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?
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2. What are some observations of the candy bar when you dropped the indenter on the chocolate
bar?
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3. What are some factors that influenced the hardness of the candy bar?
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4. Why are different types of chocolate harder than others?
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5. Compare the average diameters of the indents of your chocolate bars for height 1. Rank the
indents from smallest to largest.
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6. Compare the average diameters of the indents of your chocolate bars for height 2. Rank the
indents from smallest to largest.
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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.
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8. Why is it important that the indenter be harder than the material it is testing?
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9. Would the size of the indent change if the marble was bigger or smaller? What if you used
more or less pennies?
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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.
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11. How would the experiment change if instead of a marble, a sharp object like a needle was
used?
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12. How do you think the hardness would be affected if the chocolate was partially melted, or
completely frozen?
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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?
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