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## Mechanical Properties of Chocolate: How Hard is your 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 be able to calculate, measure and identify the hardness of various chocolate bars.
• Students will apply previous knowledge of velocity and energy to find the hardness of candy bars.

### Process Objectives

• Students will be able to determine the hardness of various substances using quantitative data.
• Students will make observations of the hardness of chocolate while dropping the indenter on various chocolate bars.
• Students will be able to determine the amount of hardness of a candy bar by first determining the potential and kinetic energy and the velocity of the indenter upon impact of the candy bar.
• Students will compare and contrast the hardness of chocolate bars.

### Suggested Time

60-90 minutes

### Part I: Hardness of Materials

1. Introduce this lesson by asking the students to share their ideas about hardness of substances.

• Show the video clip Bend, Twist and Break: The Bridge QuickTime Video (1 minute 48 seconds)
• Discuss the difference between graphite (in pencils that they use in class) and diamond (the hardest mineral on Earth) to introduce materials.
• Ask the students to discuss differences between materials around them.
• What are some ways that they have determined the hardness of substances?

2. Talk to the students about some ways they are familiar with the concept that some materials are harder than others. For example, everyday when you chew your food your teeth don?t break because your teeth are harder than the foods you chew. What are some of the foods that you eat? What would happen if you tried to eat food harder than your teeth?

3. Review hardness and the Mohs Hardness Scale.

4. In 1812, Friedrich Mohs came up with a way of ranking materials on a comparative scale ? he simply took 2 different materials and observed which one got scratched when they were rubbed together. Since then, a more quantitative measure of hardness has been developed. 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.

5. Introduce lab.

6. Show the video clip Bend, Twist and Break: Breaking Glass QuickTime Video (1 minute 58 seconds) before students make hypothesis.

7. Complete lab.

8. Complete conclusion questions.

9. Discuss lab and conclusion questions as a class.

### Part II: Video

10. Use the video clips Bend, Twist and Break: The Bridge QuickTime Video (1 minute 48 seconds) and Bend, Twist and Break: Breaking Glass QuickTime Video (1 minute 58 seconds) during this lesson.

11. The video clip Bend, Twist and Break: The Bridge QuickTime Video will go with the introduction discussion. Bend, Twist and Break: Breaking Glass QuickTime Video will be viewed before students make their hypothesis.

### Part III: Other Examples

12. Teacher-led discussion about how some materials are harder than another.

### Part IV: Extension

14. Try changing the height of the drop, the weight of the indenter, or the shape of the indenter (different size marbles, or use pencils) to see effects discussed in question 6.

15. Try the experiments with different materials. Any material which can deform under the weight of your thumb is appropriate for this lab. Some easily available materials to test would be wax (i.e. candles), silly putty, clay etc.

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