Question

How can copper(II) sulfate crystallize outside a glass vial?

Answer 1

Preparation of the solution

Take about 25-30 ml of distilled water in a beaker and add powdered crude copper sulphate while stirring the solution so that the powder dissolves.Keep adding the powdered sample till a little of it remains undissolved, even if it is stirred thoroughly.Now add 2-3 ml of dil. H2SO4 to make the solution clear. This prevents the hydrolysis of copper sulphate.

Filtration of the solution

Take a funnel and fix a filter paper in it and clamp it to a stand.Place a china dish under the funnel. The stem of the funnel should touch the wall of the china dish to avoid the solution splashing out.Pour the solution into the funnel over a glass rod and collect the filtrate in a china dish. The insoluble impurities are left in the filter paper as residue.

Concentration of the filtrate to crystallisation point

Heat the china dish on a sand bath till the solution is reduced to about one-third of its original volume.To avoid crust formation on the sides of the dish, stir the solution continuously with a glass rod. If crust is formed, it can be dissolved in the solution by removing it with the glass rod. Don’t allow the solution in the dish to boil.To know if the solution has reached its crystallisation point, use the end of the glass rod to remove a drop of the solution and cool it by blowing on it. The appearance of a crust or tiny crystals on the glass rod shows that the crystallisation point has been reached.Turn off the burner and stop heating.Transfer the hot saturated solution into a crystallising dish.

Cooling the hot saturated solution

 Place the crystallisation dish containing the hot saturated solution in a trough containing water filled to the brim and allow it to cool slowly for sometime. Deep blue crystals of copper sulphate will be formed. The crystallisation will complete after half an hour.

Separation of crystals and drying

Decant the mother liquor carefully into a beaker.Wash the crystals with a little ethyl alcohol containing a small amount of cold water.Place the crystals on a filter paper which will soak up the solution.Transfer the crystals to another filter paper and dry them by pressing gently between the folds of the filter paper or by spreading on a porous plate.Transfer the crystals into a dry test tube and cork it.