Salts Formulas and Industrial Uses

Why Salts Score Easy Marks in CDS

A salt is the ionic compound formed when the hydrogen of an acid is replaced by a metal (or an ammonium ion). In simple terms, every neutralisation reaction — acid plus base — gives a salt and water. Common table salt, sodium chloride, is only the most familiar member of a very large family.

In the CDS General Science paper, salts appear almost every year as direct, single-fact questions: the chemical formula of washing soda, the common name of NaHCO₃, the gas released by bleaching powder, or the salt used to set fractured bones. None of these need algebra, so a few hours of crisp memorisation here gives a very high marks-per-minute return.

The smart way to learn this chapter is as a three-column table in your head — common name, chemical name, formula — plus one headline use for each salt. Once those four facts lock together for a dozen salts, the objective options almost sort themselves.

One more reason salts are worth your time: they connect cleanly to the acid-base, pH and metals chapters you are already revising. The same neutralisation idea that explains why an antacid soothes the stomach also explains why washing soda softens hard water and why a salt’s solution can be acidic or basic. So the effort you spend here pays off in two or three other topics as well, which is exactly the kind of leverage a time-pressed CDS aspirant should look for.

What Exactly Is a Salt?

When an acid reacts with a base, the H⁺ of the acid and the OH⁻ of the base combine to form water, while the remaining ions join to form a salt. This is the neutralisation reaction:

So the salt’s “parents” tell you its nature. Sodium chloride comes from a strong acid (HCl) and a strong base (NaOH), so it is a neutral salt. Knowing the parent acid and base lets you predict whether the salt’s solution will be acidic, basic or neutral — a frequent CDS twist.

Classifying Salts: Acidic, Basic and Neutral

Salts are grouped by the strength of the acid and base that formed them:

• Normal / neutral salt: strong acid + strong base → pH about 7. Example: NaCl, Na₂SO₄, KNO₃.
• Acidic salt: strong acid + weak base → solution is acidic (pH < 7). Example: NH₄Cl, (NH₄)₂SO₄.
• Basic salt: weak acid + strong base → solution is basic (pH > 7). Example: Na₂CO₃ (washing soda), CH₃COONa.

There is also the term acid salt, where only part of the replaceable hydrogen has been swapped, e.g. sodium bicarbonate NaHCO₃ still has one H left.

The logic is simple once you remember a single rule: in a salt solution, whichever “parent” was the stronger one wins and decides the pH. A strong base such as NaOH overpowers a weak acid such as carbonic acid, so the carbonate solution turns out basic. A strong acid such as HCl overpowers a weak base such as ammonium hydroxide, so ammonium chloride gives an acidic solution. When both parents are strong, neither wins and the solution stays neutral. Examiners often test this with a litmus or pH clue rather than naming the salt directly, so train yourself to reason backwards from the parents.

Common Salt (Sodium Chloride, NaCl)

Common salt is the raw material for most other sodium compounds. It is obtained by the evaporation of sea water and from underground rock-salt deposits.

• Used directly to flavour and preserve food.
• Electrolysis of brine (concentrated NaCl solution) — the chlor-alkali process — gives three valuable products: sodium hydroxide (NaOH) at the cathode, chlorine (Cl₂) at the anode, and hydrogen (H₂).

From NaCl we also make washing soda, baking soda and bleaching powder — so this one salt is the hub of the whole chapter.

Common salt is also a good example of how a salt’s source is tested in CDS. Sea water yields salt by simple solar evaporation in shallow pans, but the salt of large inland deposits, called rock salt, was left behind when ancient seas dried up and is now mined like coal. A pure crystal of sodium chloride does not deliquesce, yet ordinary table salt often turns damp in the rainy season because of the magnesium chloride impurity mixed in it — a small fact that occasionally appears as a reasoning question.

Baking Soda (Sodium Hydrogencarbonate, NaHCO₃)

Baking soda is sodium hydrogencarbonate (sodium bicarbonate), made by passing carbon dioxide through brine containing ammonia.

• Formula: NaHCO₃  |  Common name: baking soda
• It is a mild, non-corrosive base, so it is safe in food.
• On heating it releases carbon dioxide, which makes cakes and bread rise: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
• It neutralises excess acid in the stomach, so it is the active ingredient in many antacids.
• Used in soda-acid fire extinguishers and as baking powder (with a mild edible acid such as tartaric acid).

Washing Soda (Sodium Carbonate, Na₂CO₃·10H₂O)

Washing soda is sodium carbonate with ten molecules of water of crystallisation. It is obtained by heating baking soda to get anhydrous Na₂CO₃, then recrystallising it.

• Formula: Na₂CO₃·10H₂O  |  Common name: washing soda
• Used in laundries and for removing permanent hardness of water.
• Used in the manufacture of glass, soap, paper and borax.
• It is a basic salt — its solution turns red litmus blue.

Another famous water-of-crystallisation salt is blue vitriol, copper sulphate pentahydrate CuSO₄·5H₂O, which turns white when heated and blue again with water.

Bleaching Powder and Plaster of Paris

Two more salts are CDS regulars because of their dramatic everyday uses.

Bleaching powder (calcium oxychloride)

• Formula: CaOCl₂  |  made by passing chlorine over dry slaked lime Ca(OH)₂.
• It releases chlorine, which acts as the bleaching and disinfecting agent.
• Used to bleach cotton and paper, to disinfect drinking water, and as an oxidiser.

Plaster of Paris (POP)

• Formula: CaSO₄·½H₂O (calcium sulphate hemihydrate).
• Made by heating gypsum (CaSO₄·2H₂O) to about 100°C / 373 K.
• On mixing with water it sets into a hard solid (gypsum again), so it is used to set fractured bones, make casts, statues and smooth surfaces.

The Must-Memorise Salt Table

This single list is the highest-yield content in the chapter. Learn common name, formula and one use together.

• Common salt — NaCl — food, raw material for NaOH, Cl₂, soda.
• Baking soda — NaHCO₃ — antacid, baking, fire extinguisher.
• Washing soda — Na₂CO₃·10H₂O — cleaning, softening hard water, glass.
• Bleaching powder — CaOCl₂ — bleaching, disinfecting water.
• Plaster of Paris — CaSO₄·½H₂O — bone casts, statues.
• Gypsum — CaSO₄·2H₂O — cement, source of POP.
• Blue vitriol — CuSO₄·5H₂O — fungicide, electroplating.
• Green vitriol — FeSO₄·7H₂O — ink, fertiliser.
• Epsom salt — MgSO₄·7H₂O — laxative, bath salt.
• Saltpetre — KNO₃ — gunpowder, fertiliser.
• Marble / limestone — CaCO₃ — cement, building, source of CO₂.

Worked Example: Identifying a Salt from Its Use

Common Mistakes to Avoid

• Writing washing soda without its water of crystallisation — it is Na₂CO₃·10H₂O, not just Na₂CO₃.
• Swapping baking soda (NaHCO₃) and washing soda (Na₂CO₃) — baking soda has the H.
• Saying bleaching powder is sodium based — it is calcium oxychloride, CaOCl₂.
• Confusing POP (½H₂O) with gypsum (2H₂O).
• Calling sodium carbonate solution neutral — it is basic (weak acid + strong base).

Previous-Year Style Question

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