Which one of the following methods can be used to remove permanent hardness of water?

Q: What is the answer to CAPF 2020 Paper I Q11?

B. Correct — B, Ion-exchange. Permanent hardness is caused by dissolved chlorides and sulphates of calcium and magnesium, which do not decompose on heating. An ion-exchange resin traps those calcium and magnesium ions and releases harmless sodium or hydrogen ions in their place, so the hardness leaves the water with the ions that caused it. This is what a domestic water softener does, and the resin is regenerated with brine once it is loaded.

(a)Boiling
(b)Ion-exchange
(c)Lime treatment
(d)Bubbling sulphur dioxide gas

Answer

Why

Correct — B, Ion-exchange. Permanent hardness is caused by dissolved chlorides and sulphates of calcium and magnesium, which do not decompose on heating. An ion-exchange resin traps those calcium and magnesium ions and releases harmless sodium or hydrogen ions in their place, so the hardness leaves the water with the ions that caused it. This is what a domestic water softener does, and the resin is regenerated with brine once it is loaded.

Why the others are wrong

(a)Boiling — Boiling removes only temporary hardness. It decomposes the bicarbonates of calcium and magnesium into insoluble carbonates, which settle out as scale; chlorides and sulphates survive boiling untouched.
(c)Lime treatment — Lime alone is Clark's method, and it too addresses bicarbonate hardness. Lime paired with soda ash does deal with permanent hardness, but the option offers lime by itself.
(d)Bubbling sulphur dioxide gas — Sulphur dioxide dissolves to give an acid. It would make the water sour and corrosive without removing any calcium or magnesium.

Concept

Hard water is water carrying dissolved calcium and magnesium salts. Where the salt is a bicarbonate the hardness is temporary, because heating drives off carbon dioxide and precipitates the carbonate. Where the salt is a chloride or a sulphate the hardness is permanent, and it has to be removed by chemistry rather than by heat.

Sorting the options by what they act on makes the answer quick. Boiling and lime treatment both work on bicarbonate, so both are temporary-hardness methods. Sulphur dioxide acts on nothing relevant. Ion exchange is the only technique offered that pulls the metal ions out regardless of which anion they arrived with, and that is exactly what permanent hardness demands. Washing soda and Calgon do the same job by different chemistry, and reverse osmosis removes both kinds today.

Key facts

Temporary hardness comes from calcium and magnesium bicarbonates and is removed by boiling or by Clark's lime method.
Permanent hardness comes from the chlorides and sulphates of the same metals and survives boiling.
Ion-exchange resins swap the calcium and magnesium ions in the water for sodium or hydrogen ions and are regenerated with brine.
Washing soda and the lime-soda process precipitate the metals as insoluble carbonates and also remove permanent hardness.
Hard water wastes soap because the calcium and magnesium form an insoluble scum, and it coats boilers and pipes with scale.

What each offered method actually removes

Boiling→Temporary hardness only — decomposes bicarbonates, leaves chlorides and sulphates

Lime treatment on its own→Temporary hardness — Clark's method, aimed at bicarbonate

Ion exchange→Both kinds — takes out the calcium and magnesium ions themselves

Bubbling sulphur dioxide→Nothing. It acidifies the water instead

Permanent hardness is defined by what boiling cannot touch, so any method that works through heat or through bicarbonate chemistry is ruled out before it is examined.

Study next

Temporary and permanent hardness, and the salts responsible for eachClark's method, the lime-soda process and Calgon treatmentWhy soap lathers poorly in hard water while synthetic detergents do notBoiler scale, its formation and its consequencesReverse osmosis and modern domestic water treatment

Common traps

Extending boiling to permanent hardness because it is the method everyone learns first.
Reading 'lime treatment' as the full lime-soda process, which is a different thing.
Forgetting that ion exchange puts sodium into the water in place of the calcium it removes.

A method-to-target item. The examiner mixes temporary-hardness methods among the options and relies on the candidate not distinguishing the two kinds of hardness.

Related PYQs

NDA_GAT_2018_II_Q782018Open & attempt →

Permanent hardness of water cannot be removed by which one of the following methods?

(a) Treatment with washing soda
(b) Calgon's method
(c) Boiling
(d) Ion exchange method

Answer(c) Boiling

The same fact asked from the other side. That paper's key rules out boiling and leaves washing soda, Calgon and ion exchange standing — which is precisely the set of methods that reach permanent hardness.

NDA_GAT_2017_I_Q572017Open & attempt →

Temporary hardness in water is due to which one of the following of Calcium and Magnesium ?

(a) Hydrogencarbonates
(b) Carbonates
(c) Chlorides
(d) Sulphates

Answer(a) Hydrogencarbonates

The other half of the pair. Fixing bicarbonates as the cause of temporary hardness is what makes chlorides and sulphates, and therefore ion exchange, the answer here.

Practice

practice — not a real PYQ
Temporary hardness of water is caused by the presence of
- (a)chlorides of calcium and magnesium
- (b)bicarbonates of calcium and magnesium
- (c)sulphates of calcium and magnesium
- (d)nitrates of sodium and potassium
Answer(b) bicarbonates of calcium and magnesium — these decompose on boiling, which is exactly why the hardness they cause is called temporary.
practice — not a real PYQ
Soap produces a scum rather than a lather in hard water because it forms
- (a)soluble sodium salts
- (b)insoluble calcium and magnesium salts
- (c)carbon dioxide
- (d)a colloidal suspension of oil
Answer(b) insoluble calcium and magnesium salts — the soap is consumed forming this scum before any lather can build up.