7. Sublimation and Crystallization

Sublimation occurs when heated, and hence it is called an endothermic process. In an endothermic process, the speed of the molecules is increased, so they move faster. In the endothermic process, the heat is absorbed from the surroundings.

 

 

 

In evaporation, the liquid state changes to a gaseous state.

 

In the above process, when the substances like camphor and ammonium chloride are heated continuously, forms vapour that is collected in the funnel with the closed end.
  

  

  

Certain solid substances like camphor, naphthalene, menthol, and dry ice (solid form of carbon dioxide) that get converted into gas directly upon heating without becoming liquid. This change in the state during the sublimation process is due to the change in temperature of the substances.

 

Sublimation is a physical change as no new substance is formed. When collected and cooled, the vapours formed will be back to their original state without any change in their chemical properties.

  

This is a better technique than evaporation to remove impurities from solids.

 

First, the solute (solid) is dissolved in a solvent and heated continuously, where the excess solvent evaporates. Then, it is allowed to cool down, which forms the crystals. If this process is repeated, it is known as recrystallisation.

 

 

The common examples of the crystallisation process are snowflakes. This process is used in the purification of chemical compounds, seawater etc,.

  

 

Crystallisation is used to separate and purify substances. In this process, large crystals are formed. The arrangement of particles in the crystal formed is well defined, as in the case of solids. Crystallisation is an example of a physical process.