NDA Chemistry · Chemistry in Everyday Life
Common Chemicals and Their Uses
The household and industrial substances the NDA names by their use — washing soda, plaster of Paris, potassium permanganate, silver salts in photography, clean fuels, biogas, and the gas divers breathe.
Why this matters
The larger of the chapter's two subtopics — about seven PYQs, all EASY/MODERATE, almost all 'which chemical is used as / for X'. A recurring shape is 'which statement is NOT correct', where the falsified statement is usually a wrong chemical formula (the plaster-of-Paris formula trap). All recall: learn the substance↔use↔formula table cold.
Concept 1 of 3
Household chemicals by use
Intuition
A cluster of common salts named by their everyday job — what cleans, what sets hard, what purifies water, what darkens in light. The bank asks the substance for a use, or falsifies one chemical formula in a list. Learn the name, the use, and the exact formula.
Definition
The high-frequency household chemicals:
- Washing soda = sodium carbonate, Na₂CO₃·10H₂O (used for cleaning and softening hard water).
- Baking soda = sodium bicarbonate, NaHCO₃ (do not confuse with washing soda).
- Plaster of Paris = calcium sulphate hemihydrate, CaSO₄·½H₂O (sets hard with water; used in casts and moulds).
- Potassium permanganate, KMnO₄, is a strong oxidizing agent — that is why it purifies (disinfects) drinking water.
- Silver bromide, AgBr (and silver chloride), darkens on exposure to light — the basis of black-and-white photography.
| Substance | Formula | Common use |
|---|---|---|
| Washing soda | Na₂CO₃·10H₂O | Cleaning; softening hard waterQ Washing soda = sodium carbonate (Na₂CO₃). Baking soda is sodium bicarbonate (NaHCO₃) — different substance. |
| Plaster of Paris | CaSO₄·½H₂O | Casts, moulds, blackboard chalkQ Plaster of Paris is CaSO₄·½H₂O (hemihydrate) — NOT CaSO₄·2H₂O. CaSO₄·2H₂O is gypsum. This wrong formula is the bank's favourite 'NOT correct' statement. |
| Potassium permanganate | KMnO₄ | Purifies drinking water (oxidizing agent)Q KMnO₄ purifies water because it is an oxidizing agent — it oxidises (destroys) organic impurities and microbes. |
| Silver bromide | AgBr | Black-and-white photography (light-sensitive)Q Silver halides (AgBr, AgCl) darken in light — used in photographic film. Not a silver oxide or nitrate here. |
Practice this concept5 quick reps
Practice — Level 1 (5 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Which chemical is used as washing soda?
- 2.What is the molecular formula of plaster of Paris?
- 3.Why is potassium permanganate used to purify drinking water?
- 4.Which compound is used in black-and-white photography?
- 5.Which silver salt darkens on exposure to light?
From the bank · past-year question
Which one of the following statements is NOT correct ?
[Q97 · Sep · 2025]
Plaster of Paris is CaSO₄·½H₂O, not CaSO₄·2H₂O
Plaster of Paris is the hemihydrate, CaSO₄·½H₂O. The dihydrate CaSO₄·2H₂O is gypsum (the raw material). A statement giving plaster of Paris as CaSO₄·2H₂O is the FALSE one in a 'which is NOT correct' question.
Washing soda ≠ baking soda
Washing soda is sodium carbonate, Na₂CO₃·10H₂O. Baking soda is sodium bicarbonate, NaHCO₃. The bank offers the bicarbonate as a distractor when it asks for washing soda.
KMnO₄ disinfects by oxidising, not by killing on contact
Potassium permanganate purifies water because it is an oxidizing agent — it oxidises organic matter and microbes. The wanted answer is 'oxidizing agent', not 'reducing agent' or 'bleaching agent'.
Concept 2 of 3
Clean fuels and biogas
Intuition
Two recall facts about everyday fuels: which gas is the 'clean' fuel, and what biogas is mostly made of. Both answer a small hydrocarbon — propane for clean LPG fuel, methane for biogas.
Definition
The fuel-recall pairs:
- Propane (C₃H₈) is a clean fuel — it burns with little soot and is a major component of LPG (liquefied petroleum gas).
- Biogas (gobar gas) is produced by anaerobic decomposition of organic waste and is mainly methane (CH₄), with some carbon dioxide.
- CNG (compressed natural gas) is also mainly methane — a cleaner vehicle fuel than petrol or diesel.
| Fuel | Main component | Note |
|---|---|---|
| Clean fuel / LPG | Propane (C₃H₈) with butane | Burns cleanly, little sootQ Propane is the clean-fuel answer; it is a key component of LPG. |
| Biogas (gobar gas) | Methane (CH₄) | From anaerobic decay of organic wasteQ Biogas is MAINLY methane (≈ 50–70%), with carbon dioxide as the next gas. The major constituent is methane. |
| CNG (vehicle fuel) | Methane (CH₄) | Compressed natural gas; cleaner than diesel |
Practice this concept3 quick reps
Practice — Level 1 (3 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Which gas is an example of a clean fuel?
- 2.Which gas is the major constituent of biogas?
- 3.CNG used as a vehicle fuel is mainly which gas?
From the bank · past-year question
Which one of the following is the major constituent of biogas?
[Q91 · Sep · 2019]
Biogas is mainly methane, not carbon dioxide
Biogas does contain carbon dioxide, but its MAJOR constituent is methane (CH₄). When asked for the major constituent, choose methane.
LPG and CNG are different gases
LPG (liquefied petroleum gas) is mainly propane and butane. CNG (compressed natural gas) is mainly methane. Do not equate them.
Concept 3 of 3
Gas mixtures for breathing
Intuition
Deep-sea divers cannot breathe ordinary air at depth — the nitrogen causes 'nitrogen narcosis'. So their cylinders mix oxygen with helium, which is inert and light and does not cause narcosis.
Definition
The breathing-gas facts:
- Deep-sea divers breathe oxygen mixed with helium (called heliox). Helium is used because it does not cause nitrogen narcosis and has low density, making breathing easier at high pressure.
- Ordinary air is about 78% nitrogen and 21% oxygen — the nitrogen is the problem at depth, so it is replaced by helium.
| Use | Gas mixture | Why |
|---|---|---|
| Deep-sea diving | Oxygen + helium (heliox) | Helium avoids nitrogen narcosis; low density eases breathingQ Divers breathe oxygen + HELIUM — not oxygen + nitrogen. Nitrogen at depth causes narcosis. |
| Hospital / medical use | Oxygen (sometimes with helium or CO₂) | Pure oxygen or controlled mixtures for patients |
Practice this concept2 quick reps
Practice — Level 1 (2 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Deep-sea divers carry oxygen mixed with which gas?
- 2.Why is helium, not nitrogen, used in diving cylinders?
From the bank · past-year question
To help deep-sea divers breathe, they carry cylinders of oxygen mixed with
[Q71 · Sep · 2022]
Divers breathe oxygen + helium, not oxygen + nitrogen
The whole point of the diver's mixture is to AVOID nitrogen, which causes narcosis at depth. The answer is helium. 'Nitrogen' is the wrong-on-purpose distractor.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Household chemicals by use4 rows
| Substance | Formula | Common use |
|---|---|---|
| Washing soda | Na₂CO₃·10H₂O | Cleaning; softening hard waterQ Washing soda = sodium carbonate (Na₂CO₃). Baking soda is sodium bicarbonate (NaHCO₃) — different substance. |
| Plaster of Paris | CaSO₄·½H₂O | Casts, moulds, blackboard chalkQ Plaster of Paris is CaSO₄·½H₂O (hemihydrate) — NOT CaSO₄·2H₂O. CaSO₄·2H₂O is gypsum. This wrong formula is the bank's favourite 'NOT correct' statement. |
| Potassium permanganate | KMnO₄ | Purifies drinking water (oxidizing agent)Q KMnO₄ purifies water because it is an oxidizing agent — it oxidises (destroys) organic impurities and microbes. |
| Silver bromide | AgBr | Black-and-white photography (light-sensitive)Q Silver halides (AgBr, AgCl) darken in light — used in photographic film. Not a silver oxide or nitrate here. |
Clean fuels and biogas3 rows
| Fuel | Main component | Note |
|---|---|---|
| Clean fuel / LPG | Propane (C₃H₈) with butane | Burns cleanly, little sootQ Propane is the clean-fuel answer; it is a key component of LPG. |
| Biogas (gobar gas) | Methane (CH₄) | From anaerobic decay of organic wasteQ Biogas is MAINLY methane (≈ 50–70%), with carbon dioxide as the next gas. The major constituent is methane. |
| CNG (vehicle fuel) | Methane (CH₄) | Compressed natural gas; cleaner than diesel |
Gas mixtures for breathing2 rows
| Use | Gas mixture | Why |
|---|---|---|
| Deep-sea diving | Oxygen + helium (heliox) | Helium avoids nitrogen narcosis; low density eases breathingQ Divers breathe oxygen + HELIUM — not oxygen + nitrogen. Nitrogen at depth causes narcosis. |
| Hospital / medical use | Oxygen (sometimes with helium or CO₂) | Pure oxygen or controlled mixtures for patients |
Watch out for (6)
- Plaster of Paris is CaSO₄·½H₂O, not CaSO₄·2H₂O→ Household chemicals by use
- Washing soda ≠ baking soda→ Household chemicals by use
- KMnO₄ disinfects by oxidising, not by killing on contact→ Household chemicals by use
- Biogas is mainly methane, not carbon dioxide→ Clean fuels and biogas
- LPG and CNG are different gases→ Clean fuels and biogas
- Divers breathe oxygen + helium, not oxygen + nitrogen→ Gas mixtures for breathing
Mastery check — 4 interleaved questions
Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.
Which one of the following compounds is used in 'black and white' photography?
[Q102 · Sep · 2021]
Which one of the following is an example of a clean fuel?
[Q62 · Apr · 2019]
Which one among the following chemicals is used as washing soda?
[Q119 · Sep · 2017]
Why is potassium permanganate used for purifying drinking water?
[Q120 · Sep · 2017]
Drill every past-year question on this subtopic
7 questions from the bank — paginated, with cart and Word-export support.