chemistry MCQs

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chemistry MCQs

Practice complete Chemistry MCQs covering Matter, States of Matter, Physical and Chemical Changes, Atomic Structure, Chemical Bonding, Periodic Table, Acids, Bases and Salts, Metals and Non-Metals, Chemical Reactions, Electrochemistry, Organic Chemistry, Hydrocarbons, Environmental Chemistry, Polymers, Biomolecules, Nuclear Chemistry, and all other important topics. Includes chapter-wise and exam-oriented multiple-choice questions with detailed answers and explanations for JKSSB, SSC, Banking, Railway, UPSC, CUET, NEET, State PSCs, and other competitive exams.

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Page 13 of 55
Question #241
The combining capacity of an element is called its:
A. Mass number
B. Atomic number
C. Valency
D. Ionization energy
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Correct Answer: Option C

Explanation:
Valency is the number of electrons an atom can lose, gain, or share to achieve a stable octet. It determines the number of chemical bonds it can form. For example, carbon has valency 4, oxygen 2, chlorine 1. Atomic number is number of protons, mass number is protons+neutrons, ionization energy is energy required to remove electron.
This question belongs to: Science chemistry
Question #242
Which of the following is a molecular solid?
A. Iron
B. Diamond
C. Ice
D. NaCl
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Correct Answer: Option C

Explanation:
Ice (solid H₂O) is a molecular solid where molecules are held together by hydrogen bonds and van der Waals forces. Diamond is a covalent network solid. NaCl is ionic solid. Iron is metallic solid. Molecular solids have low melting points, are soft, and are poor conductors. Other examples: dry ice (CO₂), iodine, sugar.
This question belongs to: Science chemistry
Question #243
Which of the following is a characteristic of a physical change?
A. Change in chemical composition
B. Formation of new substance
C. Easily reversible, no new substance
D. Irreversible and energy change
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Correct Answer: Option C

Explanation:
Physical changes alter the form, state, or appearance without changing chemical identity. They are usually easily reversible (e.g., freezing/melting). Chemical changes produce new substances and are often irreversible (e.g., burning). Physical: crushing, dissolving, boiling. Chemical: rusting, cooking, digestion.
This question belongs to: Science chemistry
Question #244
The smallest particle of a compound that retains its chemical properties is a:
A. Molecule
B. Ion
C. Electron
D. Atom
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Correct Answer: Option A

Explanation:
A molecule is the smallest particle of a compound (or element) that can exist independently and exhibit the compound's chemical properties. For covalent compounds (water, CO₂), the molecule is the discrete particle. For ionic compounds, the term 'formula unit' is used rather than molecule. Atoms are smallest particles of elements. Ions are charged atoms/molecules.
This question belongs to: Science chemistry
Question #245
Isotopes of hydrogen include protium, deuterium, and tritium. Which one is radioactive?
A. Protium
B. Deuterium
C. Tritium
D. Both protium and deuterium
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Correct Answer: Option C

Explanation:
Tritium (³H) is a radioactive isotope with a half-life of ~12.3 years, decaying by beta emission to ³He. Protium (¹H) and deuterium (²H) are stable. Tritium is used in self-luminous paints, tracer studies, and nuclear fusion. It occurs naturally in very small amounts from cosmic rays but is mainly produced in nuclear reactors.
This question belongs to: Science chemistry
Question #246
The valency of phosphorus in PCl₅ is:
A. 1
B. 3
C. 5
D. 6
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Correct Answer: Option C

Explanation:
In PCl₅, phosphorus forms five covalent bonds with five chlorine atoms. Phosphorus has 5 valence electrons (3s²3p³), and by using d-orbitals, it can expand its octet to accommodate 5 bonds. Thus its covalency is 5. In PCl₃, it is 3. Valency is the number of bonds formed. The oxidation state of P in PCl₅ is +5.
This question belongs to: Science chemistry
Question #247
Which of the following molecules has a linear shape?
A. NH₃
B. CO₂
C. CH₄
D. H₂O
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Correct Answer: Option B

Explanation:
CO₂ has sp hybridized carbon, forming two double bonds with oxygen in a linear arrangement (O=C=O) with bond angle 180°. H₂O is bent (angular, 104.5°), NH₃ is trigonal pyramidal (107°), CH₄ is tetrahedral (109.5°). Molecular shape is determined by VSEPR theory: CO₂ has 2 bond pairs, 0 lone pairs → linear.
This question belongs to: Science chemistry
Question #248
Which element was discovered by Henry Moseley through his X-ray spectra experiments?
A. New way to determine atomic number
B. Proton
C. Electron
D. Neutron
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Correct Answer: Option A

Explanation:
Henry Moseley (1913) established the concept of atomic number as the number of protons through X-ray spectra of elements. He showed that atomic number, not atomic mass, is the fundamental property for arranging elements in the periodic table, resolving anomalies in Mendeleev's table. He did not discover a particle but a principle. Modern periodic law is based on atomic number.
This question belongs to: Science chemistry
Question #249
The law of multiple proportions was proposed by:
A. John Dalton
B. Joseph Proust
C. Antoine Lavoisier
D. Jöns Jacob Berzelius
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Correct Answer: Option A

Explanation:
John Dalton formulated the law of multiple proportions: If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers. Lavoisier gave law of conservation of mass. Proust gave law of definite proportions. Dalton's atomic theory explained these laws.
This question belongs to: Science chemistry
Question #250
Which of the following acids is considered a strong acid?
A. Citric acid
B. Carbonic acid
C. Acetic acid
D. Hydrochloric acid
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Correct Answer: Option D

Explanation:
Hydrochloric acid (HCl) dissociates completely in water, making it a strong acid. Acetic acid (CH₃COOH), carbonic acid (H₂CO₃), and citric acid are weak acids—they partially ionize. Other strong acids: H₂SO₄ (first dissociation), HNO₃, HBr, HI, HClO₄. Strong acids have very large Ka values. pH of 0.1 M HCl is about 1.
This question belongs to: Science chemistry
Question #251
Baking powder is a mixture of baking soda and:
A. Sodium chloride
B. A mild edible acid like tartaric acid
C. Ammonium sulfate
D. Calcium carbonate
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Correct Answer: Option B

Explanation:
Baking powder contains sodium bicarbonate (NaHCO₃) and a weak solid acid such as tartaric acid or cream of tartar (potassium hydrogen tartrate). When moistened, they react to produce CO₂, making dough rise. Baking soda alone requires an acidic ingredient (yogurt, lemon). The acid in baking powder ensures reliable leavening. Sodium chloride and calcium carbonate do not serve this purpose.
This question belongs to: Science chemistry
Question #252
The bleaching action of bleaching powder is due to:
A. Ozone
B. Chlorine liberated in the presence of acid
C. Hydrogen peroxide
D. Sodium hypochlorite
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Correct Answer: Option B

Explanation:
Bleaching powder (CaOCl₂) reacts with dilute acids or even CO₂ from air to release chlorine: CaOCl₂ + H₂SO₄ → CaSO₄ + Cl₂ + H₂O; or with CO₂: CaOCl₂ + CO₂ → CaCO₃ + Cl₂. The chlorine gas acts as oxidizing and bleaching agent. Actually the bleaching is due to nascent oxygen produced by chlorine with water: Cl₂ + H₂O → 2HCl + [O]. But the key is chlorine liberation.
This question belongs to: Science chemistry
Question #253
Which of the following is a basic salt?
A. NH₄Cl
B. NaNO₃
C. CH₃COONa
D. NaCl
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Correct Answer: Option C

Explanation:
Sodium acetate (CH₃COONa) is a salt of weak acid (acetic acid) and strong base (NaOH). It hydrolyzes to produce OH⁻ ions: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻, making the solution basic. NaCl (strong acid-strong base) neutral. NH₄Cl (strong acid-weak base) acidic. NaNO₃ neutral. Basic salts turn red litmus blue.
This question belongs to: Science chemistry
Question #254
Which of the following metals can be cut with a knife?
A. Copper
B. Iron
C. Sodium
D. Zinc
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Correct Answer: Option C

Explanation:
Sodium is a soft alkali metal that can be easily cut with a knife, revealing a silvery surface that tarnishes quickly in air. Potassium is also soft. Iron, zinc, and copper are much harder. Alkali metals have low density and low melting points. They are stored under kerosene to prevent reaction with moisture and oxygen.
This question belongs to: Science chemistry
Question #255
The rusting of iron is accelerated by:
A. Placing in vacuum
B. Presence of salt in water
C. Dry air
D. Coating with paint
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Correct Answer: Option B

Explanation:
Salt (NaCl) dissolved in water increases the electrical conductivity of the electrolyte, facilitating the electrochemical corrosion process. This is why iron rusts faster in coastal areas and on roads treated with deicing salts. Dry air prevents rusting (needs moisture). Coating with paint blocks oxygen and moisture. Vacuum has no oxygen.
This question belongs to: Science chemistry
Question #256
Which allotrope of carbon is a good conductor of electricity?
A. Diamond
B. Charcoal
C. Graphite
D. Buckminsterfullerene
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Correct Answer: Option C

Explanation:
Graphite conducts electricity due to delocalized electrons within its layers. Diamond is an insulator. Buckminsterfullerene (C₆₀) is a semiconductor but not as good a conductor as graphite in bulk; fullerene solids can be doped to become conductors. Charcoal is a poor conductor. Graphite electrodes are used in batteries and electrolysis.
This question belongs to: Science chemistry
Question #257
Methane is the first member of the alkane series. Its molecular formula is:
A. CH
B. CH₃
C. CH₂
D. CH₄
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Correct Answer: Option D

Explanation:
Methane is CH₄, the simplest alkane with one carbon atom bonded to four hydrogen atoms. The series general formula is CₙH₂ₙ₊₂. For n=1, H=4. CH, CH₂, CH₃ are not stable neutral molecules; they are fragments (methyl radical etc.). Methane is a major component of natural gas and biogas.
This question belongs to: Science chemistry
Question #258
Which fossil fuel is found in association with petroleum deposits and is the cleanest burning?
A. Natural gas
B. Peat
C. Lignite
D. Coal
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Correct Answer: Option A

Explanation:
Natural gas (methane) is often found above petroleum deposits, trapped under cap rock. It burns cleaner than coal and oil, producing less CO₂, no SO₂, and minimal particulates. Coal (including lignite) contains more impurities. Peat is an early stage of coal. Natural gas is transported via pipelines or as LNG.
This question belongs to: Science chemistry
Question #259
The calorific value of a fuel is expressed in:
A. °C
B. atm
C. mol/L
D. kJ/kg or J/g
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Correct Answer: Option D

Explanation:
Calorific value is the energy released per unit mass or volume of fuel. Common units: kJ/kg, kJ/g, J/g, kcal/kg, Btu/lb. It measures fuel efficiency. Hydrogen has about 141.9 MJ/kg (higher), petrol ~47 MJ/kg. Temperature (°C) is not a unit for calorific value. Pressure and concentration are not appropriate.
This question belongs to: Science chemistry
Question #260
Which gas is produced at the cathode during the electrolysis of acidified water?
A. Ozone
B. Chlorine
C. Hydrogen
D. Oxygen
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Correct Answer: Option C

Explanation:
At cathode (negative electrode), H⁺ ions are reduced: 2H⁺ + 2e⁻ → H₂(g). At anode (positive), water is oxidized: 2H₂O → O₂ + 4H⁺ + 4e⁻. Overall: 2H₂O → 2H₂ + O₂. The volume of H₂ collected is double that of O₂. Acidified water (with H₂SO₄) enhances conductivity. Platinum electrodes are used.
This question belongs to: Science chemistry

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