Chemistry MCQs

Question #341

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Which of the following alcohols is toxic and can cause blindness?

A. Butanol

B. Methanol

C. Ethanol

D. Propanol

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Correct Answer: Option B

Explanation:
Methanol is metabolized to formaldehyde and formic acid, which are toxic to the optic nerve and central nervous system, leading to blindness or death. Ethanol is the alcohol in beverages; moderate consumption is intoxicating but not blinding. Propanol and butanol are also toxic but not typically associated with blindness via this mechanism.

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Question #342

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Soap scum is formed due to the reaction of soap with:

A. Calcium and magnesium ions

B. Potassium ions

C. Sodium ions

D. Chloride ions

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Correct Answer: Option A

Explanation:
Soap (sodium carboxylate) reacts with Ca²⁺/Mg²⁺ to form insoluble calcium or magnesium carboxylate (scum). This precipitates and reduces cleaning efficiency. Sodium and potassium salts are soluble. This is why detergents, which form soluble Ca/Mg salts, are used. Scum is visible in hard water areas.

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Question #343

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Cement mixed with aggregates (sand, gravel) and water forms:

A. Mortar

B. Asphalt

C. Plaster

D. Concrete

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Correct Answer: Option D

Explanation:
Concrete is a mixture of cement, fine aggregate (sand), coarse aggregate (gravel/crushed stone), and water. Mortar is cement + sand + water (no coarse aggregate). Plaster is cement/sand with water for coating walls. Asphalt is bitumen + aggregate. Concrete is strong in compression, used in buildings, bridges.

This question belongs to: Science chemistry

Question #344

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Which type of plastic is used for electrical switches and handles due to heat resistance?

A. PVC

B. Polystyrene

C. Bakelite

D. Polythene

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Correct Answer: Option C

Explanation:
Bakelite is a thermosetting phenol-formaldehyde resin with excellent heat resistance and electrical insulation properties, making it suitable for switches, handles, and electrical fittings. Polythene and PVC are thermoplastics that soften on heating. Polystyrene is brittle and not highly heat resistant. Thermosets do not melt upon reheating.

This question belongs to: Science chemistry

Question #345

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Teflon's monomer is:

A. CH₂=CH₂

B. CH₂=CHCl

C. CH₂=CH-C₆H₅

D. CF₂=CF₂

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Correct Answer: Option D

Explanation:
Teflon is polytetrafluoroethylene (PTFE) polymerized from tetrafluoroethene (CF₂=CF₂). CH₂=CH₂ gives polythene; CH₂=CHCl gives PVC; CH₂=CH-C₆H₅ (styrene) gives polystyrene. The C-F bond is extremely strong, giving Teflon its chemical inertness and thermal stability.

This question belongs to: Science chemistry

Question #346

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Distilled water is considered the purest form of water because:

A. It contains beneficial minerals

B. It is free from dissolved salts and impurities

C. It has a high pH

D. It is carbonated

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Correct Answer: Option B

Explanation:
Distilled water is produced by boiling water and condensing the steam, leaving behind non-volatile impurities like salts, minerals, and most contaminants. It is almost pure H₂O. Deionized water also has salts removed. However, it may absorb CO₂ and become slightly acidic. It is used in laboratories and batteries.

This question belongs to: Science chemistry

Question #347

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Which of the following is a secondary air pollutant?

A. Lead

B. Ozone

C. SO₂

D. CO

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Correct Answer: Option B

Explanation:
Tropospheric ozone is not emitted directly; it forms by photochemical reactions of NOx and VOCs in sunlight. So it's a secondary pollutant. CO, SO₂, and lead are primary pollutants emitted directly from sources. Secondary pollutants include ozone, PAN (peroxyacetyl nitrate), and sulfuric acid mist. Ozone at ground level is harmful.

This question belongs to: Science chemistry

Question #348

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The half-life of a radioactive element is the time required for:

A. Half of the atoms to decay

B. The mass to become zero

C. The temperature to halve

D. Complete decay

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Correct Answer: Option A

Explanation:
Half-life (t₁/₂) is the time taken for half of the radioactive nuclei in a sample to decay. It is constant for a given isotope and independent of initial amount (first-order kinetics). After one half-life, 50% remains; after two, 25%. It is a measure of stability. Short half-life = more radioactive.

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Question #349

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Which of the following is a metalloid?

A. Calcium

B. Sodium

C. Oxygen

D. Arsenic

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Correct Answer: Option D

Explanation:
Arsenic (As) is a metalloid, showing properties of both metals and non-metals. It is in group 15, period 4. Sodium and calcium are metals. Oxygen is a non-metal. Metalloids: B, Si, Ge, As, Sb, Te. They are semiconductors and form amphoteric oxides.

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Question #350

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Which of the following reactions is NOT a redox reaction?

A. 2Na + Cl₂ → 2NaCl

B. CH₄ + 2O₂ → CO₂ + 2H₂O

C. Zn + CuSO₄ → ZnSO₄ + Cu

D. CaCO₃ → CaO + CO₂

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Correct Answer: Option D

Explanation:
In CaCO₃ decomposition, the oxidation states are: Ca (+2), C (+4), O (-2) in both reactant and products, so no change—not a redox reaction. The others involve changes: Zn(0→+2), Cu(+2→0) in B; Na(0→+1), Cl(0→-1) in C; C(-4→+4), O(0→-2) in D. Redox requires change in oxidation numbers.

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Question #351

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Turmeric stain on a white cloth turns reddish-brown when rubbed with soap solution because soap is:

A. Acidic

B. Neutral

C. Salty

D. Basic

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Correct Answer: Option D

Explanation:
Turmeric contains curcumin, a natural indicator that is yellow in acidic/neutral and turns reddish-brown in basic medium. Soap solution is basic (pH ~9-10), thus the stain changes color. This is used as a test for basicity. Vinegar or lemon juice (acid) can reverse the color. Soap is alkaline due to hydrolysis.

This question belongs to: Science chemistry

Question #352

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Washing soda (Na₂CO₃·10H₂O) is obtained from sodium carbonate by:

A. Crystallisation from its aqueous solution

B. Adding acid

C. Electrolysis

D. Heating

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Correct Answer: Option A

Explanation:
Sodium carbonate decahydrate (washing soda) is prepared by dissolving soda ash (anhydrous Na₂CO₃) in water and allowing it to crystallise. Heating would remove water of crystallization (efflorescence). Adding acid would decompose it. Electrolysis would produce NaOH and Cl₂ from brine, not washing soda directly.

This question belongs to: Science chemistry

Question #353

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The metal that does not react with dilute hydrochloric acid to produce hydrogen is:

A. Mg

B. Fe

C. Al

D. Ag

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Correct Answer: Option D

Explanation:
Silver (Ag) is a noble metal below hydrogen in the reactivity series and does not displace H₂ from dilute acids. Mg, Al, Fe are above H and react: Mg + 2HCl → MgCl₂ + H₂. However, Al reacts slowly initially due to oxide layer. Copper also doesn't react. The reactivity series determines acid reactivity.

This question belongs to: Science chemistry

Question #354

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Stainless steel is an alloy of iron with:

A. Aluminium and tin

B. Chromium and nickel

C. Copper and zinc

D. Lead and tin

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Correct Answer: Option B

Explanation:
Stainless steel contains iron, chromium (min 10.5%), and often nickel. Chromium forms a passive oxide layer preventing rust. Nickel improves ductility and corrosion resistance. Molybdenum may be added for pitting resistance. Brass is Cu-Zn, bronze is Cu-Sn, solder is Pb-Sn.

This question belongs to: Science chemistry

Question #355

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Graphite is used as an electrode because it:

A. Is very hard

B. Is an insulator

C. Has free electrons and is chemically inert

D. Is transparent

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Correct Answer: Option C

Explanation:
Graphite conducts electricity due to delocalized electrons in its structure and is chemically inert, allowing its use in electrodes for electrolysis and batteries. Diamond is an insulator. Hardness is not the reason; graphite is soft. It is opaque and black. Graphite electrodes are used in arc furnaces.

This question belongs to: Science chemistry

Question #356

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The number of covalent bonds in a molecule of ethane (C₂H₆) is:

A. 6

B. 7

C. 5

D. 8

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Correct Answer: Option B

Explanation:
Ethane: H₃C-CH₃. There is one C-C single bond and six C-H single bonds, total 7 covalent bonds. Each bond is a sigma bond. The structural formula shows 7 bonds. Methane has 4, propane 10. Counting bonds is simple: each line represents a covalent bond.

This question belongs to: Science chemistry

Question #357

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The process of separating petroleum into petrol, kerosene, diesel etc. is:

A. Cracking

B. Fractional distillation

C. Reforming

D. Polymerisation

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Correct Answer: Option B

Explanation:
Fractional distillation is the primary separation method in refining. Cracking breaks larger hydrocarbons into smaller ones (gasoline). Reforming alters structure to improve octane rating. Polymerisation combines small molecules. Fractional distillation separates based on boiling points in a fractionating column.

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Question #358

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The main gas present in biogas is:

A. Nitrogen

B. Oxygen

C. Methane

D. Carbon dioxide

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Correct Answer: Option C

Explanation:
Biogas is produced by anaerobic digestion of organic matter and consists mainly of methane (50-70%) and carbon dioxide (30-40%), with traces of H₂S, NH₃. Methane is the combustible component. It is a renewable fuel. The waste slurry is used as manure.

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Question #359

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The scientist who first proposed that electrons move in specific orbits around the nucleus was:

A. Niels Bohr

B. Ernest Rutherford

C. James Chadwick

D. J. J. Thomson

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Correct Answer: Option A

Explanation:
Niels Bohr in 1913 proposed the planetary model where electrons revolve in certain allowed circular orbits without radiating energy, with quantized angular momentum. Rutherford proposed the nuclear model but did not specify quantized orbits. Thomson gave plum pudding. Chadwick discovered neutron.

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Question #360

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Which gas is evolved when zinc granules react with dilute sulfuric acid?

A. Sulfur dioxide

B. Oxygen

C. Hydrogen

D. Carbon dioxide

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Correct Answer: Option C

Explanation:
Zn + H₂SO₄ → ZnSO₄ + H₂↑. Hydrogen gas is released, which burns with a pop sound. This is a test for hydrogen. Metals above hydrogen in reactivity series displace it from acids. Dilute H₂SO₄ with Zn produces H₂; with Cu, no reaction.

This question belongs to: Science chemistry

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