The Production of Salt Crystals

Powdery salts (e.g. sodium chloride = table salt) consist of tiny crystals that dissolve in water. Thereby, the ionic lattice, which is responsible for the regular structure of the tiny crystals, is destroyed.

The higher the water temperature is, the more salt can be dissolved. When the water cannot take in any more salt, this solution is called a saturated solution.

When the solution is cooled down, it gives off the surplus salt: Salt crystals form again, which are usually considerably bigger than the original tiny crystals of the powder, though.

Recipes:

1) 100 g copper sulphate (CuSO₄) and 200 ml distilled water

2) 100 g potassium-aluminium alum (KAl(SO₄)₂ .12 H₂O) and 500 ml distilled water

3) 100g potassium-chromium alum (KCr(SO₄)₂ .12 H₂O) and 500 ml distilled water

The salt was mixed with the distilled water and heated on a heating plate. The temperature was monitored with a thermometer, and the solution had to be stirred from time to time with a glass rod. We also used a magnetic stirrer: a stirring rod - that is a piece of iron covered with a white plastic coat looking like a tic-tac - is put into the beaker with the solution. The beaker is placed on a device with a rotating magnet inside, which induces the stirring rod in the liquid to rotate as well and thus cause a whirl.

The solution of copper sulphate was dark blue, the potassium-aluminium alum solution was white and the potassium-chromium alum solution was almost black.

When the solution had reached a temperature of 50 to 60°C, it was poured into a funnel coated with filter paper. This should prevent non-dissolved crystals from remaining in the solution. The liquid was distributed into several glasses, covered with aluminium foil and placed into the fridge or onto the experiment table for cooling.

When we had a look two days later, blue crystals had formed in the copper sulphate solution and white crystals in the potassium-aluminium alum solution.

Unfortunately, in the potassium-chromium alum solution no crystals had formed at all!