Crushing Test

Can you identify the unknown crystal by crushing the different crystals and comparing them?

A hardness test is used to identify rock samples, so a similar test may provide some information about the crystals. In this activity, students will try to design a crushing test and discover that identifying and controlling the variables may be difficult. Although the crushing test may not give conclusive results, it is a good opportunity to discuss variables and why they are sometimes difficult to control. This crushing test is also a good example of how a particular test does not always give enough information to answer a question.

Materials needed for each group

• • Salt in cup

  • Epsom salt in cup

  • MSG (Accent®) in cup

  • Sugar in cup

  • (unknown)

  • Black construction paper, ½ piece

  • Plastic teaspoon

Notes about the materials

Be sure students wear properly fitting goggles.

Use the crystals in labeled cups from Activity 2.1: Curious Crystals. These will be reused again in Activity 2.3: Solubility Test.

Use the piece of black construction paper labeled in Activity 2.1: Curious Crystals.

Procedure

• 1. Use masking tape and a pen to make a small label for each of the five crystals on the black paper.

  2. Spread a little of each of the five crystals in their labeled areas on the black paper.

  3. Use your thumb in the bowl of a plastic spoon to press down on each pile of crystals. Rock the spoon back and forth to help crush the crystals.

  4. Listen to the sounds the crystals make as they break. Notice any difference in the way the crystals feel when they break. Compare the residue left behind on the black paper.

  5. Describe the physical characteristics of each crystal and write responses in all sections of part 2 of Physical Properties of Solids Assignment Sheet.

  6. Compare the unknown to the four known crystals by circling the boxes of the four known crystals that are similar to the unknown. Then add up totals for each column and answer the questions at the end of part 2 (base your answer and explanation on the evidence).

From http://www.inquiryinaction.org/