Purity

Purity

1. Pure substances have a sharp melting point and boiling point, e.g. water boils at 100 degrees C. If a substance is impure, the impurity will have an effect on the melting and boiling points.

2. Ensuring purity is important because impurities can have harmful effects on our health, e.g. impurities in medical drugs.

3. Methods of purification

   • Use of a suitable solvent
     • A second solvent is used to extract one of the solids from the first solvent. The second solvent must not mix with the first. The solution would then be shaken in a separating funnel to separate the solution into the two solutes.
     • Used to separate two solutes dissolved in a solvent
   • Filtration
     • An undissolved solid can be separated from a solution or liquid by passing it through a piece of filter paper in a filter funnel.
   • Crystallization
     • The solution is gently heated in an evaporating basin to concentrate it. The solvent is evaporated until the crystallization point has been reached. The concentrated solution is left to cool, and it then forms crystals at the bottom of the evaporating basin.
   • Distillation
     • The solution is heated until one of the solutes is boiled off - difference in boiling points. The boiled off solute condenses in a separate beaker, leaving the other solute as a separate solid.
       • E.G. salt water
     • Fractional distillation
       • Used to separate a mixture of liquids with different boiling points. A tall column in which continuous evaporation and condensation of the liquid mixture occurs.
         • This process is important in the making of petroleum and in fermentation of ethanol.