Watching Crystals Grow

Subject: Chemistry

Objectives:

·  Students will grow crystals

·  Students will learn about crystals and how crystals relate to shape of molecules

·  Students will observe some of the ways crystals grow

National Science Education Standards Grades 9-12:

·  Physical Science: Structure of atoms

·  Physical Science: Structure and properties of matter

·  Science as an Inquiry: Abilities necessary to do scientific inquiry

·  Science as an Inquiry: Understanding about scientific inquiry

·  Science in Personal and Social Perspectives: Historical perspectives

State Standards

·  INQUIRY

Ø  Apply inquiry-based and problem-solving processes and skills to scientific investigations.

o  Clarify research questions and design laboratory investigations. (DOK 3)

o  Demonstrate the use of scientific inquiry and methods to formulate, conduct, and evaluate laboratory investigations (e.g., hypotheses, experimental design, observations, data analyses, interpretations, theory development). (DOK 3)

o  Evaluate procedures, data, and conclusions to critique the scientific validity of research. (DOK 3)

o  Formulate and revise scientific explanations and models using logic and evidence (data analysis). (DOK 3)

o  Collect, analyze, and draw conclusions from data to create a formal presentation using available technology (e.g., computers, calculators, SmartBoard, CBL’s, etc.) (DOK 3)

·  PHYSICAL SCIENCE

Ø  Demonstrate an understanding of the atomic model of matter by explaining atomic structure and chemical bonding.

o  Describe and classify matter based on physical and chemical properties and interactions between molecules or atoms. (DOK 1)

§  Physical properties (e.g., melting points, densities, boiling points) of a variety of substances

§  Substances and mixtures

§  Three states of matter in terms of internal energy, molecular motion, and the phase transitions between them

Ø  Analyze the relationship between microscopic and macroscopic models of matter.

o  Analyze the nature and behavior of gaseous, liquid, and solid substances using the kinetic molecular theory. (DOK 3)

Safety/Disposal:

Students must wear safety goggles. Dispose of materials manner indicated by your instructor. Use caution when operating the hot plates.

Lesson Plan:

Engage:

Have students look at and think about the way different materials fit together. Students may extend this knowledge later and apply it to their reasoning about why crystals form particular shapes.

·  Have each student build a simple object using Legos.

·  Have students individually describe the shape of their structures and compare their structure to that of others.

·  What was similar? (square edged)

·  Why? (because the pieces fit together that way). Now challenge students to make a round object.

·  Can you do it? (No, not easily because the pieces don’t connect to form a circular shape.

Have students pour marbles into a bowl. Watch the pattern they make. You should be able to see a “flower” with six petals and a center. Can you induce the marbles to make a seven petal flower? Why?

Explore:

Explain to students that by growing crystals they will be able to macroscopically observe the internal structure of the matter added to the solution.

In small groups, students will prepare supersaturated solution one of the three substances (borax, Sodium acetate, and alum). Following the directions on the laboratory procedures provided, students will complete the chemical preparation and process to create the situation for crystal growth of that substance. Upon sharing their results, students should recognize the similar patterns and conditions necessary for the growth of these crystals. Laboratory teams will also explain their observation of the crystals structures that develop.

Materials

·  Ten small clean bowls with lids

·  Four pairs of small clean miscellaneous objects (such as rocks, nails, aluminum foil, buttons, or marbles.)

·  heat source (hot plate)

·  measuring spoons and cups

·  Borax, Alum, or sodium acetate (not table salt, but crystals of hydrated magnesium sulfate available at most pharmacies)

·  Eight seed crystals (borax, alum, or sodium acetate)

·  Glass beaker

·  Stirring rod

·  Thermometer

·  Tongs (to move heated water from heat source)

·  Magnifying glass or microscope

·  Flashlight

·  Observation sheets

·  Ruler

Procedure:

1. Using labeling tape, number each bowl and include your group number. Separate the bowls into five pairs.

2. Select one of the three types of seed crystals (sodium acetate, borax, or alum). You will need eight of the same type seed crystals.

3. Using a ruler, measure the size of each seed crystal (in millimeters). Place one seed crystal in each bowl (Leave two bowls empty). Record the size of the crystal for each corresponding bowl.

4. Put different objects (nails, marbles, etc.) into each pair of bowls containing the seed crystals.

5. Add 2 cups of water to a beaker. Heat the water on a hot plate to about 60°C. Don’t let the thermometer touch the bottom of the beaker. Don’t use thermometer as a stirring rod.

6. After water is heated, slowly add the solid material that is the same as your seed crystal. Continually stir the mixture so that the material dissolves, but don't allow it to boil! Continue to add material until material drops out of solution.

NOTE: If alum is selected, measure with a tablespoon not measuring cup.

7. CAREFULLY, divide the mixture among the ten bowls, using tongs or measuring cup. Don't worry about dividing it exactly; only add enough to cover objects. Fill the empty bowls last.

8. Put two drops of food coloring in the center of several of the bowls.

9. Put five of the bowls in a refrigerator and five leave on bench top, where they will not be touched or disturbed.

Explain:

States of matter: Explain the various states of matter and where they are present in this experiment. The solution is first a liquid and then solid crystals form. This is even followed by additional evaporation of the remaining water. Liquids have no strict molecular arrangement and occupy the shape of the container. Gases are even less ordered than liquids and their molecules are free to move in random directions. Solids have tightly packed molecules, and crystal molecules are packed in an orderly, repeating pattern.

Saturation: In order for the experiment to take place, a supersaturated solution must be formed. This occurs when the initial solution is unable to absorb anymore solute. As the supersaturated solution cools, solute will no longer be able to stay in solution and will start forming solid crystals.

Crystal Formation: A crystal is a collection of atoms/molecules structured in a specific repeatable pattern (which can be simple or complicated). Alum, borax, and sodium acetates are all examples of crystals-solids with flat sides and symmetrical shape because their molecules are arranged in a unique, repeating pattern. All crystals of the same type have the same internal structure. That’s why sodium acetates crystals may be large or small but are all needle-shaped and borax crystals are always hexagonal-shaped. How do the sodium acetates crystals grow? Hot water holds more sodium acetates crystals than cold water does. That’s because heated water molecules move farther apart, making room for more of the sodium acetates crystals to dissolve. When no more of the crystals can be dissolved, you have a saturated solution. As this solution cools, the water molecules move closer together again. Now there’s less room for the solution to hold onto as much of the dissolved salts. Crystals begin to form and build on one another as the water lets go of the excess and evaporates. A new crystal grows most easily by attaching itself to a surface of a preceding crystal. A new crystal usually doesn’t attach itself exactly to duplicate the existing crystal. This is why clusters of small crystals are more common than large perfect crystals.

During the Explain stage, students should be able to:

·  Relate the process of self-assembly to crystal growth and to other natural phenomena

·  Explain the procedure for growing crystals

·  Explain the observations of crystal growth

·  Describe the similarities and differences of a variety of crystals, detailing the unique patterns present due to the specific structure of that matter

·  Related observations made during the experimental process to an explanation of phenomena

·  Provide reasonable responses to questions related to the experiment

Elaborate:

The relationship of “building blocks” or self-assembly is applied in other disciplines of science including biology (proteins in DNA and RNA) and geology (stalactites and stalagmites in cave formation).

Evaluate:

Group Comparison of Grown Crystals

1. Examine the crystals grown by each team and share results.

2. Discuss with each team the commonalities and differences that exist between the structures formed. These include:

·  Do the crystals have any similarities in terms of shape and symmetry when observed under the microscope or with a magnifying glass?

·  How many sides (faces) does each of the crystals have?

·  Does shining a flashlight on the surface of the crystals affect their appearance?

·  When did the crystals stop growing? Did some stop growing before others?

·  Did temperature affect the rate of crystal growth or the size of the crystals? Were warm or cold temperatures more favorable?

·  How did the crystals "use" the food coloring? Did the food coloring affect the rate or size of crystal growth?

·  Did crystals grow in all the bowls? If not, what may be some of the reasons why the crystals did not grow?

·  Did crystals appear to grow more easily on rocks or metal? On smooth or rough objects?

Data Chart

Bowl Number / Type of Seed Crystal/Object / Location of bowl / Food Coloring / Appearance at the start of experiment / Appearance after 30 minutes / Appearance after a couple of days
1
2
3
4
5
6

Resources:

Doezema, M. Watching Crystals Grow. Stanford University. 2004

Kim, B., Rogan, K., Nathan, R. Lesson Plan for “Growing Sugar Crystals”. UC Berkley.

Newberry, D1 and Stanaway, J2. Growing Crystals: A Lesson on the Structure of Matter and Self-Assembly. Dakota County Technical College1. Jeannine Lansing Community College2. 2010

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