Tag: chemistry

Common salt obtained from sea water is an impure substance having many impurities. This common salt can thus be purified by crystallization.

Crystallisation is a separation technique that is used to separate. a solid that has dissolved in a liquid and made a solution. The solution is warmed in an open container, allowing the solvent to evaporate, leaving a saturated solution. A solution that has as much solid dissolved in it.

Impure copper sulphate is dissolved in water and heated till saturation. It is then cooled and crystals of pure copper sulphate are formed. These crystals are then separated by filtration and then dried.

Purification by crystallization is a better technique as compared to evaporation by dryness. Some solids like sugar decompose or get charred in evaporation to dryness. Also impurities are not removed in this method. However, all these drawbacks are overcome in crystallization.

Choosing an appropriate solvent is the important process of crystallisation, as crystallisation works only when a proper solvent is used. It is important to choose a solvent that will not dissolve the substance at room temperature. But as the temperature of the solvent increases, the solubility of the solute also increases. At the same time, the impurities that are present must either be soluble in the solvent at room temperature or must be insoluble in the solvent at a high temperature. If the solvent is not hot when the dissolution is carried out, too much solvent will be used, leading to diminished yield.

Crystallisation is one of the very important purification techniques, purifying substances by removing unwanted by-products. Crystalline compounds are generally purified via this crystallisation process. The principle behind the crystallisation is that the amount of solute that can be dissolved by a solvent increases with temperature.

To concentrate the filtrate of impure copper sulphate, the china dish containing filtrate is heated gently with constant stirring. This is done to ensure uniform evaporation and to prevent formation of a solid crust. 

Crystallization is better than evaporation to dryness because some solids get decomposed or charred during evaporation.

To crystallise copper sulphate, it is dissolved in water and a small quantity of dilute sulphuric acid is added to prevent the hydrolysis of copper sulphte. The impurities left behind in the solution are removed by filtration.

Crystallization is a method of purification by the formation of crystals.