Growing a crystal of alum or copper sulfate

Class practical

Here students have the satisfaction of growing their own crystals. This is a wonderful experience.

Apparatus and Materials

• Jars, small, e.g. jam jars, one per student
• Alum or copper sulfate
• Plastic bucket with lid

Health & Safety and Technical Notes

Strong solutions of copper sulfate are HARMFUL.

Read our standard health & safety guidance

Label the jars carefully: otherwise the best crystal always appears to belong to everyone!

The saturated solution

• Several days before, a solution of the salt which is saturated at room temperature must be prepared. This is best done by allowing a supersaturated solution to deposit its excess solid as explained below.
• To prepare a working solution, dissolve the salt in warm water (about 50°C) at a rate of 40 g per 100 ml for potassium alum. Seven litres is sufficient for a class of 32 using jam jars. Pour this solution into the bucket, close the lid, and allow to cool to room temperature. This solution is now supersaturated.
• Seed this solution with a pinch of tiny crystals and leave it in the closed vessel for two to three days, shaking occasionally, to become saturated at room temperature. Pour off the clear saturated solution into another vessel which is closed with a lid.

Seed crystals

• One method of producing the seed crystals is to dip a length of thread into the saturated solution and then to hang it up to dry. Small crystals will appear on it. The whole thread is then hung in the solution. After one or two days, examine the crystals that have developed and break off all but the best. These will then form the seeds for the next stage.
• Another method is to place about 50 ml of the prepared solution in an open beaker and allow it to evaporate overnight. Sort and dry the resulting seeds, retaining those which are perfect in shape and about 3 mm long.
• If you have many students growing crystals, you may need to collect the jam jars from students. Be warned - jars must be very clean as crystals do not grow well in a solution of raspberry jam!
• You may have storage problems. One solution is to use tapered glass tumblers that can be stacked. Wherever they are stored, remember that their temperature must remain even.

Procedure

First method

1. Suspend the thread with its perfect seed crystal from, say, a pencil laid across the top of the jar.
2. Fill the jar with the saturated solution until the seed is completely covered to a depth of at least two centimetres.
3. Cover the jar with a piece of thin cotton cloth (for example muslin or cheesecloth) which is held in place with an elastic band.

Second method

1. Tie the seed to a short length of cotton or thread. This is not an easy process. Students might be advised to prepare a slip knot and insert the crystal, rather than trying to tie a reef or granny knot around the seed.
2. Suspend the thread with its perfect seed crystal from, say, a pencil laid across the top of the jar.
3. Fill the jar with the saturated solution until the seed is completely covered to a depth of at least two centimetres.
4. Cover the jar with a piece of thin cotton cloth (for example muslin or cheesecloth) which is held in place with an elastic band.

In either case, the jar is then left undisturbed and at an even temperature for several days.

Teaching Notes

• Getting a crystal to grow provides a great sense of achievement.
• Label the jars carefully - otherwise everyone claims the best crystals belong to them.
• The most common problem encountered is that the temperature at which the jars are stored does not remain constant, day and night. Increasing temperature increases the solubility and the precious crystals disappear.
• Colourful pictures of crystals can make a good classroom display. Do not forget snowflake crystals.
• This activity could also be done at home. If you choose this option, encourage students to grow crystals of such common substances as sugar, common salt, Epsom salts and washing soda. You could also provide them with plenty of alum to take home to try. Students would be well-advised to keep their jar in the refrigerator. This activity usually gets parents involved - this can be beneficial as long as they do not take over.

This experiment was safety-tested in May 2004