Question

How can common salt be purified

Answer 1

By the process of crystallisation. 
In  this method impure salt is dissolved in a particular solvent either as such or on heating while the impurity does not dissolve. A hot concentrated solution is prepared and this upon filtration and cooling gives the crystals of the pure compound NaCl.

Answer 2

Rock salt or halite is a mineral the contains sodium chloride (table salt) as well as other minerals and impurities. You can remove most of these contaminants using two simple purification techniques: filtration andevaporation.

Materials

Rock saltWaterSpatulaFilter paperFunnelEvaporating dishBeaker or graduated cylinderTripodBunsen burner

Filtration

If the rock salt is one large chunk, grind it into a powder using a mortar and pestle or a coffee grinder.