Everyday and Industrial Chemistry PYQs – Chemistry, SSC & Railway

Answer: Option 1 — Allicin
Allicin is the organosulphur compound released when a garlic clove is crushed.
It gives garlic both its pungent smell and its antibacterial and antifungal action.
Revision note — Active compounds in common foods
• Garlic (Allium sativum) stores a stable, odourless compound called alliin; crushing releases the enzyme alliinase, which converts it into allicin within seconds.
• Allicin is unstable and breaks down on heating, which is why raw crushed garlic is the form credited with antimicrobial value; it was first isolated by Chester J. Cavallito in 1944.
• Ascorbic acid is vitamin C, found in amla, guava and citrus fruits; its deficiency causes scurvy. It is an antioxidant, not garlic's antimicrobial agent.
• Curcumin is the yellow pigment of turmeric (Curcuma longa), valued as an anti-inflammatory; Lakadong turmeric of Meghalaya is prized for its high curcumin content.
• Eugenol is the chief compound of clove oil, used as a dental analgesic and in the laboratory as an olfactory indicator.
• Other pairs worth memorising: capsaicin (chilli), piperine (black pepper), cinnamaldehyde (cinnamon), allyl isothiocyanate (mustard), lycopene (tomato), zingiberene (ginger).
Do not mix these up
• Alliin vs allicin — alliin is the odourless compound stored inside the intact clove; allicin appears only when the clove is crushed or cut.
• Allicin vs capsaicin — allicin is the pungency of garlic; capsaicin is the compound that makes chillies hot.
Also asked from this topic: the compound giving garlic its sharp taste and smell (allicin); the state whose Lakadong turmeric is high in curcumin (Meghalaya); clove oil's lab use (olfactory indicator).

Correct Answer:
Correct Answer: Option 3 – A blue, non-luminous flame is observed with no residue left behind
Explanation:
Complete combustion of methane (a hydrocarbon fuel) occurs when it burns withsufficient oxygen supply, producing carbon dioxide and water vapour only: CH₄ + 2O₂ → CO₂ + 2H₂O. This reaction gives aclean blue, non-luminous flamewith no soot or residue. An orange/yellow flame with black residue (soot/carbon particles) indicates incomplete combustion due to insufficient oxygen.
Important Facts:
• Complete combustion → blue flame, produces CO₂ + H₂O.
• Incomplete combustion → yellow/orange flame, produces CO and soot (carbon).

Correct Answer:
Correct Answer: Option 2 – a yellow flame with lots of black smoke
Explanation:
Unsaturated hydrocarbons (containing double/triple bonds) have a higher proportion of carbon relative to hydrogen. When burnt in air, they undergo incomplete combustion, producing ayellow, sooty flame with lots of black smokedue to unburnt carbon particles.
Important Facts:
• Saturated hydrocarbons (alkanes) generally burn with a clean blue flame.
• Unsaturated hydrocarbons (alkenes, alkynes) burn with a smoky yellow flame.
• This property is used to test for saturation/unsaturation of a compound.

Answer: Option 2 — Unsaturated hydrocarbons have a higher carbon-to-hydrogen ratio, leading to incomplete combustion and soot
Their higher C:H ratio causesincomplete combustion → sooty yellow flame→ Option 2.

Answer: Option 2 — Ethanol
Ethanol(a saturated, oxygen-rich compound) burns with a cleannon-luminousblue flame → Option 2.

Answer: Option 2 — A is true, but R is false
A: unsaturated hydrocarbons do give ayellow, sooty flame— true.
R: this happens due toincompletecombustion (insufficient O₂), NOT complete burning — the reason as stated ("burn completely when oxygen is insufficient") is false → Option 2.

Answer: Option 1 — Complete combustion produces a blue flame due to sufficient oxygen
Withenough oxygen, ethanol undergoescomplete combustiongiving a cleanblue flame(CO₂ + H₂O). Insufficient oxygen gives a yellow/sooty flame → Option 1.

Topic: Antacids and Indigestion
Indigestion occurs when excessHCl (hydrochloric acid)is produced in the stomach.
Antacids containbasic compoundslike Mg(OH)₂, Al(OH)₃, NaHCO₃.
These basesneutralise excess acidthrough acid-base reaction.
Reaction: HCl + Base → Salt + Water
This reduces acidity and relieves pain.
Answer: Option 4 → Neutralising excess stomach acid

Topic:Chemical Process in Milk
When lemon juice (acid) is added to milk, the milk proteins (casein) clump together and separate from the liquid (whey).
This process of protein precipitation due to acid is calledCoagulation.
Other processes:
• Hydrolysis - Breaking down with water
• Crystallization - Formation of crystals
• Sedimentation - Settling of particles
Answer: Option 2 → Coagulation

Topic:Starch and Heat Treatment
Starchis a polysaccharide found in flour, potatoes, rice.
When starch is heated with water:
- Starch granules absorb water and swell
- Granules rupture and release starch molecules
- Forms a thick, gel-like consistency
- This process is calledGelatinization
Other processes:
- Fermentation: breakdown by microorganisms
- Saponification: fat + alkali → soap
- Caramelization: sugar heating (no water)
Answer: Option 1 → Gelatinization

Correct Answer: Yellow — Option (b)
The middle zone of partial combustion in a candle flame is Yellow in colour . About the Zones of a Candle Flame: A candle flame has three distinct zones: Zone 1 — Innermost zone (Dark/Black zone): Location: Core/center of the flame; Nature: No combustion; Contains unburned wax vapors (volatile hydrocarbon gases); The hottest in terms of fuel vapors but no burning occurs here; Relatively cool; Appears dark/sooty blue at the base; Zone 2 — Middle zone (Yellow/Luminous zone): Location: Middle part of the flame; Nature: PARTIAL combustion — incomplete burning of carbon; The wax vapors burn partially, producing incandescent carbon particles (soot/glowing carbon particles); The yellow/orange glow is due to these hot, glowing carbon particles; Temperature: Moderate (~1000°C); This is the luminous part of the flame — carbon particles glow yellow/orange when heated; Zone 3 — Outermost zone (Blue/Non-luminous zone): Location: Outer periphery of the flame; Nature: COMPLETE combustion — carbon burns completely with oxygen from air; Products: Carbon dioxide (CO₂) and water (H₂O); Hottest zone (approximately 1200-1500°C); Appears blue/almost invisible; Why yellow? In the middle zone, carbon particles are heated but not completely oxidized — they emit yellowish light when incandescent, similar to how a piece of iron glows yellow/orange when heated.
Option (a) Red — Incorrect: The middle zone of partial combustion in a candle flame is NOT red — it is yellow/orange; red might be seen in some flames but is NOT the characteristic color of the middle zone of a candle; the yellow color of the middle zone comes from incandescent carbon particles undergoing incomplete combustion. The candidate selected this incorrect option.
Option (c) Green — Incorrect: The middle zone of a candle flame is NOT green; green flames are produced by certain chemical compounds (like copper salts, boric acid, or barium compounds) when burned; a simple candle wax flame produces yellow (middle zone) due to incandescent carbon particles and blue (outer zone) due to complete combustion — NOT green.
Option (d) Brown — Incorrect: The middle zone of a candle flame is NOT brown; brown is not a characteristic color of any zone of a candle flame; the three zones of a candle flame are: dark/no-combustion (innermost), yellow/luminous/partial combustion (middle), and blue/complete combustion (outermost); none of these zones appears brown.

Correct Answer: Fuel for home and industry — Option (d)
LPG (Liquefied Petroleum Gas) is primarily used as fuel for home cooking (domestic use) and industrial applications. LPG is a mixture of propane and butane that is stored and transported in liquid form under moderate pressure. It is used in: domestic cooking stoves and ovens, water heaters, and home heating systems; industrial furnaces, cutting and welding torches, and heating applications; commercial catering and restaurants. In India, LPG is widely distributed through Pradhan Mantri Ujjwala Yojana (PMUY) for domestic cooking fuel. LPG burns cleanly with a high calorific value, making it an efficient fuel for home and industrial use.
Option (a) Ointments, candles, vaseline etc. — Incorrect: These products are made from paraffin wax and petroleum jelly — heavier fractions/derivatives of petroleum, not from LPG (propane/butane gas).
Option (b) Fuel for heavy motor vehicles, electric generators — Incorrect: Heavy vehicles and generators typically run on diesel (a heavier petroleum fraction); LPG is used in some vehicles as CNG/LPG conversion kits, but its primary use is for home and industrial fuel, not heavy vehicles.
Option (c) Paints, road surfacing — Incorrect: Paints use mineral spirits or turpentine as solvents; road surfacing uses bitumen/asphalt (residual heavy fractions of petroleum). These are not uses of LPG.

Correct Answer: To cut off the supply of air, reduce the temperature of the fuel, or both — Option (d)
Fire requires three elements — fuel, oxygen (air), and heat — known as the 'Fire Triangle'. A fire extinguisher works by eliminating one or more of these elements. It can: (1) Cut off the supply of oxygen/air (e.g., CO₂ extinguishers displace oxygen), (2) Reduce the temperature of the fuel below its ignition point (e.g., water extinguishers cool the fuel), or (3) Both simultaneously (e.g., dry chemical powder extinguishers both smother and cool). Different types of extinguishers (water, CO₂, foam, dry powder, wet chemical) work on different mechanisms based on the type of fire.
Option (a) To add more fuel to the fire — Incorrect: This would increase the fire; extinguishers are designed to suppress fire by removing fuel, heat, or oxygen.
Option (b) To increase the temperature of the fuel — Incorrect: Increasing fuel temperature would intensify the fire; extinguishers reduce temperature.
Option (c) To increase the supply of air — Incorrect: Increasing air (oxygen) supply would intensify combustion; extinguishers cut off or displace oxygen.

Correct Answer: In manufacturing of steel — Option (d)
Coke (metallurgical coke) is a solid carbonaceous material produced by heating coal in the absence of air (destructive distillation). Its primary use is in the manufacturing of steel and iron in blast furnaces. In the blast furnace, coke serves three purposes: (1) Fuel — it burns to generate the high temperatures required; (2) Reducing agent — CO produced from burning coke reduces iron ore (Fe2O3) to metalite iron; (3) Structural support — it supports the burden (iron ore + limestone) in the blast furnace.
Option (a) As fuel for residential cooling — Incorrect: Coke is not used for residential cooling; air conditioning uses refrigerants, and coke is an industrial fuel.
Option (b) As food preservative — Incorrect: Coke has no role as a food preservative; food preservatives include salt, vinegar, citric acid, etc.
Option (c) In production of pharmaceuticals — Incorrect: Pharmaceuticals are produced using chemical synthesis and biological processes; coke is not a pharmaceutical ingredient.

Correct Answer: Both I and II — Option (c)
Combustion (burning) occurs in several types: (1) Rapid Combustion — combustion that takes place quickly and produces heat and light (e.g., burning of LPG gas, a candle flame). (2) Spontaneous Combustion — combustion that occurs without the application of external heat; a substance self-ignites due to internal chemical reactions (e.g., white phosphorus ignites spontaneously in air; fires in coal mines caused by spontaneous oxidation). Both are recognized types of combustion in chemistry and fire science.
Option (a) Only I — Incorrect: Both rapid combustion and spontaneous combustion are valid types of combustion; only stating rapid combustion is incomplete.
Option (b) Only II — Incorrect: Rapid combustion is equally a valid type of combustion; excluding it makes this option incorrect.
Option (d) Neither I nor II — Incorrect: Both rapid combustion and spontaneous combustion are well-recognized types of combustion; this option is completely wrong.

Topic: Food Science - Thickening Agents
Starchis a polysaccharide that absorbs water when heated.
Process: Starch granules swell and gelatinize, forming a thick gel.
Common sources: Cornstarch, flour, arrowroot, potato starch.
Salt, sugar, and oil do not have gelling properties.
Answer: Option 3 → Starch

Topic:Combustion and Soot Formation
Blackening of vessel exterior = Carbon deposits (soot)
Complete combustion: Fuel + O₂ → CO₂ + H₂O (clean, no residue)
Incomplete combustion: Insufficient oxygen → Unburned carbon particles
These carbon particles deposit as black soot on vessel surface
Common causes: Poor air supply, low flame temperature, improper fuel-air ratio
Answer: Option 2 → The combustion of the fuel is incomplete.

Mayonnaise is a thick, creamy sauce made by mixing oil and water-based ingredients (egg yolk, vinegar/lemon juice).
Emulsification: Process of mixing two immiscible liquids (oil + water) to form a stable mixture.
Egg yolk contains lecithin (emulsifier) that stabilizes oil droplets in water, creating thick mayonnaise.
Other options: Oxidation (chemical reaction with oxygen), Maillard (browning of proteins), Caramelisation (sugar browning).
Answer: Option 2 → Emulsification

The coal formation sequence follows increasing carbon content.
It starts from Peat, which is the initial stage with lowest carbon.
Then it progresses to Lignite, which has more carbon than Peat.
Next is Bituminous coal with further increased carbon content.
Finally, Anthracite coal has the highest carbon content.
Hence, the correct order is Peat → Lignite → Bituminous → Anthracite.
Final answer: Option 4

Alcohol is considered a cleaner fuel than petrol because it produces less smoke on burning.
Less smoke means fewer pollutants and particulates are released into the air.
This makes alcohol a cleaner option for the environment compared to petrol.
Hence, option 4 is correct.
Final answer: Option 4

Correct Answer: Combustion (Option 3)
Combustion is the chemical process in which a substance (fuel) reacts with oxygen to produce heat and light. Example: CH₄ + 2O₂ → CO₂ + 2H₂O + heat (burning of methane).
Why other options are wrong:
Option 1 – Oxidation is the broader process of loss of electrons or gain of oxygen, but it does not specifically mean heat release.
Option 2 – Reduction is the opposite of oxidation — it involves gain of electrons or loss of oxygen.
Option 4 – Sublimation is a physical process (not chemical) in which a solid directly converts to a gas without passing through the liquid phase (e.g., dry ice, camphor, iodine).

The correct answer is option 3.
Petroleum (crude oil) is a complex mixture of hydrocarbons. On fractional distillation, it yields several useful products at different temperature ranges. Diesel (a medium-weight fuel fraction) and Paraffin wax (obtained from the heavier fractions) are both well-established products/constituents of petroleum refining.
Option 1 is incorrect: While Diesel is indeed a petroleum product, Paraffin wax is equally a petroleum product; 'Only I' is incomplete.
Option 2 is incorrect: While Paraffin wax is a petroleum product, Diesel is equally so; 'Only II' is incomplete.
Option 4 is incorrect: Both Diesel and Paraffin wax are universally recognised petroleum products; stating that neither is a constituent is factually wrong.

Kerosene (also known as mitti ka tel in India) is a liquid fuel derived from petroleum and is used both as fuel for stoves (kerosene stoves/primus stoves) and as fuel for lamps (kerosene lanterns/hurricane lamps). Both uses have been historically significant, especially in rural areas.
Why other options are incorrect:
- Only I: Kerosene is also widely used as fuel for lamps; only saying 'Only I' is incomplete.
- Only II: Kerosene is equally used as fuel for stoves; only saying 'Only II' is incomplete.
- Neither I nor II: Both uses are well-established and practical applications of kerosene.

Coke is produced by the destructive distillation (carbonisation) of coal in the absence of air. This process drives off water, tar, and volatile gases, leaving behind a solid residue that is almost pure carbon (about 85-90% carbon content). Coke is widely used as a fuel and reducing agent in the iron and steel industry.
Why other options are incorrect:
- Phosphorus: Coke contains no significant amount of phosphorus as its primary constituent.
- Sulphur: While coal may contain sulphur, coke is not a pure form of sulphur.
- Nitrogen: Nitrogen is a gaseous element; coke is a solid carbon-based material.

Correct Answer: 2 (The water absorbs the heat, preventing the paper from reaching its ignition temperature)
Water has a very high specific heat capacity (4200 J/kg/K), meaning it absorbs large amounts of heat without a significant rise in temperature. When a paper cup filled with water is heated, the water continuously absorbs the heat being supplied, keeping the temperature of the paper below its ignition temperature (approximately 233°C or 451°F). Since paper cannot reach its ignition point, it does not burn.
Option 1 (Water evaporates and cools) is partially true but not the primary scientific reason - the main reason is heat absorption below ignition point.
Option 3 (Paper becomes fireproof when wet) is incorrect - wet paper is not fireproof; once dried it can burn normally.
Option 4 (Steam extinguishes flame) is incorrect - steam from boiling water at 100°C does not extinguish the external flame.

Correct Answer: I, II and III
Early matches (Lucifer matches, invented in the 1820s-1830s) contained all three substances in their heads:
Antimony trisulphide (Sb2S3) - Acted as a fuel.
Potassium chlorate (KClO3) - Acted as an oxidising agent to support combustion.
White phosphorus (P4) - Acted as the ignition agent (lit by friction).
White phosphorus was later banned due to its extreme toxicity (caused 'phossy jaw' disease in workers) and replaced with red phosphorus in safety matches.

Option 2 is the incorrect statement. Mosquito repellant creams do NOT work by forming a physical barrier on the skin. They work chemically — by releasing volatile compounds (like DEET or citronella) that interfere with the mosquito's chemical sensory systems (olfactory receptors), confusing or deterring the mosquito from detecting and landing on the host. A thick cream layer does not stop a mosquito from penetrating it to bite.
• Option 1 is correct: DEET is one of the most widely used repellents and works by blocking/confusing the mosquito's olfactory (smell) receptors that detect human-emitted compounds like CO₂ and lactic acid.
• Option 3 is correct: Natural repellents like citronella mask human scent, making it harder for mosquitoes to locate and identify a human host.
• Option 4 is correct: Repellent effectiveness depends on concentration (higher DEET % lasts longer), formulation (lotion vs. spray), and environmental factors (temperature, humidity, sweating).

Correct Answer: Option 2 – Only Statements 1 and 3 are correct
Statement 1 is CORRECT: Ethanol 100 was launched by India's Ministry of Petroleum and Natural Gas as a cleaner, greener fuel alternative. It has a high octane number (around 108–113), which improves engine performance and reduces harmful emissions compared to petrol.
Statement 2 is INCORRECT: Ethanol 100 is NOT 100% pure ethanol. While the name suggests high ethanol content, it includes additives such as motor spirit and co-solvents. Pure 100% ethanol is called 'anhydrous ethanol' and has practical limitations for direct engine use.
Statement 3 is CORRECT: Ethanol 100 contains motor spirit (petrol) and co-solvents to improve flame visibility (pure ethanol burns with an almost invisible flame, which is a safety hazard) and to enhance combustion efficiency in engines.
Why other options are wrong:
Option 1 (Only 1 and 2): Statement 2 is incorrect.
Option 3 (Only Statement 2): Statement 2 is incorrect.
Option 4 (All 3 correct): Statement 2 is incorrect.

Correct Answer: Allicin
Allicin (diallyl thiosulfinate ) is the primary organosulfur compound responsible for garlic's pungent smell and sharp taste. It is produced when garlic is crushed or chopped, triggering an enzymatic reaction. Allicin also has antibacterial and medicinal properties.
Option 1 (Allyl isothiocyanate): Incorrect. This compound gives mustard and horseradish their pungency.
Option 3 (Capsaicin): Incorrect. Capsaicin is responsible for the heat/spiciness of chilli peppers.
Option 4 (Thymol): Incorrect. Thymol is the active compound in thyme, used as an antiseptic.

Metanil yellow is a non-permitted synthetic dye sometimes used to adulterate turmeric. Its detection is carried out by a specific chemical test: when turmeric adulterated with Metanil yellow is treated with alcohol and concentrated hydrochloric acid , it gives a pink or violet-red coloration . This reaction confirms the presence of the dye. The other options such as mixing with saltwater, adding vinegar, or boiling do not specifically detect Metanil yellow. Thus, the correct test is the alcohol + hydrochloric acid test .

The correct answer is option 2. A simple household method to detect the presence of chalk powder (which is calcium carbonate) in wheat flour is by mixing the flour with vinegar (acetic acid). If chalk powder is present, it reacts with the acid and produces effervescence (fizzing/bubbles) due to the release of carbon dioxide gas. This reaction is a clear indicator of adulteration. Other listed methods such as adding iodine (used to detect starch), heating (odor test), or soaking in alcohol are not effective for chalk detection.

Correct Answer: Because friction produces heat (Option 2)
When a matchstick is struck against a rough surface, the friction between the match head and the rough surface generates heat. This heat ignites the chemicals in the match head — typically red phosphorus (on the striking surface) and potassium chlorate (in the match head). The heat causes a rapid exothermic chemical reaction, producing a flame. This is a practical application of the conversion of mechanical energy (friction) into heat energy.
Matchsticks do not contain petrol. The match stick is not hollow — it is a solid wooden stick. Absorbing moisture would actually prevent ignition. The correct scientific reason is friction producing heat.

Liquefied Petroleum Gas (LPG) is used in homes for cooking. LPG is primarily composed of propane (C3H8) and butane (C4H10). It is stored under pressure in cylinders and when released, it vaporizes and burns cleanly, making it ideal for cooking purposes. Why other options are wrong: Option 1 - Argon gas: An inert (noble) gas, used in welding and lighting, not for cooking. Option 2 - Nitrogen gas: An inert gas used in food packaging and industrial purposes, not for cooking. Option 4 - Oxygen gas: Used in medical and industrial applications; pure oxygen is dangerous and is not used as cooking fuel.

Correct Answer: Because it has the highest energy output per kilogram.
Liquid hydrogen is used as fuel in cryogenic engines (like those used in rockets) primarily because it has the highest energy output (specific impulse) per unit mass of any chemical fuel - approximately 120 MJ/kg, which is nearly three times that of conventional rocket fuels like kerosene. This high energy density makes it ideal for space launch vehicles. ISRO uses cryogenic engines in its GSLV rockets.
Why other options are wrong:
- Easy to store at room temperature: Hydrogen must be stored at -253°C (cryogenic temperatures); it is NOT easy to store at room temperature.
- Non-flammable: Hydrogen is highly flammable; this is incorrect.
- Very cheap: Liquid hydrogen is actually expensive to produce and store; not the reason for its use.

Correct Answer: Option 3 – H₂O
Water (H₂O) is a molecule composed of two hydrogen atoms covalently bonded to one oxygen atom. Its molecular formula is H₂O. Water has a bent/V-shaped molecular geometry with a bond angle of approximately 104.5°. It is a polar molecule due to the electronegativity difference between oxygen and hydrogen, giving water its unique properties: high surface tension, high specific heat capacity, universal solvent, and ability to form hydrogen bonds.
Option 1 (CO₂): Carbon dioxide — one carbon atom and two oxygen atoms. A greenhouse gas, not water.
Option 2 (O₂): Molecular oxygen — two oxygen atoms. Essential for respiration, not the formula for water.
Option 4 (NaCl): Sodium chloride (common salt) — an ionic compound of sodium and chlorine. Not water.

Correct Answer: Option 3 – Its combustion produces only water vapour and no greenhouse gases
Hydrogen (H₂) is considered a clean fuel because when it burns (combustion: 2H₂ + O₂ → 2H₂O), the only by-product is water vapour — no carbon dioxide (CO₂), no carbon monoxide (CO), no particulate matter, and no greenhouse gases. This makes hydrogen a zero-emission fuel at the point of use. When produced using renewable energy (green hydrogen), it represents a completely clean energy carrier. This is why hydrogen fuel cells are being developed for electric vehicles, power generation, and industrial applications.
Option 1 (No energy loss): All energy conversion processes have some losses (thermodynamic efficiency limits). Hydrogen combustion also has energy losses.
Option 2 (Easily available in pure form): Hydrogen is NOT freely available in pure form in the atmosphere. It must be produced through electrolysis, steam methane reforming, or other processes — it is not naturally available as a free gas.
Option 4 (Lower ignition temperature): Hydrogen actually has a LOWER minimum ignition energy than many fuels, making it somewhat more flammable — this is a safety concern, not its advantage as a clean fuel.

Correct Answer: Option 1 — Magnesium hydroxide
Our stomach produces hydrochloric acid (HCl) to digest food. When too much of it is secreted the result is acidity or indigestion, and the pain is relieved by an antacid — a mild, non-corrosive base that neutralises the excess acid without harming the stomach lining.
Magnesium hydroxide, Mg(OH)₂, is exactly such a base. Its suspension in water is the familiar milk of magnesia, a white liquid which is only slightly soluble, so it is weakly basic and perfectly safe to swallow. It neutralises the excess HCl to give magnesium chloride and water.
⚗️ The Reaction
Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
• A neutralisation reaction: Base + Acid → Salt + Water.
• The OH⁻ of the base combines with the H⁺ of the stomach acid, so the pH rises towards neutral.
• Milk of magnesia is also used as a mild laxative.
❌ Why the other options are incorrect
✗ Calcium oxide — CaO (quicklime) is a strong, corrosive base that reacts violently with water giving out heat; it would burn the stomach and is used in whitewashing and cement, never as a medicine.
✗ Sodium chloride — common salt is a neutral salt (pH 7) of a strong acid and a strong base; having no OH⁻ to spare, it cannot neutralise anything.
✗ Calcium hydroxide — Ca(OH)₂ (slaked lime) is a fairly strong base used in whitewashing, bleaching powder and in treating acidic soil; it is too caustic to be taken internally.
📌 SSC & Railway Exam Facts
• Common antacids: Mg(OH)₂ (milk of magnesia), Al(OH)₃, NaHCO₃ (baking soda), CaCO₃.
• Milk of magnesia = suspension of magnesium hydroxide; its pH is about 10.
• Baking soda — NaHCO₃; washing soda — Na₂CO₃·10H₂O; quicklime — CaO; slaked lime — Ca(OH)₂.
• Stomach acid is HCl, with a pH of about 1.2–3.0.
• Tooth decay begins when the mouth's pH falls below 5.5; toothpastes are basic to correct it.

Correct Answer: Option 1 — Carbon dioxide
Coke is an almost pure form of carbon — a grey, hard, porous solid obtained by the destructive distillation of coal (heating coal strongly in the absence of air). Since coke is essentially carbon, burning it in air simply oxidises that carbon, and the gas produced in the main is carbon dioxide (CO₂).
Coke burns without a flame, giving only a red glow, because it is a solid fuel that produces no gaseous vapours. It has a high calorific value, so it is widely used as a fuel and, more importantly, as a reducing agent in the blast furnace for extracting iron from its ore.
⚗️ The Reaction
C(s) + O₂(g) → CO₂(g) + heat
• One carbon atom joins one oxygen molecule, so the equation is already balanced: C 1 = 1, O 2 = 2.
• It is at once a combination, an oxidation and a combustion reaction, and is strongly exothermic.
• In a limited supply of air the product is poisonous carbon monoxide (CO) instead — 2C + O₂ → 2CO.
❌ Why the other options are incorrect
✗ Nitrogen dioxide — NO₂ comes from the oxides of nitrogen formed at very high temperatures in engines, or from burning nitrogen-rich matter; coke contains carbon, not nitrogen, as its main element.
✗ Oxygen — oxygen is consumed during burning, not produced; burning is a reaction with oxygen.
✗ Sulphur dioxide — SO₂ is produced when sulphur-bearing fuels (such as raw coal or crude petroleum) burn; coke is purified carbon, and SO₂ is only a trace impurity, not the main gas.
📌 SSC & Railway Exam Facts
• Coke = pure carbon, obtained by the destructive distillation of coal; other products are coal tar and coal gas.
• Coke burns without a flame (only a glow), like charcoal; substances that vaporise (LPG, kerosene, wax) burn with a flame.
• Coke is the reducing agent in the blast furnace: Fe₂O₃ + 3CO → 2Fe + 3CO₂.
• SO₂ and NO₂ cause acid rain, producing H₂SO₄ and HNO₃.
• Calorific value is measured in kJ/kg; hydrogen has the highest of all fuels (≈ 150,000 kJ/kg).

Correct Answer: Option 1 — Compressed Natural Gas
CNG stands for Compressed Natural Gas. It is natural gas stored under a high pressure of about 200–250 bar so that a large quantity can be packed into a small cylinder and carried in a vehicle.
Its main constituent is methane, CH₄ (about 80–90%), which makes it a clean fuel: it burns almost completely and releases far less carbon monoxide, unburnt hydrocarbons and particulate matter than petrol or diesel. It also produces no sulphur dioxide, so it does not contribute to acid rain.
Because of this, the Supreme Court ordered all public transport in Delhi to switch to CNG, and CNG is now used widely in buses, autos and cars, and as a domestic and industrial fuel (PNG — piped natural gas).
❌ Why the other options are incorrect
✗ Combustible Nitrogen Gas — nitrogen is not combustible at all; it is in fact an inert gas used to fill food packets precisely because it does not support burning.
✗ Carbonated Natural Gas — "carbonated" means dissolved CO₂, as in soft drinks; CO₂ is a non-supporter of combustion and is used in fire extinguishers, so such a fuel is impossible.
✗ Chemical Natural Gas — not a real term; every gas is a chemical, so the expansion carries no meaning.
📌 SSC & Railway Exam Facts
• CNG — Compressed Natural Gas, mainly methane (CH₄).
• LPG — Liquefied Petroleum Gas, mainly butane (C₄H₁₀) with propane; the smell is from ethyl mercaptan added as a warning.
• The calorific value of CNG ≈ 35,000–50,000 kJ/kg; that of LPG ≈ 55,000 kJ/kg; of hydrogen ≈ 150,000 kJ/kg, the highest of all fuels.
• A good fuel has a high calorific value, low ignition temperature, low ash and causes no pollution.
• Hydrogen is the cleanest fuel because burning it gives only water.

Correct Answer: Option 1 — Compressed Natural Gas and Liquefied Petroleum Gas
CNG stands for Compressed Natural Gas. Natural gas is mainly methane (CH₄) — about 90% — and is compressed to roughly 200–250 bar before being stored in cylinders. It burns almost without smoke or soot, which is why CNG is used for buses, autos and cars in cities such as Delhi; it is also a raw material for making hydrogen and fertilisers.
LPG stands for Liquefied Petroleum Gas. It is obtained during the refining of petroleum and is essentially a mixture of butane (C₄H₁₀) and propane (C₃H₈), liquefied under pressure inside the cylinder. LPG itself is odourless, so a strong-smelling compound, ethyl mercaptan, is deliberately added so that a leak can be detected.
❌ Why the other options are incorrect
✗ Compressed Neutral Gas and Liquefied Petroleum Gas — the N in CNG stands for Natural, not "Neutral"; a common mis-expansion.
✗ Compressed Natural Gas and Liquefied Pressured Gas — the P in LPG stands for Petroleum, not "Pressured".
✗ Compressed Neutral Gas and Liquefied Pressured Gas — both expansions are wrong here.
📌 SSC & Railway Exam Facts
• The chief constituent of CNG / natural gas is methane (CH₄).
• LPG is mainly butane (C₄H₁₀) + propane (C₃H₈); ethyl mercaptan gives it the warning smell.
• Calorific value: LPG ≈ 55 kJ/g, CNG ≈ 50 kJ/g, kerosene ≈ 45 kJ/g.
• Hydrogen has the highest calorific value of all fuels — about 150 kJ/g.
• CNG is called a clean fuel because it releases almost no SO₂ or particulate matter.

Answer: Option 4 — Milk of magnesia
Acidity is caused by excess hydrochloric acid in the stomach.
An antacid is a mild base that neutralises this acid; milk of magnesia is a suspension of magnesium hydroxide, Mg(OH)_(2).
Revision note — Antacids and neutralisation
• Acid + base → salt + water is neutralisation.
• The stomach makes hydrochloric acid (HCl); too much of it causes indigestion / acidity.
• Common antacids: milk of magnesia (Mg(OH)_(2)), baking soda (NaHCO_(3)), aluminium hydroxide.
• Antacids must be mild bases — strong alkalis like NaOH or quick lime (CaO) would burn the gut.
• Other neutralisation uses: baking soda / calamine on an ant or bee sting (formic acid); quick lime or slaked lime for acidic soil.
Do not mix these up
• Lime water vs quick lime — lime water is a solution of Ca(OH)_(2) used to test CO_(2); quick lime is CaO. Neither is used as an antacid.
Also asked from this topic: how antacids relieve indigestion (by neutralising excess stomach acid); a kitchen substance that acts as an antacid (baking soda).