Sublimation and Distillation

Sublimation

(Separation of a Mixture of Salt and Ammonium Chloride)

Some solids have a tendency to sublime on heating, i.e., they convert directly from solid to gaseous/vapour phase on heating through the liquid phase.

Chromatography

(Separation of Two Colour of a Dye)

This method is used to separate a mixture of different dyes. The coloured component that is more soluble in water, rises faster and in thisway the colours get separated. This technique is generally used for the separation of those solutes that dissolve in the some solvent.

Distillation

(Separation of two Miscible Liquids)

Two miscible liquids that boil without decomposition and have sufficient difference in their boiling point (> 25° C) can la separated by simple distillation. Fractional distillation is used to separate a mixture of miscible liquids the difference it the boiling point of whichis less than 25° C.

Fractional Distillation

More than components can be separated. Difference in boiling points of components may be less than 25oC. A long fractionating column is also used (where evaporation and condensation take place side by side) along with distillation flask distillation flask, condenser and receiver.

Separation of Different Gases Present in Air

Air is a homogeneous mixture of a number of gases. These can be separated from air by fractional distillation.

Flow diagram showing the separation of gases of air

Air (Free from CO2 and water vapours) High pressure and low (temperature)

  • Liquefied Air (BP = – 200oC)Fractional distillation (Slow warming in fractionating column)
  • Nitrogen Separates firs(BP = – 196oC)
  • Oxygen (BP = – 183oC) Separation of constituents of air

Crystallisation

(Separation of Pure Substance from its impure Form)

Crystals are the purest form of a substance and have definite geometrical shapes. The process by which an impure compound is converted into its crystals is known as crystallisation.

Advantages of Crystallisation over Evaporation

Crystallisation is a better technique than evaporation to purify a solid due         to the following reasons.

  • During evaporation, the solution is heated to dryness. Some solids may decompose or some substances like sugar may get charred during direct heating.
  • Some impurities may still be present in the solution after filtration and they make the solid impure.
  • Direct heating or evaporation does not give crystals. Only a solid residue is left in the dish.

Purification of Drinking Water

In cities, drinking water is supplied from water works. The various processes used in water works for purification are:-

  • Sedimentation to remove suspended solids.
  • Loading with alum to remove negatively charged colliodal clay particles.
  • Filtration to remove dissolved solids.
  • Chlorination to kill bacteria.

Purification of Drinking Water

Pure Substance

A substance that consists of only a single type of constituent particles is called pure substance. e.g., gold, water etc.

Element

An element consists of only one type of atoms. e.g., gold, silver, hydrogen, oxygen etc. An element is a pure substance. 

Elements are further classified into three groups: metals, non-metals and metalloids.