Question

how will we purify common salt

Answer 1

we can purify the common salt by boiling the water and by boling the water you can see small tiny porses called common salt

Answer 2

Sea water contains a large amount of common salt and the salts of other metals dissolved in it. Near the sea-shore, the sea water is collected in shallow pits and allowed to evaporate in sunshine. In a few days, the water evaporates, leaving behind salt. The salt so obtained is collected and transported to big factories, where it is purified and packed for consumtions.