Question 3
Why is salt from the third pond heaped up and kept for six months?
The right answer is the one marked above.
The magnesium salts absorb water vapour and are removed. Magnesium chloride and magnesium sulphate are deliquescent, so during storage they take up water from the air, dissolve and drain away, leaving a solid mass of salt.
Pure sodium chloride is not hygroscopic — it is precisely the impurities that are, and that difference is what the six months exploits. Those same impurities are why unrefined salt tastes bitter.