Part 1: Appearance Test - Curious Crystals

Can you identify an unknown crystal by comparing its appearance to other known crystals?

In this activity, students will carefully look at four known household crystals. After observing and describing the crystals, students will be given an unknown crystal, which is chemically the same as one of the four known crystals but looks different. The other activities in this investigation are examples of tests students can conduct on the crystals. After a series of these tests, students will gather enough evidence to identify the unknown crystal.

Materials needed for each group

• • Salt

  • Epsom salt

  • MSG (Accent®)

  • Sugar in cup

  • (unknown)

  • 5 Small cups

  • Black construction paper, ½ piece

  • Magnifier

  • Masking Tape

  • Pen

  • Plastic Spoon

Notes about the materials

Be sure you and the students wear properly fitting goggles.

Procedure

• 1. Use masking tape and a pen to label four corners of a piece of black construction paper: sugar, salt, Epsom salt, and MSG. Label the center unknown.

  2. Place small samples of Epsom salt, table salt, sugar, and MSG on the labeled areas of the construction paper.

  3. Use a magnifier to look carefully at each type of crystal. Describe the physical characteristics of each crystal and write responses in all sections of part 1 of Physical Properties of Solids Assignment Sheet.

  4. Get the “unknown crystal” and follow the previous step. Note: this unknown is chemically the same as one of the other crystals you just looked at.

  5. 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 1 (base your answer and explanation on the evidence).

Have students discuss their observations.

Part 2: 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).

Part 3: Solubility test

Question to investigate: Can you identify the unknown crystal by the amount that dissolves in water?

• 1. Have students weigh equal amounts of the crystals.

     1. The amounts of crystal and water used in this solubility test are specific and should be used because they give clear results. There are a variety of methods students could use to weigh equal amounts of each crystal. They could construct a balance themselves, or they could use any scale that can weigh 0.5 grams.

     2. Procedure

     3. If the cups are not already labeled, use your masking tape and pencil to label five small cups salt, sugar, epsom salt, MSG, and unknown. Also label five larger clear plastic cups in the same way. You should have two labeled cups (one large, one small) for each type of crystal.

     4. Using a balance, get 0.5 grams of salt into the newly-labeled small cup for salt.

     5. Weigh the other four crystals in the same way so that you have equal amounts of all five crystals in their small labeled cups.

        1. Discuss the variables that need to be controlled in the solubility test.

           1. How might we mix the crystals into water to compare how they dissolve?

• • • • • How many cups do we need?

        • Should the cups all have the same amount of water?

        • What else about the water should be the same?

        • What is a good way to mix the crystals into water in each cup?

• 1. 5. 2. Students dissolve the crystals in water.

• • • • 1. The following procedure is also listed in Activity sheet 2.3—Solubility test. The amount of water used in the procedure is specific and should be used because it gives clear results. Swirling the crystals in water is a good way of mixing them to help them dissolve. The class works together at the same step so that all groups pour their crystal samples into the water at the same time. Teacher tells students when to swirl the water and crystals and when to stop and observe. There will be three 20-second intervals.

        2. Procedure

        3. Carefully fill a large clear plastic cup with hot tap water and then place 1 teaspoon of hot tap water into each labeled empty clear plastic cup.

        4. Match up each pair of cups so that each cup of crystal is near its corresponding cup of water. With the help of your lab partners, listen for your teacher’s instructions, and pour the weighed amount of each crystal into its cup of water at the same time.

        5. With the help of your lab partners, swirl each cup at the same time and in the same way for about 20 seconds and observe. Swirl again for 20 seconds and observe and then for 20 more seconds and make your final observations.

        6. Slowly and carefully pour the solution from each clear plastic cup back into its small empty cup. Try not to let any undissolved crystal go into the small cup. Compare the amount of crystal remaining in each clear plastic cup.

        7. Students should use their observations during the solubility test to help them answer the questions about the possible identity of the unknown in part 3 of the Physical Properties of Solids Assignment Sheet.

Part 4: Recrystallization Test: 24-Hour method

• 1. Students will reuse the large clear plastic cups and solutions from Activity 2.3 according to the procedure below.

     1. Procedure

     2. Dip a cotton swab into one of the solutions. Apply the solution in a circular motion to its labeled area on the activity sheet. Repeat until as much of the circle is covered with the solution as possible.

     3. Using clean cotton swabs, repeat Step 2 for the other four solutions. Set the paper aside and check it in about an hour. If not much crystal has formed; check it again in another hour.

     4. Allow the solutions to sit overnight.

  2. The next day, have students observe the crystals and fill in part 4 of the Physical Properties of Solids Assignment Sheet

From http://www.inquiryinaction.org/