Chemistry MCQs

Question #61

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The number of valence electrons in an atom with electronic configuration 2, 8, 2 is:

A. 10

B. 12

C. 2

D. 8

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Correct Answer: Option C

Explanation:
Valence electrons are the electrons in the outermost shell. Configuration 2,8,2 means K=2, L=8, M=2. The outermost shell M has 2 electrons, so valency is 2, typical of Group 2 metals (Mg). These atoms lose 2 electrons to form M²⁺. The number of valence electrons determines the group in the periodic table. Elements with 2 valence electrons belong to Group 2 (alkaline earth metals).

This question belongs to: Science chemistry

Question #62

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Which molecule contains a triple bond?

A. O₂

B. H₂O

C. N₂

D. CH₄

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Correct Answer: Option C

Explanation:
Nitrogen molecule (N₂) has a triple bond between the two nitrogen atoms (N≡N), with bond order 3. This makes it very stable and relatively inert. O₂ has a double bond (O=O). CH₄ has four single C–H bonds. H₂O has two single O–H bonds. The triple bond in N₂ requires high energy to break, which is why N₂ is used as an inert atmosphere. Bond energy N≡N ≈ 946 kJ/mol.

This question belongs to: Science chemistry

Question #63

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The bond in HCl is best described as:

A. Pure ionic

B. Coordinate

C. Pure covalent

D. Polar covalent

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Correct Answer: Option D

Explanation:
Hydrogen chloride is a polar covalent bond because the shared electron pair is unequally shared due to the higher electronegativity of chlorine (3.0) compared to hydrogen (2.1). The bond has a dipole moment, making HCl molecule polar. In water, HCl ionizes to H⁺ and Cl⁻, but the gaseous molecule is covalent. Pure ionic bonds exist in compounds like NaCl where electronegativity difference is large (>1.7).

This question belongs to: Science chemistry

Question #64

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In the formation of ammonium ion (NH₄⁺) from NH₃ and H⁺, the type of bond formed is:

A. Covalent bond

B. Hydrogen bond

C. Ionic bond

D. Coordinate bond

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Correct Answer: Option D

Explanation:
NH₃ has a lone pair on nitrogen, which it donates to the empty 1s orbital of H⁺, forming a coordinate (dative) bond where both electrons come from nitrogen. The resulting NH₄⁺ has four equivalent N–H covalent bonds and a +1 charge. Coordinate bonds are common in complex ions: e.g., [Ag(NH₃)₂]⁺. Once formed, a coordinate bond is indistinguishable from a covalent bond.

This question belongs to: Science chemistry

Question #65

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Which element has the highest electronegativity?

A. Nitrogen

B. Fluorine

C. Chlorine

D. Oxygen

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Correct Answer: Option B

Explanation:
Fluorine is the most electronegative element with a Pauling electronegativity of 3.98. Electronegativity increases across a period and decreases down a group. Thus, among the halogens, F > Cl > Br > I. Oxygen (3.44) is second. Electronegativity determines bond polarity. Because of its small size and high effective nuclear charge, fluorine strongly attracts bonding electrons. Noble gases are not assigned electronegativity values (except Kr, Xe).

This question belongs to: Science chemistry

Question #66

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Mendeleev's periodic table had a gap for an element with properties similar to aluminum, later discovered and named:

A. Scandium

B. Germanium

C. Boron

D. Gallium

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Correct Answer: Option D

Explanation:
Mendeleev named the missing element eka-aluminium. When gallium was discovered (1875), its properties (density, oxide formula) closely matched Mendeleev's predictions, validating his table. Germanium was eka-silicon; scandium was eka-boron. These predictions were a triumph of the periodic law. Gallium has a low melting point (~30°C) and is used in semiconductors. Mendeleev corrected atomic masses based on group properties.

This question belongs to: Science chemistry

Question #67

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Which of the following metals can displace copper from copper sulfate solution?

A. Au

B. Zn

C. Hg

D. Ag

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Correct Answer: Option B

Explanation:
According to the reactivity series, zinc is more reactive than copper, so Zn displaces Cu: Zn + CuSO₄ → ZnSO₄ + Cu. Silver, gold, and mercury are less reactive than copper and cannot displace it. Reactivity: K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au. Displacement reactions are used in metallurgy and galvanic cells. The blue color of CuSO₄ fades as copper is deposited.

This question belongs to: Science chemistry

Question #68

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Rusting of iron requires:

A. Only water

B. Carbon dioxide and water

C. Both oxygen and water

D. Only oxygen

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Correct Answer: Option C

Explanation:
Rusting is an electrochemical process requiring both oxygen and water (moisture). The overall reaction: 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O. In the absence of either, rusting does not occur. Salt accelerates rusting by increasing conductivity. Prevention methods include painting, greasing, galvanization, and cathodic protection. Rust is hydrated iron(III) oxide, a reddish-brown flaky substance, not protective.

This question belongs to: Science chemistry

Question #69

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Diamond and graphite are allotropes of carbon. The difference arises because of:

A. Different number of protons

B. Presence of impurities

C. Different arrangement of atoms

D. Different chemical composition

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Correct Answer: Option C

Explanation:
Allotropes are different structural forms of the same element. Diamond has a tetrahedral sp³ network, while graphite has a planar sp² layered structure. This difference in atomic arrangement results in distinct physical properties: diamond is hard, transparent, insulating; graphite is soft, black, conducting. Allotropes have same chemical composition (pure carbon). Other allotropes include fullerenes and graphene.

This question belongs to: Science chemistry

Question #70

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The compound with the molecular formula C₂H₆ belongs to which homologous series?

A. Alkenes

B. Alkanes

C. Alkynes

D. Alcohols

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Correct Answer: Option B

Explanation:
C₂H₆ (ethane) follows the general formula CnH₂n+2 (n=2 gives C₂H₆), which is the formula for alkanes. Alkanes are saturated hydrocarbons with single bonds. Alkenes have CnH₂n (e.g., C₂H₄), alkynes CnH₂n-2 (e.g., C₂H₂), alcohols have –OH group. Ethane is a component of natural gas and is used to produce ethylene. Methane (CH₄) is the simplest alkane.

This question belongs to: Science chemistry

Question #71

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Which fossil fuel is formed from the remains of ancient marine plants and animals under high pressure and temperature?

A. Coal

B. Biogas

C. Petroleum

D. Charcoal

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Correct Answer: Option C

Explanation:
Petroleum (crude oil) and natural gas are formed from the remains of tiny marine organisms (plankton) that settled on the seabed, buried under sediment, and transformed by heat and pressure over millions of years. Coal is formed from terrestrial plant material in swampy conditions. Biogas is produced by anaerobic decomposition of organic waste. Charcoal is produced by destructive distillation of wood.

This question belongs to: Science chemistry

Question #72

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The electrolyte used in lead-acid battery is:

A. Dilute HCl

B. Dilute H₂SO₄

C. Ammonium chloride paste

D. KOH solution

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Correct Answer: Option B

Explanation:
Lead-acid battery uses dilute sulfuric acid (H₂SO₄) as the electrolyte with lead dioxide (PbO₂) as positive plate and spongy lead (Pb) as negative plate. Discharge reaction: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O. It is a secondary (rechargeable) battery used in automobiles. The specific gravity of electrolyte indicates the state of charge. KOH is used in Ni-Cd and alkaline batteries.

This question belongs to: Science chemistry

Question #73

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Charles' law states that at constant pressure, volume of a fixed mass of gas is directly proportional to:

A. Density

B. Pressure

C. Absolute temperature

D. Molar mass

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Correct Answer: Option C

Explanation:
Charles' law: V ∝ T (Kelvin) at constant P and n. V/T = constant. This means if temperature is doubled (in Kelvin), volume doubles. Temperature must be in Kelvin (K = °C + 273). Graph of V vs T is a straight line passing through origin. This law is used in hot air balloons: heated air expands, decreasing density, providing lift. Combined with Boyle's law gives PV = nRT.

This question belongs to: Science chemistry

Question #74

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Which of the following is a negative catalyst (inhibitor)?

A. Phosphoric acid in decomposition of H₂O₂

B. Pt in oxidation of SO₂

C. MnO₂ in decomposition of H₂O₂

D. Fe in Haber process

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Correct Answer: Option A

Explanation:
A negative catalyst (inhibitor) slows down a reaction. Phosphoric acid (or acetamide) retards the decomposition of H₂O₂ by stabilizing it. MnO₂ is a positive catalyst for H₂O₂ decomposition, speeding it up. Platinum catalyzes SO₂ oxidation in contact process. Iron (Fe) catalyzes ammonia synthesis in Haber process. Negative catalysts are often used to prevent undesirable reactions, e.g., antioxidants in food preservation.

This question belongs to: Science chemistry

Question #75

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Which of the following statements about enzymes is INCORRECT?

A. Enzymes work optimally at high temperatures (>100°C)

B. Enzymes are highly specific in action

C. Enzymes are proteins

D. Enzymes lower the activation energy of a reaction

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Correct Answer: Option A

Explanation:
Enzymes are biological catalysts that function optimally at moderate temperatures (around 35-40°C for human enzymes). Most enzymes denature and lose activity at temperatures above 50-60°C, as the protein structure unfolds. They are highly specific (lock-and-key model), protein in nature, and lower activation energy without being consumed. Some thermophilic bacteria have enzymes active at high temperatures, but typical enzymes are not stable at >100°C.

This question belongs to: Science chemistry

Question #76

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In the extraction of aluminium from bauxite, the reagent used to remove silica as an impurity is:

A. NaOH

B. HCl

C. Cl₂

D. CO

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Correct Answer: Option A

Explanation:
Bauxite contains SiO₂ (silica) as impurity. In the Bayer's process, powdered bauxite is treated with hot concentrated NaOH solution. Alumina dissolves forming sodium aluminate (NaAlO₂), while silica does not dissolve and is filtered off. Alternatively, silica can be removed by adding lime during refining. The purified alumina is then electrolyzed in cryolite. NaOH selectively dissolves Al₂O₃ due to its amphoteric nature.

This question belongs to: Science chemistry

Question #77

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DAP (diammonium phosphate) provides which primary nutrients to plants?

A. N and K

B. N, P, and K

C. N and P

D. P and K

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Correct Answer: Option C

Explanation:
DAP, (NH₄)₂HPO₄, supplies nitrogen (18% N) and phosphorus (46% P₂O₅). It is a widely used phosphorus-rich fertilizer. Potassium is not present. NPK complex fertilizers provide all three. DAP is highly water-soluble, making phosphorus readily available. In soil, DAP temporarily increases pH around granules due to ammonia, but later nitrification lowers pH. Application: usually at sowing time.

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Question #78

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Sulphuric acid is manufactured by Contact process. Which reaction involves the catalyst?

A. S + O₂ → SO₂

B. SO₂ + O₂ → SO₃

C. H₂S₂O₇ + H₂O → 2H₂SO₄

D. SO₃ + H₂SO₄ → H₂S₂O₇

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Correct Answer: Option B

Explanation:
In the Contact process, the key catalytic step is oxidation of SO₂ to SO₃ using V₂O₅ (vanadium pentoxide) catalyst at 450°C: 2SO₂ + O₂ ⇌ 2SO₃. Sulfur burning is non-catalytic. The SO₃ is absorbed in 98% H₂SO₄ to form oleum (H₂S₂O₇), then diluted to get concentrated H₂SO₄. Direct dissolution of SO₃ in water is avoided due to mist formation.

This question belongs to: Science chemistry

Question #79

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Ethanol is used as a fuel additive. The complete combustion of ethanol yields:

A. CO₂ and H₂

B. CO and H₂

C. CO and H₂O

D. CO₂ and H₂O

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Correct Answer: Option D

Explanation:
Complete combustion of ethanol: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heat. In sufficient oxygen, carbon burns to CO₂, hydrogen to H₂O. Incomplete combustion (lack of O₂) produces CO and soot (C). Ethanol is a biofuel, reducing CO₂ emissions if derived from biomass. It has high octane number and is blended with petrol (gasohol). CO₂ is a greenhouse gas, but bioethanol is carbon-neutral over cycle.

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Question #80

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Which of the following detergents is non-biodegradable?

A. Soap

B. Sodium lauryl sulfate

C. Branched alkylbenzene sulfonate (ABS)

D. Linear alkylbenzene sulfonate (LAS)

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Correct Answer: Option C

Explanation:
Detergents with branched hydrocarbon chains (ABS) resist bacterial degradation and persist in water bodies causing foam and pollution. Linear alkylbenzene sulfonates (LAS) are biodegradable and preferred. Soap is biodegradable but forms scum with hard water. Sodium lauryl sulfate is used in shampoos and is biodegradable. Environmental regulations now mandate the use of linear chain detergents.

This question belongs to: Science chemistry

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