LESSON 4: HOW CAN WE TELL THE HARDNESS OF ROCKS?

AIMS:  1.  What is Moh's Scale of Hardness?

2.  What is the scratch test?

MOTIVATION:  Have a sample of the following rocks on display for the children to touch: The first two rocks (talc and feldspar) were taken from

http://ivyhall.district96.k12.il.us/4th/kkhp/RocksandMinerals/PinkFeldspar.html

and the third picture, quartz, is from http://casdn.neu.edu/~geology/department/staff/colgan/iceland/minerals/quartz.htm

The fourth sample, mica, is from http://cobweb.net/~bug2/rock4.htm

The fifth sample, granite, is from http://ivyhall.district96.k12.il.us/4th/kkhp/RocksandMinerals/rocks.html

### PROCEDURE:

1.  Have students use their fingernails to see if they can make a scratch mark on any of the rock samples.  Have them log on to http://ivyhall.district96.k12.il.us/4th/kkhp/RocksandMinerals/RockQuiz.html and read how they can judge the hardness of the various objects they will use to test their rocks.

### The following Moh's Scale of Hardness was taken from

and the picture samples are from http://ivyhall.district96.k12.il.us/4th/kkhp/RocksandMinerals/RockQuiz.html

## topaz

#### spinel

(available in rock shops)

## diamond

### 2.  Students will perform a scratch test on the five rock samples.  See the following lab activity:

LAB ACTIVITY:

Problem:  How hard is a rock?

Hypothesis:  We think if we use our fingernail, penny, and an iron nail, then we can determine how hard a rock is.

Materials:  penny, nail, crayons, 5 rock samples, fingernail

Procedure:  1.  Label each rock 1, 2, 3, 4, 5.  2.  Try to scratch each rock.  Use your fingernail first, then the penny, and last, the nail.  3.  Record what happened.  Put an X on the chart of the rock was scratched.

Observations:  Use the chart to record your observations.

 Rock Fingernail Penny Nail #1 #2 #3 #4 #5

1.  How many rocks could you scratch with your fingernail?_________ Penny?________ Nail?_________

Conclusion:

1.  Which rock was the hardest?  How do you know? ________________________________

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2.  Which rock is the softest?  How do you know?___________________________________

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