops of said liquid on the penny until it spills over, and see how their predictions fared.

How it fits to Standard

 * Different kinds of materials display different properties (K-1)
 * The properties of an object depend on its shape and the material it is made from (2-3)
 * Substances have unique properties based on their atomic structures (6-8)

Follow Up
In order to explore more about polarity, have students look up molecular structures of the substances they have just observed. Which molecules are symmetrical and which ones are not?