Rock hard: how rocks are classified by their hardness.

THINK: The popular phrase "hard as a rock" isn't always clear cut. Rock hardness varies and is one characteristic that rockhounds use to classify rocks. Can you think of any particularly soft or hard rocks?

PREDICT: The Mohs hardness scale ** ranks 10 minerals from softest to hardest. A mineral on the scale can scratch any mineral softer than itself. Where do you think some common household objects belong on the scale?

** Teachers: Students can refer to the Mohs hardness scale in the activity on T3, in the Teacher's Edition.

You'll Need *

** Pencil and paper

** Chalk

** Copper penny

** Metal bolt

** Rock

** Glass jar or microscope slide

** Stainless-steel spoon

* Be sure to only use materials that you wouldn't mind leaving scratch marks on. Also, feel free to add any other rocks or household items whose hardness you

Procedure:

[] 1. Divide into groups of three or four students.

[] 2. On a separate piece of paper, make a data table with an equal number of rows and columns.' Label the first column "Test Object." Label the rows in the first column with the names of the objects to be tested, starting with the stainless-steel spoon.

[] 3. Label the second column "stainless-steel spoon" and the third "copper penny." Continue labeling the columns with the names of the remaining objects.

[] 4. Using the spoon, try to scratch the surface of the penny. Wipe any residue off the surface of the penny and note if the spoon made any scratches on the penny. If the spoon did scratch the penny, write a "Yes" in the chart. If the spoon did not scratch the penny, write "No." (Note: Be sure that the object actually has scratch marks and not just a streak of residue.)

[] 5. Continue scratching the rest of the objects with the spoon and marking the results beneath the corresponding columns.

[] 6. Repeat steps 4 and 5 with the rest of the test objects, and record your results in the data table.

([dagger]) Teachers: A printable data chart for this activity is available at www.scholastic.com/ superscience.

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Conclusions

1. List your test objects in order from softest to hardest. (Hint: Harder objects are more difficult to scratch.)

2. The Mohs Hardness Scale goes from 1 for the softest minerals to 10 for the hardest. If a penny has a hardness of 3.5 and a glass slide has a hardness of 5.5, where do you think chalk's hardness ties on this scale?

3. If you were going rock hunting and could bring three household items to help test the hardness of the rocks that you find, which three would you bring? Why?

Hands-On: Rock Hard (Student Edition, p.15)

Conclusions: 1. Answers may vary depending on the test objects. For the test objects listed, the order from softest to hardest is: chalk, penny, bolt, spoon, rock, jar/slide, tile. 2. Chalk's hardness would be below a penny's and a glass slide's, so lower than 3.5. 3. Answers will vary.