Objectives
- Observe and record factors that influence crystal growth.
- Identify the structure of crystals.
Materials
- Ten or more shallow bowls (such as petri dishes)
- Five small, clean rocks
- Four small miscellaneous objects (such as nails, aluminum foil, shells, or marbles.)
- Pan for heating, heat source
- Spoon and measuring cup
- Four cups Epsom salts (not table salt, but crystals of hydrated magnesium sulfate available at most pharmacies)
- Two cups water
- Magnifying glass or microscope
- Flashlight
- Observation sheets
Subjects
Procedure
1. Separate the bowls into five pairs. Place small rocks in one of each pair. Let the students choose several small, miscellaneous objects to put into four of the other bowls. Leave one bowl empty. Number each bowl.
2. Heat two cups of water in the pan, slowly adding the four cups of Epsom salts. Continually stir the mixture so that the salts dissolve, but don't allow it to boil!
3. Divide the mixture among the ten bowls. Don't worry about dividing it exactly, but make sure that the mixture completely covers the objects in the bowls.
4. Put two drops of food coloring in the center of several of the bowls.
5. Put five of the bowls in a cool part of the room and five in a warm part, where they will not be touched or disturbed.
Observation
1. Prepare an observation sheet for the students. Make sure that it has enough space for observations and sketches of each bowl. See sample observation chart
2. Students should observe the bowls at the start (immediately after the water is poured into the bowl) and write down what they see. At this point the liquid should be clear and not have any solid particles in it.
3. Have students observe the bowls again after a few hours if possible and write down any changes they see. At this stage the crystals should begin forming, but there will also be a lot of liquid left.
4. Continue the observation for several days until crystal formation has stopped. As the crystals form, remove a few for observation under a magnifying glass or microscope. Ask students to draw a series of pictures of the crystals as they grow, encouraging them to focus on the shape and growth patterns.
5. Use these questions to shape an in-class discussion:
- 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?
6. Compare and contrast: Have the students look through a magnifying glass at commonly occurring crystals such as table salt and refined sugar.
- How are these crystals similar to - and different from - those grown in class?
This exercise will generate a number of questions about crystal growth. Have students compile their observation sheets into a lab notebook and make a list of questions generated from the experiments. Consider inviting a local geologist or amateur rockhound to help answer the students' questions. There may not be an easy answer for every inquiry - the important thing is that the students learn to ask good questions?
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