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9-12

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## How Hard is Chocolate?

You already know that certain materials are harder than others; in fact, you prove it everyday when you chew your food and your teeth don’t break (because your teeth are harder than the foods you chew).

### Overview

Hardness is probably a concept you are well familiar with. You already know that certain materials are harder than others; in fact, you prove it everyday when you chew your food and your teeth don?t break (because your teeth are harder than the foods you chew). Hardness can be defined as a material's ability to resist a change in shape. Modern hardness testers take a well-defined shape and press it into a material with a certain force, observing the indent it leaves in the material when it is removed. In this lesson, you will be performing hardness testing on different bars of chocolate.

### Content Objectives

• Students will conduct an experiment mimicking a hardness test.
• Students will infer reasons for various levels of hardness among chocolate bars.
• Students will determine the hardness of chocolate bars.
• Students will examine indentations to determine the hardness of chocolate bar and infer reasons for the differences in hardness.

### Process Objectives

• Students will calculate the hardness of various chocolate bars.
• Students will calculate the potential and kinetic energy of an indenter.
• Students will compare the hardness of various chocolate bars.

### Suggested Time

45-50 minutes

### Materials

• Lab - How Hard is your Chocolate? PDF Document

• Teachers Guide PDF Document

• Computer with Internet access
• 4 different Hershey chocolate bars, for example:
• Regular milk chocolate (1.55 oz.)
• Dark chocolate (1.45 oz.)
• Hershey Mr. Goodbar (1.75 oz.)
• Nestle Crunch bar (1.55 oz.)
• Roll of pennies
• Tape
• Metric ruler or tape measure.
• Analytical balance
• Sheet of blank paper

### Part I:

1. Students should view video clip Bend, Twist and Break: Fracture Surfaces QuickTime Video (1 minute 43 seconds).

2. Teacher should lead a discussion on various mechanical experimental designs that would test different physical properties of the chocolate bars.

3. Students should create a hypothesis which ranks the various chocolate bars in order form hardest to softest.

### Part II:

4. Students should complete the laboratory activity.

### Part III:

5. Students should watch video clip Bend, Twist and Break: Beyond the Laboratory QuickTime Video (1 minute 41 seconds)

6. Teacher should lead a discussion around various objects and careers in which the experiment would be useful.

### Part IV: Extension

7. Using the Virtual Microscope (http://virtual.itg.uiuc.edu/), students can view the candy bar samples under an electron microscope.

8. Students can compare results and average the data.

9. Some of the candy bars can be frozen and the same experiment conducted. One major source of error here would be heating of the bar while the experiment in being conducted.

## Educational Standards

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