Crystals come in different sizes, shapes, and colors. Some, such as diamonds, are more coveted than others, but they all have their own quality of beauty. By definition, crystals are solids bound by flat surfaces evenly and regularly arranged. Most solids are crystalline, even though their external shapes do not always indicate their orderly internal patterns.
In this project, you will grow crystals that exhibit external structures characteristic of crystals. You will also test the effect of temperature, evaporation rate, and purity of solution on crystal information.
Purpose: To grow sucrose crystals in a gelatin solution.
- 1/2 cup (125 ml) of distilled water
- small saucepan
- 0.25 ounce (7-g) of unflavored gelatin
- 1-1/4 cups (313 ml) of table sugar (sucrose)
- 1-pint (500-ml) glass jar
- Pour the water into the saucepan.
- Sprinkle the gelatin on the surface of the water and let it stand undisturbed for two minutes.
- Stir the liquid continuously over medium heat until the gelatin in completely dissolved.
- Add the sugar slowly while stirring.
- Continue to stir until all of the sugar is dissolved.
- Remove the saucepan from the heat when the liquid starts to boil.
- Allow the solution to cool for five minutes.
- Pour the cooled solution into the jar.
- Place the jar where it can remain undisturbed for two weeks.
- 10. Make daily observations of the contents of the jar.
The liquid gels when it reaches room temperature. After two or three days, tiny, clear, glistening crystals appear suspended throughout the gel (see Figure 25.1) The crystals grow larger each day and form white, feathery, cloudlike clumps throughout the gel.
Solutions contain a solute (what is dissolved) and a solvent (what a solute dissolves in ). More solute will dissolve in a hot solvent than would dissolve in a cool solvent. In this experiment, extra sugar is dissolved in the water by heating the water. As the solution cools, more solute is dissolved in the solvent than would normally dissolve at the cooler temperature. This cooled solution with excess solute is called a super-saturated solution.
As water evaporates from the solution, the solution becomes even more supersaturated. Supersaturated solutions are unstable, and disturbances will cause the microscopic molecules of solute to stick together and form large, visible crystals. The solute is said to "fall out of solution" when the crystals of solute appear in the solution. The first crystals that fall out of solution are too small to be seen, but as more molecules leave the solution, they bind together and form larger and larger crystals. The gel in this experiment allows the crystals to stay suspended; thus, many clusters of crystals can form.
Try New Approaches
- Will the crystals continue to grow? Observe the size of the crystals in the jar over a two-month period. Science Fair Hint: Make weekly observations and draw diagrams of the crystals. Display the diagrams along with photographs.
- Crystalline particles arrange themselves in positions that require the least amount of energy for their formation. Crystals are able to do this if they form slowly. The slower the crystal forms, the larger and more perfect is its shape.
- Do crystals form more slowly if the supersaturated solution cools slowly? Is there a difference in the sugar crystal formation if the solution is cooled at a slower rate by insulating the jar? Repeat the experiment placing the jar inside a large, insulated thermos. Compare the crystals formed in this experiment with those formed in the original experiment.
- Does evaporation rate affect the speed at which crystals are formed? Repeat the original experiment placing the solution in a small-mouthed bottle, such as a soda bottle. The smaller mouth of the bottle slows the evaporation rate of the water from the solution.
- Does the gel alter the shape of the sugar crystals? Repeat the original experiment omitting the unflavored gelatin. Tie a paper clip to the end of a cotton string. Cut the string so that it is just long enough to be attached to a pencil and hung inside the jar with the paper clip resting on the bottom of the jar (see Figure 25.2).
- Repeat this experiment testing the effect that slow cooling has on crystal growth by placing the jar of hot liquid inside a thermos bottle.
- Determine whether the rate of evaporation affects the growth of crystals in this watery solution. Repeat this experiment and, as before, use a soda bottle to reduce the exposed surface area of the liquid, thus reducing the rate of evaporation.