Purification of Impure Samples by Crystallization

Crystallisation of Copper sulphate

Materials Required

The Procedure

Real lab procedure

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.  

Crystallisation of Potash Alum

Materials required

The Procedure

Real lab procedure

Preparation of solution

  • Take about 25-30 ml of distilled water in a beaker. Add about 5-6 g of crude sample of potash alum to it and stir the contents of the beaker with a glass rod until the solution is clear.
  • Warm to dissolve all of the alum present in the sample.  

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

  • 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 a crust is formed, dissolve it back into 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 beaker containing water filled to the brim and allow it to cool slowly for some time.
  • Colourless transparent crystals of potash alum begin to separate.
  • After about half an hour, the crystallisation is complete. 

5. Separation of crystals and drying 

  • Decant off the mother liquor carefully into a beaker.
  • Wash the crystals with cold solution of alcohol and 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.
  • Transfer the crystals into a dry test tube and cork it. 

Crystallisation of Benzoic Acid

Materials required

The Procedure

Real lab procedure

Preparation of solution 

  • Take about 150 ml of distilled water in the beaker and boil the water over a Bunsen burner by placing the beaker over a piece of wire gauze placed over a tripod stand.
  • In another beaker, take 2-3 g of crude sample of benzoic acid.  While constantly stirring, gradually add a minimum quantity of boiling water just sufficient to dissolve the benzoic acid. If required, heating can be used to dissolve the benzoic acid. 

Filtration of the solution 

  • Take a funnel and fix a filter paper in it and clamp it to a stand.
  • Fix a fluted filter paper into the funnel and filter the hot solution into a beaker.
  • Insoluble impurities will be left in the filter paper. 

Cooling the hot saturated solution 

  • Let the filtered solution come to room temperature by itself.
  • Now cool it by placing it in a cold water trough.
  • Opaque white crystals of benzoic acid begin to separate. 

Separation of crystals and drying 

  • Fix a filter paper in a funnel that is clamped to a stand and separate the crystals by filtration.
  • Wash the crystals with cold water.
  • Transfer the crystals to another filter paper and dry them by pressing gently between the folds of the filter paper.
  • Transfer the crystals into a dry test tube and cork it. 

Simulator Procedure (as performed through the Online Labs)

You can select any one of the sample to perform the experiment by clicking on the corresponding sample icon. 

Copper Sulphate

  • Drag the spatula containing copper sulphate towards the beaker containing distilled water and drop it in.
  • Click on the glass rod to stir the solution.
  • Drag the measuring cylinder containing conc. H2SO4 towards the beaker to pour it into the solution.
  • Click on the ‘Next’ button.
  • Drag the beaker towards the funnel to filter the copper sulphate solution.
  • Click on the inference icon to see the inference.
  • We can now click on the ‘Next’ button to go to the next process.
  • Click on the knob of the burner to turn it on.
  • To stir the solution in the china dish, click on the glass rod.
  • Click on the inference icon to see the inference.
  • Drag the china dish towards the crystallisation dish to transfer the hot saturated solution of copper sulphate into it.
  • To cool the hot solution, drag the crystallisation dish and place it in the trough containing water.
  • Wait for some time for the copper sulphate to recrystallise from the solution.
  • Drag the crystallisation dish out of the water trough to place it back.
  • Click on the inference icon to see the inference.
  • We can click on the ‘Next’ button to go to the next process.
  • Drag the bottle containing ethyl alcohol and water mixture towards the crystallisation dish to wash the crystals.
  • Drag the crystallisation dish towards the trough to decant the upper solution.
  • Click on the ‘Next’ button to go to the next process.
  • Click on the inference icon to see the inference.
  • To transfer the crystals into the test tube, drag the filter paper containing the crystals toward it.
  • Drag the cork towards the test tube and close it.
  • To redo the experiment, click on the ‘Reset’ button.

Potash alum

  • Drag the spatula containing potash alum towards the beaker containing distilled water and drop it in.
  • Click on the glass rod to stir the solution.
  • To go to the next process, click on the ‘Next’ button.
  • Drag the beaker towards the funnel to filter the potash alum solution.
  • Click on the inference icon to see the inference.
  • To go to the next process, click on the ‘Next’ button.
  • Click on the knob of the burner to turn it on.
  • To stir the solution in the china dish, click on the glass rod.
  • Click on the inference icon to see the inference.
  • Drag the china dish towards the crystallisation dish to transfer the hot saturated solution of potash alum into it.
  • To cool the hot solution, drag the crystallisation dish and place it in the trough containing water.
  • Wait for some time for potash alum to recrystallise from the solution.
  • Drag the crystallisation dish out of the water trough to place it back.
  • Click on the inference icon to see the inference.
  • We can click on the ‘Next’ button to go to the next process.
  • Drag the bottle containing alcohol and water mixture towards the crystallisation dish to wash the crystals.
  • Drag the crystallisation dish towards the trough to decant the upper solution.
  • Click on the ‘Next’ button to go to the next process.
  • Click on the inference icon to see the inference.
  • To transfer the crystals into the test tube, drag the filter paper containing the crystals toward it.
  • Drag the cork towards the test tube and close it.
  • To redo the experiment, click on the ‘Reset’ button.

Benzoic acid

  • Drag the spatula containing benzoic acid towards the beaker containing hot water and drop it in.
  • Click on the glass rod to stir the solution.
  • To go to the next process, click on the ‘Next’ button.
  • Drag the beaker towards the funnel to filter the benzoic acid solution.
  • Click on the inference icon to see the inference.
  • To go to the next process, click on the ‘Next’ button.
  • To cool the hot solution, drag the beaker and place it in the trough containing water.
  • Wait for some time for benzoic acid to recrystallise from the solution.
  • Drag the beaker out of the water trough to place it back.
  • Click on the ‘Next’ button to go to the next process.
  • Drag the beaker towards the funnel to filter its contents.
  • Click on the inference icon to see the inference.
  • Click on the ‘Next’ button to go to the next process.
  • Drag the bottle containing cold water towards the crystallisation dish to wash the crystals.
  • Drag the crystallisation dish towards the trough to decant the upper solution.
  • To go to the next process, click on the ‘Next’ button.
  • Click on the inference icon to see the inference.
  • To transfer the crystals into the test tube, drag the filter paper containing the crystals toward it.
  • Drag the cork towards the test tube and close it.

Precautions

  •  The filtrate should be evaporated slowly by gently heating during concentration.
  •  The filtrate is to be evaporated only up to the crystallisation point. It should never be heated to dryness. Avoid overheating the  solution.
  •  The solution should be cooled slowly without disturbing it. It should never be cooled rapidly.
  •  Wash the crystals with the washing liquid 3-4 times, using very small amount of the liquid each time.
  •  If the crystals obtained are very small, it means that the solution has been concentrated more than that required for the  crystallisation stage. 

 

Cite in Scientific Research:

Nedungadi P., Raman R. & McGregor M. (2013, October). Enhanced STEM learning with Online Labs: Empirical study comparing physical labs, tablets and desktops. In Frontiers in Education Conference, 2013 IEEE (pp. 1585-1590). IEEE.

Cite this Simulator: