Handbook of Nature-Study · Lesson CCXI · Topic 33: Crystals, minerals and rocks
211. Crystal growth
Goal: Your child grows crystals on a string and sees that each substance makes crystals of its own shape.
You'll need
- Alum (from the spice aisle), table salt and sugar
- Three clean glass jars
- Cotton string, pencils or craft sticks, small washers or paper clips for weights
- Hot water from the kettle (an adult pours)
- A hand lens (magnifying glass)
- A nature notebook and a pencil (colored pencils if you have them)
What's in the book
The lesson has a leading thought, a method with three solutions (blue vitriol, potassium bichromate and alum) grown on strings in vials on the window sill, and six observations on the colors and shapes of the crystals, ending with rock candy.
The lesson, step by step
Watch only: the book's chemicals
The book uses blue vitriol (copper sulfate) and potassium bichromate. Both are poisonous, and potassium bichromate can burn the skin and cause cancer. Don't try these at home. The book itself warns:
Care should be taken to prevent the children from trying to eat these beautiful crystals, by telling them that the red and blue crystals are poisonous.
If you want to see copper sulfate's deep blue crystals, look up a video of a teacher growing them. We use three safe kitchen substances instead.
The leading thought
Different substances when dissolved in water will re-form as crystals; each substance forms crystals of its own peculiar color and shape.
Make the solutions
An adult pours very hot water into each jar (hot water dissolves more). Stir in alum in one, salt in another and sugar in the third, a spoonful at a time, until no more will dissolve. Label the jars. Hot water can scald: children stir only after the adult has poured.
Hang the strings
Suspend from the mouth of each test tube or vial, a piece of white twine, the upper end tied to a tooth pick, which is placed across the mouth of the vial; the other end should reach the bottom of the vial.
Use a pencil across the jar, and a paper clip on the bottom of the string. Set the jars where they won't be bumped.
Watch them grow
Check each day for a week.
In which bottle did the crystals form first? Which string is the heaviest with the crystals?
Do you think that vitriol and potassium bichromate and alum will, under favorable circumstances, always form each its own shape of crystal wherever it occurs in the world?
Rock candy
What is rock candy? Do you think you could make a string of rock candy if you dissolved sugar in water and placed a string in it?
Your sugar jar is the answer. (Sugar crystals are slow; give them a week or two.)
Nature notebook
Draw one crystal from each jar as it looks under the lens. Label: alum (eight-sided, like two pyramids base to base), salt (cubes), sugar (long slanted blocks).
Afterwards
Check the jars daily. Pull the strings out after one to two weeks and dry the crystals on a paper towel. Keep the alum crystals out of reach of little children; they are not for eating.
Answers to the book's problems
Each substance always grows crystals of its own shape: alum makes eight-sided crystals, salt makes cubes, sugar makes slanted blocks. Crystals can also form without water, for example when melted rock cools slowly, which is how the crystals in granite formed.
Worked out for this site from the scan. If a number in your copy differs, trust the book.