Crystal Structure: Will varying shape crystals form from varying substances?
Crystals come in many shapes and sizes. The substance used to make a crystal and how this substance bonds together dictates the crystal's unit cell and, thus, its shape.
In this experiment, you will compare the unique crystal formations that grow from four different substances. The four crystal substances you will use are alum, Epsom salt, sugar, and salt. You will create supersaturated solutions out of the four substances and examine the crystals that form.
What Are the Variables
Variables are anything that might affect the results of an experiment. Here are the main variables in this experiment:
The substances that make up the crystal
- the temperature of the beginning solution
- the temperature of the water
- the environment the crystal is grown in
In other words, the variables in this experiment are everything that might affect the growth of the crystals. If you change more than one variable at the same time, you will not be able to tell which variable had the most effect on the crystal's structure.
Level of Difficulty
Moderate
Materials Needed
- alum (small jar, found in the spice section of the grocery store)
- Epsom salt
- sugar
- salt
- water
- black saucers (or any color saucers, black construction paper, and scissors)
- hot plate or stove
- saucepan
- four stirring spoons
- measuring cup
- measuring spoons
- glass cup or jars
- magnifying glass (optional)
- masking tape
- marking pen
Approximate Budget
- $5 (most materials are common household items).
Timetable
45 minutes initial time; 30 minutes over the next week.
How to Experiment Safely
This experiment requires using very hot water to make a supersaturated solution. Ask an adult to help you when using the stove or hot plate. Do not put anything in your mouth, such as a sugar crystal, before checking with an adult.
Step-by-Step Instructions
- If you do not have black saucers, cut the black construction paper to fit tightly in the bottom of each saucer and place inside.
- Make a supersaturated solution with the Epsom salt by bringing half a cup of water to the almost-boiling point, then transferring the hot water to a glass. Add 5 tablespoons Epsom salt and stir. Keep adding Epsom salt until no more salt can be absorbed by the water. You will know this when the salt begins to fall to the bottom no matter how hard you stir.
- Pour the solution into a saucer and label the saucer accordingly on masking tape.
- Repeat this process with each of the other substances. Make sure to rinse the pot and use a clean spoon. For the alum, begin with 3 tablespoons; for the salt begin with 1 tablespoon, and for the sugar begin with 4 tablespoons. The sugar solution should be thick.
- Set the saucers in a quiet place and observe them over the next week until all the liquid evaporates. When all the liquid is gone you should see crystals coating the sides and bottoms of the saucers.
- Examine the crystals with the magnifying glass.
Summary of Results
Draw the results of each of the crystals and write a written description. Was your hypothesis correct? How does the Epsom salt differ from the salt? How does the salt differ from the sugar? Compare the crystal formations with the physical shape of the substance they were made from. Can you identify to which of the seven basic crystal structures the four crystals belong?
Troubleshooter's Guide
Problem: No crystals grew in one or more of the solutions.
Possible Causes:
- The solution may not have been saturated when the water was hot. You may not have stirred enough to dissolve the solids. Pour the solution back in the saucepan. Reheat the solution, adding more of the substance and stirring well after each addition until you see bits of the substance fall to the bottom.
- The water may not have been hot enough. It should not be at the boiling point but it does need to be very hot. Pour the solution back in the saucepan. Reheat the solution, adding more of the substance and stirring well after each addition until it is saturated.