Which of the following is used for purification of water? a) Sodium bicarbonate
b) Alum
c) Copper sulphate
d) Sodium Hydroxide

A. Sodium bicarbonate is a water treatment method used to soften water (removing calcium and magnesium impurities from it) in older water softener systems. The process uses a chemical reaction to convert the calcium hydroxide (or magnesium hydroxide) into calcium carbonate, which isn't water soluble.

B. Alum, also known as potassium aluminum sulfate, can be used to purify water. ... Add 1.4 ounces of alum powder to the contaminated water in the reaction vessel and carefully stir the mixture with the stirring stick for at least five minutes. Sediment will form as the phosphorous in the water binds with the alum powder.

C. If Copper sulphate crystals are added to water it is called 'hydrated copper sulphate solution'.

If it is added to cold water then:-

it is more saturated

but if it is added to warm water then:-

it is unsaturated

and it turns bluish in colour in dissolving in water

D.  Sodium hydroxide is used in the treatment of drinking water to raise the pH of the water to a level that minimizes the corrosion. Raising the pH remains one of the most effective methods for reducing lead corrosion and minimizing lead levels in drinking water.