Crystal Investigation

by Ian McMahon

The substance, alum, is almost the principal factor for achieving some degree of success in growing good crystals.

The percentage purity of the alum needs to be high so that contaminant particles do not disrupt the proper crystal formation. The source of alum therefore must be considered before anything else.

Chemical supply companies such as Omega Scientific, Ace Chemical Co and Aim Scientific all have high purity alum available to purchase. The minimum quantity is 500g and the cost makes it difficult for smaller schools to afford if involved for only one year.

If your school has connections with a neighbourhood secondary school, they may be able to supply some to you.

For students who wish to grow crystals at home, obtaining good alum is the issue – hopefully your school will support you. For home-schooled students, it may be best to contact one of the chemical suppliers direct to see what they can do for you. They know of this competition and may be able to supply a smaller quantity thus less cost to you.

Amounts of alum that are can be purchased from hardware stores and garden centres are less pure sometimes tainted with insoluble particles as well as other soluble compounds. Some work will need to be done to have this alum ready for crystal growing.      

This will involve dissolving a small quantity of alum in hot water e.g 1 teaspoon per half a glass of hot water and keep adding until some alum remains undissolved. If you have measuring devices, then 15g per 25 mL of hot water is a fair ratio

Here is the second factor in crystal growing – the quality of water. Tap water is no substitute for distilled water or deionized water. Outside of schools, distilled water can be purchased from supermarkets.

It is important that students use the quantities given above. Using too much solid will prevent all the alum from fully dissolving and a powdered, rather than a crystalline product will form. Using too much water means that on cooling, no crystals will appear, since the solution is still unsaturated.

Students should understand that this method works because alum dissolves in water whilst most of the foreign matter is insoluble. They should also realise that the alum eventually crystallises out since it is far less soluble in cold water than in hot.

One method to improve the purity of the alum can be noted in the diagram below. If the source of alum is of lesser quality, then filtering a hot alum solution will provide the saturated solution left behind in the evaporating dish. This can allow students to ‘grow’ crystals from the saturated solution left behind in the evaporating dish .

Seed crystals can be obtained from the cooled saturated solution.

Traditionally, this is carried out by suspending crystals from a length of cotton, but equally good results are obtained by choosing a well-formed crystal and placing it in some of the filtered saturated solution (bottom grown). Dust-free conditions are essential.

The third principal factor is temperature control (to be covered in the next blog edition)

The following diagram is provided courtesy of the Royal Society of Chemistry