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Growing Crystals

based on 50 ratings
Author: Cy Ashley Webb

Grade Level: 5th - 9th; Type: Chemistry

Objective:

The goal of this experiment is to learn about how crystals form.

Research Questions:

  • What is a crystal? What is the difference between a crystal and glass? Do the molecules in glass form a distinct pattern? What about the molecules in a crystal?
  • What conditions are necessary for a molecule to form? Is there anything you can change to make crystals form faster? Does temperature affect how rapidly crystals form? 
  • Once a crystal has formed, what makes the crystal continue to grow? Do crystals grow from the inside out or from the outside in?

Man has long been fascinated by crystals. Recognizing their beauty, we may wear them as ornaments and purchase crystal-growing kits for our children. Some people even believe that certain types of crystals are associated with particular states of mind. However, some of the best crystals are grown without using store-bought kits.

The molecules in crystals are different from the molecules in other substances because they are aligned in a recognizable, repetitive pattern. This is very different from glass because the molecules in solid glass have the same random pattern that they did when the glass was in a liquid state. This regular alignment occurs when crystals emerge from a supersaturated solution. A super-saturated solution is one that contains more molecules of a dissolved crystalline solid that than the liquid can hold. Since you can dissolve more of a substance in hot water than you can in cold water, a supersaturated state is achieved by dissolving a substance in hot water. As the solution cools, it will hold less of the dissolved substance. As it cools, crystals form. They will continue to grow for as long as there liquid is present. The characteristic shape of a particular type of crystal is called its habit.  

Materials:

  • Lab book and pencil (all experiments)
  • Water (all experiments)
  • Tablespoon (all experiments)
  • Small glass bowl (all experiments)
  • Thermometer (all experiments)
  • Measuring cup (all experiments)
  • Charcoal briquettes (experiment #1)
  • Salt (experiment #1)
  • Ammonia (experiment #1)
  • Water (experiment #1)
  • Bluing (experiment #1) - Mrs. Stewart’s is best
  • Food coloring or colored ink (experiment #1)
  • Any of the following: alum, borax, sodium bicarbonate, potassium chromate, potassium dichromate, ferrous sulfate, ammonium chloride (experiment #2, optional)
  • Clean jars with screw-on lids (pint-size)
  • Small saucers or custard dishes (all experiments)
  • Nylon filament fishing line. (experiment #2) 

Experimental Procedure:

EXPERIMENT #1

  1. Place two or three charcoal briquettes in the bottom of the bowl.
  2. Mix the following substances together and stir well: ¼ cup water ¼ cup bluing ¼ cup table salt 1 tablespoon ammonia
  3. If you wish to make colored crystals place several drops of food coloring or colored ink at various locations on the briquettes.
  4. Carefully pour the solution you made in step #2 over the briquettes.
  5. Set the dish aside where it will not be disturbed. Examine the dish twice daily. Record the temperature of the air where the dish is located and what you see when you inspect the dish. Continue making observations twice daily until the liquid has evaporated (usually less than one week).

 EXPERIMENT #2

  1. Boil a gallon of water.
  2. Being careful not to burn yourself, measure 1 cup of the water and put it in a pint-size jar.
  3.  Add level tablespoonfuls of any one of the following chemicals: alum, borax, sodium bicarbonate, potassium chromate, potassium dichromate, ferrous sulfate or ammonium chloride. After each addition, stir until the chemical is dissolved. Keep adding the chemical until no more will dissolve.
  4. Pour roughly ¼ cup of the super-saturated solution that you made in step 3 into a small saucer or custard dish and set aside. Cover the jar containing the solution. You will use this solution again in step 6.
  5. Inspect the saucer or custard dish twice a day for crystals. When you see a perfectly regular shape of a single crystal, remove the crystal with tweezers or a toothpick. Very carefully, tie one end of a nylon fishing line around the crystal. It helps if you make a loose slip knot that you can loop over the crystal and gently tighten it.
  6. Tie the loose end of the fishing line around the middle of a pencil and suspend the crystal into the stock solution that you prepared in step 2. By bathing the individual crystal, it can grow freely. Leave the lid off, but leave the dish in a place where dust will not fall into the jar.
  7. Continue inspecting your crystals in the saucer and in the super saturated solution. Document your observations, including the temperature of the air. 

Terms/Concepts: Crystals; Super-saturation; Polycrystalline mass; Molecular patterns; Crystal habit; Seed crystal; Crystal growth

References:

 

 

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