Chemical Compounds

1. Compounds vs Mixtures vs Elements (Quick Recap)

• Element: pure substance made of one type of atom (cannot be broken down further chemically).
• Compound: two or more elements chemically combined in a fixed ratio, with properties different from the constituent elements (e.g., Na + Cl₂ → NaCl; sodium is a reactive metal and chlorine a poisonous gas, but salt is safe to eat).
• Mixture: physical combination of substances in variable ratio, retaining individual properties, separable by physical means.

2. Common Inorganic Compounds — Names, Formulas, Uses

Common/Chemical Name	Formula	Key Uses/Facts
Water	H₂O	Universal solvent
Hydrogen peroxide	H₂O₂	Antiseptic, bleaching agent
Sodium chloride (common salt)	NaCl	Food preservative, raw material for chlor-alkali industry
Sodium hydroxide (caustic soda)	NaOH	Soap making, paper industry
Sodium bicarbonate (baking soda)	NaHCO₃	Cooking, antacid, fire extinguishers
Sodium carbonate (washing soda)	Na₂CO₃	Glass/soap manufacture, water softening
Sodium thiosulphate (hypo)	Na₂S₂O₃	Photography (fixing agent)
Calcium oxide (quicklime)	CaO	Cement, whitewashing
Calcium hydroxide (slaked lime)	Ca(OH)₂	Whitewashing, limewater (CO₂ test)
Calcium carbonate (limestone/chalk/marble)	CaCO₃	Cement, construction, antacids
Calcium sulphate (Plaster of Paris)	CaSO₄·½H₂O	Casts for fractures, sculpture
Calcium oxychloride (bleaching powder)	Ca(OCl)Cl	Disinfectant, water treatment, bleaching
Ammonium nitrate	NH₄NO₃	Fertiliser, explosives
Ammonium chloride	NH₄Cl	Dry cell batteries, flux in soldering
Potassium nitrate (saltpetre)	KNO₃	Gunpowder, fertiliser
Potassium permanganate	KMnO₄	Disinfectant, water purification, oxidising agent
Silver nitrate	AgNO₃	Photography, silver mirror test, antiseptic
Silver bromide/chloride	AgBr / AgCl	Light-sensitive coating on photographic film
Copper sulphate (blue vitriol)	CuSO₄·5H₂O	Fungicide (Bordeaux mixture), electroplating
Zinc oxide	ZnO	Ointments, sunscreen, paint (white pigment)
Magnesium hydroxide (milk of magnesia)	Mg(OH)₂	Antacid, laxative
Magnesium sulphate (Epsom salt)	MgSO₄·7H₂O	Laxative, bath salts
Sulphuric acid	H₂SO₄	Fertilisers, batteries, "king of chemicals"
Nitric acid	HNO₃	Fertilisers (ammonium nitrate), explosives
Hydrochloric acid	HCl	Present in gastric juice, industrial cleaning
Carbon dioxide	CO₂	Photosynthesis, fire extinguishers, carbonated drinks
Carbon monoxide	CO	Poisonous gas, product of incomplete combustion
Sulphur dioxide	SO₂	Bleaching agent, causes acid rain
Nitrous oxide (laughing gas)	N₂O	Anaesthesia
Ammonia	NH₃	Fertilisers, refrigerant, household cleaners
Chlorine	Cl₂	Water disinfection, bleaching, PVC manufacture

3. Organic Compounds and Everyday Chemistry

Compound	Formula/Class	Notes
Methane	CH₄	Simplest hydrocarbon, major component of natural gas/biogas, greenhouse gas
Ethanol	C₂H₅OH	Alcoholic beverages, fuel additive, antiseptic
Acetic acid	CH₃COOH	Main component of vinegar
Citric acid	C₆H₈O₇	Found in citrus fruits, food preservative
Formaldehyde	HCHO	Preservative (formalin), disinfectant
Urea	CO(NH₂)₂	Nitrogenous fertiliser, first organic compound synthesised artificially (Wöhler, 1828)
Glucose	C₆H₁₂O₆	Simple sugar, product of photosynthesis, energy source
Sucrose	C₁₂H₂₂O₁₁	Table sugar
Chloroform	CHCl₃	Former anaesthetic, solvent
Naphthalene	C₁₀H₈	Mothballs
Benzene	C₆H₆	Simplest aromatic hydrocarbon, industrial solvent
PVC (polyvinyl chloride)	Polymer	Pipes, cables
Teflon	Polymer (PTFE)	Non-stick cookware coating

4. Allotropes (Same Element, Different Structural Forms)

• Carbon: Diamond (hardest natural substance, tetrahedral structure, poor conductor), Graphite (soft, slippery, good conductor, layered hexagonal structure, used as a lubricant and in pencils), Fullerene (C₆₀, "buckyball," cage-like structure), Graphene (single layer of graphite, extremely strong and conductive), Charcoal/Coal (amorphous forms).
• Oxygen: Dioxygen (O₂, the gas we breathe), Ozone (O₃, protects from UV radiation in the stratosphere).
• Sulphur: Rhombic sulphur, Monoclinic sulphur.
• Phosphorus: White phosphorus (highly reactive, poisonous, stored under water), Red phosphorus (less reactive, used in matchboxes), Black phosphorus.

5. Important Gas Laws and Properties (Compound Behaviour)

• Photosynthesis equation: 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂ (chlorophyll as catalyst).
• Respiration (cellular): C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (reverse of photosynthesis in terms of reactants/products).
• Rusting equation: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ (further dehydrates to Fe₂O₃·xH₂O).
• Neutralisation: Acid + Base → Salt + Water.
• Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O + energy.
• Test for CO₂: turns limewater milky (Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O); excess CO₂ makes it clear again (forms soluble Ca(HCO₃)₂).
• Test for H₂: burns with a pop sound near a flame.
• Test for O₂: relights a glowing splint.

6. Fertilisers — Chemical Names (High-Yield for Agriculture/GS Overlap)

Fertiliser	Chemical Composition	Nutrient Supplied
Urea	CO(NH₂)₂	Nitrogen (46% N, highest among solid N fertilisers)
Ammonium sulphate	(NH₄)₂SO₄	Nitrogen
Superphosphate	Ca(H₂PO₄)₂	Phosphorus
Muriate of potash	KCl	Potassium
NPK fertiliser	Mixed	Nitrogen, Phosphorus, Potassium

7. Frequently Tested One-Liners

• "Dry ice" is solid CO₂, used as a refrigerant that sublimates without leaving residue.
• Chemical formula of gypsum: CaSO₄·2H₂O; heating it partially gives Plaster of Paris (CaSO₄·½H₂O).
• LPG (cooking gas) is mainly a mixture of butane and propane.
• CNG (compressed natural gas) is mainly methane.
• Vinegar is a dilute (4–8%) solution of acetic acid in water.
• Photographic film uses silver halides (AgBr, AgCl) because they are light-sensitive.
• Chlorophyll, the green pigment in plants essential for photosynthesis, contains magnesium at its core.
• Haemoglobin, the oxygen-carrying pigment in blood, contains iron at its core.

8. Polymers and Materials Chemistry (Higher-Level Additions)

• Polymers: large molecules made of repeating smaller units (monomers). Natural polymers: cellulose, starch, proteins, natural rubber (polyisoprene). Synthetic polymers: polythene, PVC, nylon, polyester, Teflon, Bakelite.
• Plastics classification: Thermoplastics (soften on heating, can be remoulded — polythene, PVC, PET) vs Thermosetting plastics (harden permanently on heating, cannot be remoulded — Bakelite, melamine).
• Vulcanisation: process of heating natural rubber with sulphur to improve elasticity, strength, and resistance to temperature change — discovered by Charles Goodyear (1839).
• Microplastics and plastic pollution are a live environment-chemistry crossover theme (Single-Use Plastics Ban in India, 2022, under the Plastic Waste Management Rules) — frequently tested in current-affairs-linked Mains GS3 and Prelims environment questions.

9. Industrial and Strategic Compounds (UPSC/Higher-Level Additions)

• Ammonia (NH₃) and urea trace back to the Haber process — central to India's fertiliser policy, Nutrient-Based Subsidy (NBS) scheme, and the push for "Nano Urea" (a liquid nanotechnology-based urea developed by IFFCO, reducing bulk-fertiliser use) — a very current, testable dynamic layer on this static compound.
• Sulphuric acid, phosphoric acid — feedstocks for DAP (di-ammonium phosphate) and SSP (single superphosphate) fertilisers; India imports significant rock phosphate and finished phosphatic fertiliser, making this relevant to Mains GS3 questions on agricultural input security.
• Potassium chloride/Muriate of Potash (MOP) — India has negligible domestic potash reserves and imports almost all of it (from Canada, Russia, Belarus, Jordan) — a classic resource-dependency fact tested alongside geopolitics of fertiliser supply chains (e.g., Russia-Ukraine war's impact on global potash/fertiliser prices, a 2022–23 current-affairs theme).
• Silver halides (AgBr, AgCl) in traditional photography are being replaced by digital sensors — a good example of a "sunset technology" static fact.
• Hydrogen as a strategic compound: the National Green Hydrogen Mission (2023) targets India as a global hub for green hydrogen production (H₂ produced via electrolysis using renewable energy) — connects basic compound chemistry (H₂O electrolysis: 2H₂O → 2H₂ + O₂) to India's energy-transition and net-zero (2070 target) policy commitments, a high-probability current-affairs-cum-static question source.

10. UPSC Angle: Static-Dynamic Linkages

• Fertiliser compounds (urea, DAP, MOP, NPK) are the static base for a cluster of dynamic Mains GS3 themes: fertiliser subsidy burden on the Union Budget, the shift toward Nano fertilisers, and Direct Benefit Transfer (DBT) in fertiliser subsidy.
• Green hydrogen, green ammonia, and battery chemicals (lithium compounds) are the compound-chemistry static base for India's energy security and net-zero commitments — a virtually guaranteed theme across UPSC cycles given the pace of policy announcements in this space.
• Ozone (O₃) as a compound bridges directly into the ecology notes' section on ozone layer depletion and the Montreal Protocol — examiners often test the same fact (ozone is triatomic oxygen) from a pure-chemistry angle in one paper and an environmental-treaty angle in another.

11. PYQ-Style Practice Pointers

• SSC/RRB pattern: matching common/chemical names of compounds (baking soda, washing soda, bleaching powder) — appears almost every cycle in some form; treat the tables in this note as a checklist to memorise cold.
• SSC/RRB pattern: "Which gas is responsible for the 'pop' sound test / turns limewater milky / relights a glowing splint?" — these three tests (H₂, CO₂, O₂ respectively) are asked interchangeably across papers.
• UPSC Prelims pattern: a statement-based question layering a compound's basic chemistry with a current scheme (e.g., "Nano Urea is a liquid formulation of urea nanoparticles developed to reduce consumption of conventional granular urea — with reference to this, consider the following statements...") — the fact itself is static, but the question wrapper is always dynamic/current.

12. Practice Questions (With Answers)

SSC/RRB style (direct recall):

1. Plaster of Paris is chemically: (a) CaSO₄·2H₂O (b) CaSO₄·½H₂O (c) CaCO₃ (d) Ca(OH)₂ Answer: (b) CaSO₄·½H₂O — gypsum is CaSO₄·2H₂O; partial dehydration gives Plaster of Paris.

2. Which gas is used in fire extinguishers and also turns limewater milky? (a) Oxygen (b) Carbon dioxide (c) Nitrogen (d) Hydrogen Answer: (b) Carbon dioxide

3. The first organic compound to be synthesised artificially in a laboratory was: (a) Glucose (b) Urea (c) Methane (d) Acetic acid Answer: (b) Urea — by Friedrich Wöhler in 1828.

4. Hypo, used in photography, is chemically: (a) Sodium thiosulphate (b) Sodium sulphate (c) Silver nitrate (d) Silver bromide Answer: (a) Sodium thiosulphate

5. LPG (cooking gas) mainly consists of: (a) Methane and ethane (b) Butane and propane (c) Hydrogen and oxygen (d) Nitrogen and carbon dioxide Answer: (b) Butane and propane

UPSC Prelims style (statement-based/applied):

6. Consider the following statements regarding green hydrogen:
7. It is produced through electrolysis of water using renewable energy.

8. India's National Green Hydrogen Mission was launched with the aim of making India a global hub for its production and export. Which of the statements given above is/are correct? (a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2 Answer: (c) Both 1 and 2

9. With reference to allotropes of carbon, consider the following statements:

10. Diamond and graphite are both allotropes of carbon with identical electrical conductivity.

11. Graphene is a single layer of graphite. Which of the statements given above is/are correct? (a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2 Answer: (b) 2 only — diamond is a poor conductor while graphite conducts electricity, so statement 1 is incorrect.

12. Chlorophyll and haemoglobin both are complex organic pigments. Which of the following correctly identifies their central metal atoms respectively? (a) Iron, Magnesium (b) Magnesium, Iron (c) Zinc, Iron (d) Magnesium, Copper Answer: (b) Magnesium, Iron — chlorophyll has magnesium at its core, haemoglobin has iron.