How to Make Crystals in the Classroom
Crystals form when a solute is mixed with a solvent past the solute's saturation point. When the surface area ratio of solvent molecules to solute molecules, moving past each other in solution, passes equilibrium, the extra solute molecules condense. This process can be demonstrated in a lab or classroom by supersaturating a disaccharide sucrose---a molecule consisting of one glucose and one fructose molecule---and "seeding" the crystals with some sugar on a water-soaked string.
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Things You'll Need
- Bunson burner and stand
- 2 250 ml beakers
- 1 100 ml graduated cylinder
- 100 ml water
- 80 g sucrose sugar (common table sugar)
- Glass stirring rod
- 5 to 6 inches string
- Scissors
- 2 paperclips: 1 large, 1 small
Instructions
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1
Sanitize all equipment.
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2
Weigh out 40 g sucrose, place it in the first 250 ml beaker with 100 ml water and mix thoroughly. Place solution on the stand over the lit Bunsen burner and bring to a boil, stirring often with the stirring rod. Weigh out 40 g sucrose and place that in the second 250 ml beaker.
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3
Tie a paperclip to each end of the string. The large paperclip will serve to suspend the string in the graduated cylinder, and the small one will weight it to the bottom. Tie and cut the string so that the small paperclip barely touches the bottom of the graduated cylinder.
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4
Soak the string with water and coat with a small amount of sugar by dipping it into the second beaker (the amount used is negligible). These will serve to seed the crystals. Set aside.
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5
When the water/sucrose solution begins to boil, slowly introduce the sucrose from the second beaker into the solution, a little at a time, stirring throughout. Continue to boil until the mixture becomes clear.
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6
Using the appropriate safety equipment, transfer the boiling solution into the 100 ml graduated cylinder and quickly drop in the string, suspending the string above the solution by the large paperclip.
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7
Crystals should begin to form almost immediately, though it takes about 20 minutes to achieve ideal results, and five to seven days to fully crystallize.
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Tips & Warnings
A simpler version of this experiment can be performed using household items by simply boiling 1 part water to a little less than 4 parts sugar, divided in halves for steps 2 and 5. Use a shooter glass for the graduated cylinder, a mason jar to boil the mixture in, and a microwave. Combine one shot of water and two shots of sugar in the mason jar, mix and put in the microwave for about 2 minutes (microwave times may vary). Take out and slowly stir in two more shots of sugar. Put back in the microwave for 1 minute 30 seconds. Remove and pour into the tall shooter glass. Suspend string (as detailed above) into the mixture and let sit.
Safety and safety equipment should always be used when working around fire and boiling liquids.
