1. To prepare crystals of pure copper sulfate (CuSO4.5H­2O) from a given impure sample of the blue vitriol

Aim:- To prepare crystals of pure copper sulfate (CuSO₄.5H­₂O) from a given impure sample of the blue vitriol.

Theory:- The given sample is shaken with water. A few drops of dilute sulfuric acid are added to it in order to prevent hydrolysis of copper sulfate. Copper sulfate present in the sample gets dissolved while the insoluble impurities are left behind. The solution is filtered. The filtrate is concentrated to the crystallization point and then cooled. On cooling, crystals of copper sulfate (CuSO₄.5H₂O) separate out.

Requirements:- Crude sample of copper sulfate, a 400 ml beaker, a china dish, a funnel, an evaporating dish and a policeman(glass rod).

Procedure:-

Stirring with a glass rod

1. Preparation of solution: Take about 25-30 ml of water and add to it small quantity of the powdered crude sulfate. Stir well to dissolve it. Make several additions of the powdered sample till a little of it remains undissolved even if it is stirred for sometimes. Now add 2-3 ml of dilute sulfuric acid to make the solution clean. This prevents hydrolysis of the copper sulfate.

2. Filtration of the solution and concentration of the filtrate to crystallization: Filter the solution and collect the filtrate in a china dish. The insoluble impurities are left as residue on the filter paper.                                                                  

Heating of solution

Heat the china dish on a sand bath till the solution is reduced to about one-third of the original volume. As the solution gets heated up, it is stirred well with a glass rod to avoid crust formation on the side of the dish. If the crust is formed, it is dissolved into the solution by remaining it with glass rod. Do not allow the solution in the dish to boil. Remove a drop of the solution at the end of a glass rod and cool it by blowing. The appearance of a crust or tiny crystals on the glass rod shows that the crystallization point has reached. Now turn off the burner and stop heating. Transfer the hot saturated solution in a crystallization dish.

3. Cooling the hot saturated solution: Place the crystallization dish

Cooling the solution

containing hot saturated solution on a beaker containing water filled to the brim and allow it to cool slowly for sometime. Deep blue crystals of copper sulfate will appear. After about half an hour, the crystallization is complete.

4. Separation of crystals and drying: Decant off the mother liquor carefully. Wash the crystals with a little ethyl alcohol containing small amount of cold water. Remove the crystals on a filter paper which soaks the solution. Transfer the crystals on another filter paper and dry them by pressing gently between the folds of the filter papers or by spreading on a porous plate. Transfer the crystals to a dry test tube and cork it.

Removing insoluble impurities by filtration

The crystals of pure copper sulfate (CuSO₄.5H₂O) are triclinic, transparent and blue.

Precautions:-

1. The filtrate should be evaporated slowly by gently heating during concentration.
2. The filtrate is to be evaporated only up to the crystallization point. It should never be heated to dryness. Avoid over heating of the solution.
3. The solution should be cooled slowly without disturbing it. It should never be cooled rapidly.