Chemistry MCQs

Question #41

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Acid rain is caused by the presence of which gases in the atmosphere?

A. O₂ and N₂

B. CO₂ and CO

C. SO₂ and NOₓ

D. H₂ and He

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Correct Answer: Option C

Explanation:
Acid rain results from SO₂ and NOₓ (from burning fossil fuels) reacting with water vapor to form sulfuric and nitric acids. Normal rain is slightly acidic (pH ~5.6) due to CO₂ forming carbonic acid, but acid rain has pH < 5.6. Effects include corrosion of monuments (Taj Mahal), damage to aquatic life, and soil acidification. CO₂ causes global warming, not primarily acid rain.

This question belongs to: Science chemistry

Question #42

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The ozone layer protects us from:

A. Infrared radiation

B. Ultraviolet radiation

C. Visible light

D. Radio waves

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Correct Answer: Option B

Explanation:
The stratospheric ozone layer absorbs the majority of the sun's harmful ultraviolet (UV-B and UV-C) radiation, preventing it from reaching Earth's surface. UV can cause skin cancer, cataracts, and damage to ecosystems. Ozone depletion is caused by CFCs (chlorofluorocarbons). Infrared is associated with heat, visible light is mostly harmless, radio waves are not absorbed by ozone.

This question belongs to: Science chemistry

Question #43

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Who proposed the 'plum pudding' model of the atom?

A. Niels Bohr

B. J. J. Thomson

C. Ernest Rutherford

D. John Dalton

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Correct Answer: Option B

Explanation:
J.J. Thomson proposed the 'plum pudding' model (also called watermelon model) where the atom was considered a sphere of positive charge with electrons embedded like seeds. Rutherford's gold foil experiment disproved this by showing a dense, positively charged nucleus. Dalton proposed the indivisible atom theory. Bohr gave the model with discrete energy levels. Thomson discovered the electron but couldn't explain the nucleus.

This question belongs to: Science chemistry

Question #44

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Alpha particles are identical to the nucleus of:

A. Hydrogen

B. Helium

C. Deuterium

D. Lithium

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Correct Answer: Option B

Explanation:
An alpha particle (α) consists of 2 protons and 2 neutrons, which is the same as a helium-4 nucleus (²⁴He²⁺). It has a +2 charge. Beta particles are electrons (β⁻) or positrons (β⁺). Gamma rays are electromagnetic waves with no mass or charge. Alpha particles have low penetration power and are stopped by a sheet of paper or skin. Rutherford used alpha particles in his scattering experiment.

This question belongs to: Science chemistry

Question #45

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The half-life of a radioactive isotope is 10 days. What fraction of the original sample remains after 30 days?

A. 1/2

B. 1/4

C. 1/3

D. 1/8

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Correct Answer: Option D

Explanation:
After each half-life, half of the remaining atoms decay. Number of half-lives n = total time / half-life = 30/10 = 3. Fraction remaining = (1/2)ⁿ = (1/2)³ = 1/8. Thus 12.5% remains. Radioactive decay follows first-order kinetics. Exam trick: fraction remaining = (0.5)^(t/T), where T is half-life. After 4 half-lives, 1/16 remains.

This question belongs to: Science chemistry

Question #46

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The scientist who discovered the neutron was:

A. J. J. Thomson

B. James Chadwick

C. Niels Bohr

D. Ernest Rutherford

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Correct Answer: Option B

Explanation:
James Chadwick discovered the neutron in 1932 by bombarding beryllium with alpha particles, producing a neutral radiation that he identified as neutrons. Rutherford had predicted the existence of a neutral particle in 1920. Chadwick's discovery explained isotopes' mass differences. Neutron: mass ≈ 1.00866 u, no charge. It is stable inside the nucleus but decays (half-life ~10 min) when free.

This question belongs to: Science chemistry

Question #47

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Which element is classified as a metalloid?

A. Sodium

B. Calcium

C. Silicon

D. Chlorine

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Correct Answer: Option C

Explanation:
Silicon is a metalloid, possessing properties intermediate between metals and non-metals. It has a metallic luster but is brittle and a semiconductor. Other metalloids include boron, germanium, arsenic, antimony, tellurium. Sodium and calcium are metals (good conductors, malleable). Chlorine is a non-metal (gas, poor conductor). Metalloids are found along the zigzag line in the p-block of the periodic table.

This question belongs to: Science chemistry

Question #48

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The atomic radius of elements decreases across a period because:

A. Nuclear charge increases and electrons are added to the same shell

B. Atomic mass decreases

C. Number of shells increases

D. Electron-electron repulsion decreases

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Correct Answer: Option A

Explanation:
Across a period, the number of electron shells remains the same, but the nuclear charge (protons) increases. The added electrons enter the same valence shell, so the effective nuclear attraction on the electrons increases, pulling them closer and shrinking the atomic radius. Down a group, radius increases due to addition of shells. Electron-electron repulsion alone does not account for the trend. Atomic mass generally increases across a period.

This question belongs to: Science chemistry

Question #49

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Which of the following represents a double displacement reaction?

A. Zn + CuSO₄ → ZnSO₄ + Cu

B. CaCO₃ → CaO + CO₂

C. 2Mg + O₂ → 2MgO

D. AgNO₃ + NaCl → AgCl↓ + NaNO₃

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Correct Answer: Option D

Explanation:
AgNO₃ + NaCl → AgCl↓ + NaNO₃ is a double displacement (precipitation) reaction where ions exchange partners. Ag⁺ pairs with Cl⁻ to form a white precipitate of AgCl, and Na⁺ remains with NO₃⁻. Option A is combination, B is single displacement, D is decomposition. Double displacement generally forms a precipitate, gas, or water. Neutralization is also a double displacement: HCl + NaOH → NaCl + H₂O.

This question belongs to: Science chemistry

Question #50

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In the redox reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, the oxidizing agent is:

A. Cl₂

B. H₂O

C. MnO₂

D. HCl

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Correct Answer: Option C

Explanation:
MnO₂ oxidizes HCl (Cl⁻ to Cl₂) while itself being reduced (Mn⁴⁺ to Mn²⁺). Oxidizing agent (oxidant) accepts electrons and gets reduced. Here, MnO₂ accepts electrons from Cl⁻. HCl is the reducing agent (reductant) as it loses electrons. This reaction was used by Scheele to discover chlorine. Exam tip: The substance reduced is the oxidizing agent; the substance oxidized is the reducing agent.

This question belongs to: Science chemistry

Question #51

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A solution with pH = 3 is how many times more acidic than a solution with pH = 5?

A. 10 times

B. 1000 times

C. 100 times

D. 2 times

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Correct Answer: Option C

Explanation:
pH is a logarithmic scale: pH = -log[H⁺]. A difference of 1 pH unit corresponds to a tenfold change in [H⁺]. pH 3 vs pH 5 → difference = 2, so [H⁺] at pH 3 is 10² = 100 times greater than at pH 5. Thus, the solution is 100 times more acidic. Similarly, pH 10 is 100 times more basic than pH 8. Always remember: ΔpH = n, ratio = 10ⁿ.

This question belongs to: Science chemistry

Question #52

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Washing soda (Na₂CO₃·10H₂O) is manufactured by Solvay process. Its aqueous solution is:

A. Basic

B. Acidic

C. Neutral

D. Amphoteric

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Correct Answer: Option A

Explanation:
Sodium carbonate is a salt of strong base (NaOH) and weak acid (H₂CO₃). On hydrolysis, it gives OH⁻ ions: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻, making the solution basic. pH ~11. It turns red litmus blue. It is used for softening hard water and in detergents. NaHCO₃ solution is also mildly basic. Salts of strong acid + strong base are neutral (NaCl). Salts of weak base + strong acid are acidic (NH₄Cl).

This question belongs to: Science chemistry

Question #53

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Bleaching powder is produced by the action of chlorine on:

A. Limestone

B. Gypsum

C. Slaked lime

D. Quicklime

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Correct Answer: Option C

Explanation:
Bleaching powder is calcium oxychloride, CaOCl₂, made by passing chlorine gas over dry slaked lime (Ca(OH)₂): Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Quicklime is CaO, limestone is CaCO₃, gypsum is CaSO₄·2H₂O. Bleaching powder is used for bleaching cotton and paper, disinfecting water, and as an oxidizing agent. It releases chlorine on exposure to air or acids.

This question belongs to: Science chemistry

Question #54

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Which alloy is composed of iron, nickel, and chromium and is known for its resistance to corrosion?

A. Stainless steel

B. Brass

C. Solder

D. Bronze

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Correct Answer: Option A

Explanation:
Stainless steel is an alloy of iron with chromium (minimum 10.5%), often nickel, and sometimes molybdenum. Chromium forms a passive oxide layer (Cr₂O₃) that prevents rusting. Brass is Cu + Zn. Bronze is Cu + Sn. Solder is Pb + Sn. Stainless steel is used in kitchen utensils, surgical instruments, and architecture. The most common type is 304 stainless (18% Cr, 8% Ni).

This question belongs to: Science chemistry

Question #55

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LPG mainly contains:

A. Methane and ethane

B. Acetylene and propane

C. Hydrogen and methane

D. Propane and butane

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Correct Answer: Option D

Explanation:
Liquefied Petroleum Gas (LPG) consists primarily of propane (C₃H₈) and butane (C₄H₁₀) liquefied under pressure. A strong-smelling mercaptan is added as odorant to detect leaks. CNG is mainly methane. LPG is used for cooking and as automotive fuel (autogas). It has higher calorific value than natural gas. It is heavier than air, so it settles in low areas, posing a safety hazard if leaked.

This question belongs to: Science chemistry

Question #56

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Assertion (A): Graphite is a good conductor of electricity. Reason (R): In graphite, each carbon atom uses three of its valence electrons to form covalent bonds, leaving one free delocalized electron.

A. Both A and R are true, and R is the correct explanation of A

B. A is false, but R is true

C. A is true, but R is false

D. Both A and R are true, but R is not the correct explanation of A

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Correct Answer: Option A

Explanation:
Graphite has a layered structure where each carbon is sp² hybridized, forming three sigma bonds with neighboring carbons, leaving one unhybridized p orbital electron delocalized over the entire layer. These mobile electrons allow electrical conduction parallel to layers. Diamond, with all four electrons in covalent bonds, is an insulator. Both the assertion and reason are correct, and the reason correctly explains the conductivity of graphite.

This question belongs to: Science chemistry

Question #57

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Which of the following does NOT involve a chemical change?

A. Fermentation of grapes

B. Souring of milk

C. Evaporation of water

D. Making curd from milk

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Correct Answer: Option C

Explanation:
Evaporation of water is a physical change—liquid water changes to water vapor, but the chemical composition (H₂O) remains unchanged. Souring of milk and curd formation involve lactic acid bacteria converting lactose to lactic acid (chemical change). Fermentation of grapes involves yeast converting sugars to ethanol and CO₂. Chemical changes produce new substances. Physical changes only alter the state or form.

This question belongs to: Science chemistry

Question #58

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Which of the following is an example of a sublimable solid?

A. Camphor

B. Sodium chloride

C. Sugar

D. Sand

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Correct Answer: Option A

Explanation:
Camphor sublimes—it directly changes from solid to vapor on heating without passing through the liquid phase. Other sublimable solids include naphthalene, iodine, ammonium chloride, and dry ice (solid CO₂). Sodium chloride, sugar, and sand melt or decompose on strong heating. Sublimation is used to separate sublimable substances from non-sublimable impurities. The process requires careful heating.

This question belongs to: Science chemistry

Question #59

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Two miscible liquids with a boiling point difference of less than 25 K can be separated by:

A. Crystallization

B. Simple distillation

C. Separating funnel

D. Fractional distillation

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Correct Answer: Option D

Explanation:
Fractional distillation is used when the boiling points differ by less than 25 K. A fractionating column provides repeated condensation–vaporization cycles, improving separation. Example: separation of petrol, kerosene from crude oil. Simple distillation works if difference > 25 K. Separating funnel separates immiscible liquids. Crystallization purifies solids. Exam note: Fractionating column packed with glass beads increases surface area.

This question belongs to: Science chemistry

Question #60

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Which of the following pairs are isobars?

A. ³⁵Cl and ³⁷Cl

B. ⁴⁰Ar and ⁴⁰Ca

C. ¹²C and ¹⁴C

D. ¹H and ²H

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Correct Answer: Option B

Explanation:
Isobars are atoms of different elements with the same mass number but different atomic numbers. ⁴⁰Ar (atomic number 18) and ⁴⁰Ca (atomic number 20) both have mass number 40. Options A and D are isotopes (same element, different mass number). Option C are isotopes of hydrogen. Isobars have different chemical properties because of different proton numbers. Example: ¹⁴C and ¹⁴N.

This question belongs to: Science chemistry

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