Salts and Their Uses
~9 min read
- Salt: Product of acid + base reaction. Ionic compound.
- Common examples: Sodium chloride (table salt), sodium bicarbonate (baking soda), sodium carbonate (washing soda), calcium sulphate (plaster of Paris), calcium hypochlorite (bleaching powder).
- Uses: Cooking, baking, cleaning, water purification, plaster casts, mining (gunpowder = potassium nitrate).
Salts are everywhere — kitchen, cleaning cupboard, hospital. NDA tests common salts, their chemical names, and uses.
Common Salt (Sodium Chloride, NaCl)
- Source: Sea water (evaporation), rock salt mines (Khewra in Pakistan; Mandi in India).
- Uses: cooking, food preservation, manufacture of sodium metal, NaOH, washing soda, baking soda, bleaching powder, chlorine.
- Iodised salt: NaCl + small amount of KI/KIO₃ — to prevent iodine deficiency (goitre).
Baking Soda and Washing Soda
- Baking soda — Sodium bicarbonate (NaHCO₃):
- Formula: NaHCO₃. White crystalline powder. Slightly alkaline.
- Decomposes on heating: 2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The CO₂ released makes cakes rise.
- Uses: baking powder (mixed with cream of tartar), antacid (relieves acidity), fire extinguishers (releases CO₂).
- Washing soda — Sodium carbonate decahydrate (Na₂CO₃·10H₂O):
- Formula: Na₂CO₃·10H₂O.
- Uses: detergent (softens hard water), glass making, paper making, removing permanent hardness of water.
Plaster of Paris (POP) and Gypsum
- Plaster of Paris: CaSO₄·½H₂O. Made by heating gypsum to ~150°C.
- When mixed with water, sets hard rapidly (within ~15-20 min): CaSO₄·½H₂O + 1.5H₂O → CaSO₄·2H₂O.
- Uses: setting fractured bones (casts), making sculptures, walls, ceilings, models.
- Gypsum (CaSO₄·2H₂O): Naturally occurring. Used in: cement industry, plaster, school chalks.
Bleaching Powder
- Calcium hypochlorite — CaOCl₂ (or Ca(OCl)Cl).
- Made by passing chlorine gas over slaked lime [Ca(OH)₂].
- Releases chlorine on contact with water — chlorine bleaches and kills microbes.
- Uses: bleaching cotton/paper in textile/paper industries, disinfectant for water (urban water treatment), oxidising agent in chemistry.
Other Important Salts
| Salt | Formula | Common name | Use |
|---|---|---|---|
| Calcium oxide | CaO | Quick lime | Whitewashing, steel making |
| Calcium hydroxide | Ca(OH)₂ | Slaked lime / Lime water | Whitewashing, neutralising soil acidity, CO₂ test |
| Potassium nitrate | KNO₃ | Saltpetre | Gunpowder, fertilisers |
| Sodium hydroxide | NaOH | Caustic soda | Soap, paper, drain cleaner |
| Copper sulphate | CuSO₄·5H₂O | Blue vitriol | Electroplating, fungicide |
| Iron(II) sulphate | FeSO₄·7H₂O | Green vitriol | Ink, dye, fertiliser |
| Aluminium potassium sulphate | KAl(SO₄)₂·12H₂O | Alum / Fitkari | Water purification, aftershave |
| Magnesium sulphate | MgSO₄·7H₂O | Epsom salt | Laxative, bath salt |
| Silver nitrate | AgNO₃ | Lunar caustic | Photography (historically), medical use |
| Mercury chloride | HgCl₂ | Corrosive sublimate | Antiseptic (toxic, rarely used) |
NDA PYQ Examples
Q: The chemical name of baking soda is:
(a) Sodium carbonate (b) Sodium hydroxide (c) Sodium bicarbonate (d) Sodium hydrochloride
Answer: (c) Sodium bicarbonate (NaHCO₃).
Q: Plaster of Paris is chemically:
(a) CaSO₄·2H₂O (b) CaSO₄·½H₂O (c) CaO (d) Ca(OH)₂
Answer: (b) CaSO₄·½H₂O — calcium sulphate hemihydrate.
Q: Bleaching powder is:
(a) Calcium oxide (b) Calcium carbonate (c) Calcium hypochlorite / chloride of lime (d) Calcium phosphate
Answer: (c) Calcium hypochlorite (chloride of lime).
Q: Gunpowder contains:
(a) Sodium chloride (b) Potassium nitrate (c) Calcium carbonate (d) Magnesium sulphate
Answer: (b) Potassium nitrate (saltpetre) — along with charcoal and sulphur.
Q: Alum is used in:
(a) Manufacture of glass (b) Water purification (c) Petrol refining (d) Soap making
Answer: (b) Water purification — coagulates fine impurities.
Drill Salts and Their Uses for NDA
NDA-pattern items on Salts and Their Uses with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why does Plaster of Paris set hard?
When mixed with water, the hemihydrate (CaSO₄·½H₂O) takes up more water to form the dihydrate (CaSO₄·2H₂O = gypsum). This crystallisation produces a rigid network. Sets in ~15-20 minutes.
Why is iodised salt important?
Iodine is needed for thyroid hormone synthesis. Iodine-deficient diets cause goitre and impaired mental development in children. Adding KI/KIO₃ to common salt (iodisation) is a simple, cheap public-health intervention.
How does washing soda soften water?
It precipitates calcium and magnesium ions (which cause hardness) as insoluble carbonates. Soft water then lathers easily with soap. Same principle in some commercial water softeners.
Why is alum used in water purification?
Alum is a flocculating agent. When added to muddy water, it forms a gelatinous precipitate of aluminium hydroxide that traps fine suspended particles. The aggregate settles down, leaving clean water above.
How is baking powder different from baking soda?
Baking soda = pure sodium bicarbonate. Needs acid (yogurt, vinegar) in the recipe to release CO₂. Baking powder = sodium bicarbonate + a dry acid (cream of tartar) + corn starch. Self-contained — releases CO₂ when wet.