LABORATORY ACTIVITY: STUDENT VERSION LABORATORY ACTIVITY: TEACHER NOTES
WB01542_2.gif (729 bytes)Activity 1: Making Crystals WB01542_2.gif (729 bytes)Activity 1: Making Crystals
WB01542_2.gif (729 bytes)Activity 2: Identification of Minerals by Borax Bead Tests          and Flame Tests WB01542_2.gif (729 bytes)Activity 2: Identification of Minerals by Borax Bead Tests               and Flame Tests

 

 

 

 

 

 

 

 

 

 

LABORATORY ACTIVITY: STUDENT VERSION

Activity 1: Making Crystals

Introduction

Atoms, molecules, and ions can fit together in a regular pattern to form a crystal.   There are several types of crystals and crystalline structures. One of the ways to grow crystals is to allow water to evaporate so that the ions or molecules can come together to form crystals.

Purpose

To investigate sugar and salt crystals. To produce salt crystals by allowing sodium ions and chloride ions to come together in solution. To produce sucrose crystals (sometimes called rock candy) by allowing sugar molecules to come together in solution.

Safety

Wear protective goggles throughout the laboratory activity.

Procedure

Part I. Preparing Salt Crystals

  1. Fill a 250-mL beaker about three-fourths full of water.
  2. Add salt (about 20 g) to the water until no more will dissolve.
  3. Heat the solution (to about 90 °C), but do not boil it.
  4. While the solution is hot, add salt (about 15 g) until no more will dissolve. Remove the beaker from the hot plate. Be sure that no undissolved salt remains in the bottom of the beaker.
  5. Suspend a string in the solution by tying the string onto a pencil, soda straw or wood splint and suspending it across the top of the beaker.
  6. Cover the beaker with a watch glass but do not seal it. Put the beaker where it will not be disturbed for a few days. Make observations daily. Record your observations.

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Part II. Preparing Sugar Crystals

  1. Boil 150mL water in another 250-mL beaker.
  2. Add as much sugar (about 110 g) as will dissolve in the hot water.
  3. Repeat Steps 5 through 7 from Part A.
  4. Compare both types of crystals. Look at them with a microscope if available.
  5. Thoroughly wash your hands before leaving the laboratory.

Data Analysis and Concept Development

  1. Describe the results of this activity. What did the crystals look like? Draw diagrams to show the crystals.
  2. What are the characteristics of each type crystal?
  3. What purpose does the string serve?

Implications and Applications

  1. Imagine how the particles of each substance must be arranged to produce the crystals you described. Draw a "particle nature of matter" picture to describe what you imagine.
  2. If sugar crystals are made of sugar molecules, and salt crystals are made up of sodium ions and chloride ions, how would the particle pictures differ?
  3. Explain how the process of crystallization might have been involved in forming deposits of minerals.

LABORATORY ACTIVITY: TEACHER NOTES

Activity 1: Making Crystals

Major Chemical Concept

Substances crystallize in predictable, regular patterns.

Level

General or Basic

Expected Student Background

None.

Time

45 min the first day and 5 min on successive days (for observations).

Safety

Students should be reminded not to taste anything in the laboratory, even things that can ordinarily be ingested.

Materials (For 24 students working in pairs)

Nonconsumables

Consumables

Advance Preparation

No advance preparation is needed. NOTE: A heavy-gauge pre-soaked string works best since it will tend to sink.

Pre-Laboratory Discussion

This activity might be preceded by showing shapes of some naturally occurring crystals and speculating about what might happen when solids are crystallized by evaporation. A discussion of the changes in concentration of a solute that occur when the water evaporates will help students understand what happens in the activity.  The terms "dissolve" and "saturated" should be reviewed.

Teacher-Student Interaction

This activity is easy to perform and little interaction is needed. If necessary, students can be reminded that if solid remains in the beaker after heating, the solution is saturated, and no more solute should be added.

Anticipated Student Results

Both sugar and salt crystallize in the cubic form. Students may observe several smallcrystals or fewer larger ones.

Answers to Data Analysis and Concept Development

  1. Small cubes.
  2. The particle at the lattice points for sugar is a molecule of sugar; in the sodium chloride crystal the particles are sodium ions and chloride ions.
  3. To suspend the crystal so that it has a greater opportunity to serve as a growth site, and to minimize crystal distortion by resting in the bottom of the beaker.

Answers to Implications and Applications

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3. As water evaporates, the solution becomes saturated and crystals begin to form.

Post-Laboratory Activities

After the crystals begin to grow, a discussion of equilibrium would be appropriate if this topic was previously introduced. The six crystal systems (Figure 4) and examples from the mineral world may be introduced.

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Extensions

  1. Students may wish to continue crystal growing with other types of compounds, e.g., copper sulfate, alum, and Epsom salts. If students wish to keep the crystals they make, cover them with clear nail polish to prevent reaction with the atmosphere.
  2. Have students compare the basic crystal systems and show how they differ from one another.

LABORATORY ACTIVITY: STUDENT VERSION

Activity 2: Identification of Minerals by Borax Bead Tests and Flame Tests

Introduction

Two simple tests that can be used to identify the metal ion in minerals are flame tests and borax bead tests. Both use only simple equipment and require a short amount of time. Borax beads can be made in different colors depending on whether a reducing or an oxidizing flame is used.

Purpose

To identify metal ions through flame tests and borax bead tests.

Safety

Wear protective goggles throughout the laboratory activity.

Procedure

Prepare a data table to record your results.

Part 1. Flame Tests

  1. Place pea-sized amounts of the salts KCl, NaCl, LiCl, and CuCl 2 in separate watch glasses.
  2. Adjust the burner so that a distinct blue cone is present.
  3. Clean a platinum wire by dipping it into 6MHCl and then holding it in the flame until no color is produced.
  4. Dip the platinum wire in the KCl, and hold it in the flame. Record the color.
  5. Clean the wire as before, and repeat the tests with the other salts.

Part II. Borax Bead Tests

  1. Twist the end of the platinum wire to form a loop as shown.

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  1. Place about 1 gram of CuCl 2, CoCl 2, Fe(NO 3 ) 3 MnO 2 , and borax in separate watch glasses.
  2. Heat wire first in flame and then dip the loop of the wire in the borax, picking up a small amount. Heat it until a glassy bead is formed.
  3. Dip the bead in the CuCl 2, and return to the flame’s outer core (oxidizing portion).  This portion of the flame is inside the luminous part of the flame about a centimeter above the inner blue cone.  Record the color, and repeat the process in the reducing portion of the flame. This region is at the tip of the inner blue cone.

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  1. Crush the bead from the platinum wire and clean the wire with 6MHCl as described above.
  2. Bring a magnet close to the bead. Observe for attraction.
  3. Repeat the bead test with the remaining solids.
  4. Obtain an unknown from your teacher. Perform flame tests and bead tests.
  5. Thoroughly wash your hands before leaving the laboratory.

Data Analysis and Concept Development

Use your data table to determine the identity of your unknown.

Implications and Applications

Can a mineral be identified by flame and bead tests alone? Why or why not?

LABORATORY ACTIVITY: TEACHER NOTES

Activity 2: Identification of Minerals by Borax Bead Tests and Flame Tests

Major Chemical Concept

Metal ions can be identified by flame and borax bead tests.

Expected Student Background

Students should be able to:

  1. use a burner safely.
  2. make a data table.
  3. interpret data in a table.

Time

45 minutes

Safety

Remind students about hazards of operating burners and using 6MHCl.

Materials (For 24 students working in pairs)

Nonconsumables

Consumables

Advance Preparation

  1. Prepare 6 M hydrochloric acid. Be sure to add acid to water.
  2. As the wire deteriorates with use, cut off a portion to get a fresh sample.
  3. Prepare unknowns from samples used.

Pre-Laboratory Discussion

Remind students of safety hazards. Demonstrate the oxidizing and reducing portions of the flame.

Teacher-Student Interaction

Students sometimes need to be reminded to make observations as quickly as possible after the wire has been put into the flame. When the wire becomes red hot, the flame will appear orange regardless of the substance being tested.

Anticipated Student Results

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Answers to Data Analysis

Will depend on identity of student’s unknown.

Answers to Implications and Applications

These tests identify only the metal ion. Other tests such as density, color, cleavage, and scratch test will give further information to help in identification (see MECC computer simulation in Media).

Post-Laboratory Discussion

Relate these tests to similar tests that might be made on minerals.

Extension

If time permits, obtain mineral samples from the geology or earth science teacher and allow students to perform the tests with them. Check any minerals for possible hazards.

Assessing Laboratory Learning

The extension may be used for assessment. If you do not wish to use actual minerals, prepare unknowns from the salts used in the activity.