Mechanical Properties of Chocolate- How Hard is your Chocolate? 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 materials ability to resist a change in shape. 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. Today, you will be performing hardness testing on different bars of chocolate.

While there are many types of indenter heads used in measuring hardness, one of the common one is called is simply a hard sphere. We will be able to mimic this indenter tip using a marble.

. You will be testing four types of chocolate bars: Hershey’s Milk chocolate Hershey’s Dark chocolate Nestle’s crunch bar Hershey’s Mr. Goodbar (peanuts) Hypothesis Which do you think will be the softest chocolate? Why? Which do you think will be the hardest chocolate? Why?

Procedure a. Equipment needed:

i. 4 different Hershey’s chocolate bars, for example:

1. Regular milk chocolate 2. Dark chocolate 3. Hershey’s Mr. Goodbar 4. Nestle Crunch bar

ii. A roll of Pennies iii. A marble iv. Ruler or tape measure.

v. Some tape vi. A sheet of paper

vii. 2 – 4 people per group b. Work Steps:

Note: during the actual experiment procedures, be sure record all observations (ie – surface smoothness, any cracking in the chocolate after an indent, etc.)

1. Measure the diameter of your marble. You may wish to take several measurements and average them for added accuracy.

2. Tape a marble to the end of a roll of pennies. This will be your indenter (and

hereby referred to as).

3. Measure the mass of your indenter. If your lab does not have the equipment to weigh these items, calculate the approximate mass of the pennies, using 1 penny = 2.35 grams = 0.00518 lb, and get an approximation of the mass of the marble by comparing its weight to several pennies.

4. Roll up a sheet of paper and tape it in place so it creates a hollow tube that would

allow your indenter to fall through. This will serve to make sure your indenter will fall straight down and not waiver.

5. Unwrap one of your

chocolate bars and set it on its wrapper face down, so that the flatter end sticks up (lettering or grooves facing down). Make observations of your chocolate bar (notice cracks, indents, nuts, etc)

6. Measure the height of your sheet of paper. This height is important because it

will be the height that your indenter falls and will determine the force of your indentation.

7. You are now ready to start the test. Place the sheet of paper on top of the chocolate. Hold the indenter so that the marble just barely hovers over the opening of the paper. DROP YOUR INDENTER. After it has fallen and impacted your chocolate, carefully remove it.

8. Measure the diameter of the indent it left behind. If the indent is not visible, try increasing the mass of the indenter by taping an additional stack of pennies to the indenter, or drop it from a taller height, but make sure to record these modifications. Be sure to write down any relevant observations.

9. You may wish to indent

each chocolate bar more than once. If you do, be sure to space the indents appropriately. You can get more than 10 indents per bar!

10. Make observations of your chocolate bar after it has been indented (are there cracks? Can you see any nuts that were not there before? Etc.)

11. Repeat steps above for each chocolate bar to be tested. 12. Use a larger piece of paper (poster sized paper) to test a fall from a different

height. Measure the height of your sheet of paper. This height is important because it will be the height that your indenter falls and will determine the force of your indentation.

13. Repeat steps 1-4 to determine the force of indentation from your new height.

Questions 1. How did the chocolate bars differ from each other before you dropped the indenter on them? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 2. What are some observations of the candy bar when you dropped the indenter on the chocolate

bar? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 3. What are some factors that influenced the hardness of the candy bar? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 4. Why are different types of chocolate harder than others? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 5. Compare the average diameters of the indents of your chocolate bars for height 1. Rank the

indents from smallest to largest. ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

6. Compare the average diameters of the indents of your chocolate bars for height 2. Rank the indents from smallest to largest.

______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 7. Based on your answers to 5 and 6, you will determine which of the chocolate bars is the

hardest and which is the least hard.

a. Hardest chocolate bar: _________________________________ b. Least hard chocolate bar: _______________________________

c. Explain your answers to 7a and 7b.

______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 8. Why is it important that the indenter be harder than the material it is testing? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 9. Would the size of the indent change if the marble was bigger or smaller? What if you used

more or less pennies? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

10. How did the height of the drop of the indenter affect the size of the dent in the candy bar? Compare the data from the fall from height 1 to the fall from height 2.

______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 11. How would the experiment change if instead of a marble, a sharp object like a needle was

used? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 12. How do you think the hardness would be affected if the chocolate was partially melted, or

completely frozen? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 13. Modern hardness testers don’t actually drop the indent from a height, but instead, slowly and

carefully press the indenter into the material, at either a certain force, or to a certain depth. Why do you suppose they do this?

______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________