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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Question #301
Soaps are biodegradable because their hydrocarbon chains are:
A. Branched
B. Linear and unbranched, can be broken down by bacteria
C. Fluorinated
D. Aromatic
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Correct Answer: Option B

Explanation:
Natural soap molecules have straight-chain fatty acid salts that are easily broken down by microorganisms through β-oxidation. This makes them biodegradable. Some synthetic detergents with branched chains are non-biodegradable and cause pollution. Linearity allows enzymes to act. Aromatic and fluorinated compounds resist biodegradation.
This question belongs to: Science chemistry
Question #302
The setting of plaster of Paris involves:
A. Carbonation
B. Dehydration
C. Hydration to form gypsum
D. Oxidation
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Correct Answer: Option C

Explanation:
Plaster of Paris (CaSO₄·½H₂O) sets by rehydrating to gypsum (CaSO₄·2H₂O) when mixed with water: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. The interlocking crystals of gypsum provide hardness. It expands slightly on setting, making it ideal for casting. Dehydration is the opposite. Oxidation and carbonation are not involved.
This question belongs to: Science chemistry
Question #303
Glass is made by heating sand with:
A. Potassium nitrate and lime
B. Alumina and silica
C. Sodium chloride and calcium carbonate
D. Sodium carbonate and calcium carbonate
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Correct Answer: Option D

Explanation:
Soda-lime glass is produced by fusing silica (SiO₂) with sodium carbonate (Na₂CO₃) and calcium carbonate (CaCO₃) at high temperature. Na₂CO₃ acts as flux to lower melting point, CaCO₃ provides stability. Reaction: Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO₂. Sodium chloride is not used. Potassium compounds make harder glass.
This question belongs to: Science chemistry
Question #304
PVC is used in making pipes because it is:
A. Strong, rigid, and chemically resistant
B. Biodegradable
C. Flexible and elastic
D. Transparent and brittle
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Correct Answer: Option A

Explanation:
Rigid PVC (unplasticized, uPVC) is hard, strong, and resistant to chemicals and weathering, making it ideal for pipes, window frames, and fittings. Flexible PVC uses plasticizers. It is not biodegradable. Transparency depends on additives; generally it's translucent. Its chemical resistance is due to the chlorine content.
This question belongs to: Science chemistry
Question #305
Temporary hardness of water is caused by:
A. Sodium chloride
B. Calcium chloride
C. Calcium sulfate
D. Magnesium bicarbonate
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Correct Answer: Option D

Explanation:
Temporary hardness is due to dissolved bicarbonates of Ca and Mg. These can be removed by boiling, which precipitates carbonates. Calcium sulfate and chloride cause permanent hardness. Sodium chloride does not cause hardness. Boiling: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂. Bicarbonates are formed when water containing CO₂ passes over limestone.
This question belongs to: Science chemistry
Question #306
Which of the following is NOT a method of water purification?
A. Electrolysis
B. Filtration
C. Boiling
D. Chlorination
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Correct Answer: Option A

Explanation:
Electrolysis of water decomposes it into hydrogen and oxygen, not purification. Boiling kills microbes, filtration removes particulate matter, chlorination disinfects. Other methods: UV treatment, ozonation, reverse osmosis, distillation. Electrolysis is a chemical decomposition technique.
This question belongs to: Science chemistry
Question #307
The main source of chlorofluorocarbons (CFCs) in the atmosphere is:
A. Volcanoes
B. Fertilizers
C. Automobile exhaust
D. Refrigerants and aerosol propellants
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Correct Answer: Option D

Explanation:
CFCs were widely used as refrigerants (Freon), propellants in aerosol sprays, foam blowing agents, and solvents. They are chemically stable in the troposphere, drift to the stratosphere where UV breaks them, releasing Cl atoms that destroy ozone. Automobiles emit CO₂, NOx. Volcanoes emit SO₂. Fertilizers lead to N₂O.
This question belongs to: Science chemistry
Question #308
The half-life of Iodine-131 is 8 days. If a sample has initial activity of 100 counts per minute, after 24 days the activity will be:
A. 6.25
B. 50
C. 25
D. 12.5
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Correct Answer: Option D

Explanation:
Number of half-lives = 24/8 = 3. Fraction remaining = (1/2)³ = 1/8. Activity = 100 × 1/8 = 12.5 counts per minute. Radioactive decay is exponential. After 1 half-life: 50, 2: 25, 3: 12.5. This isotope is used in thyroid treatment.
This question belongs to: Science chemistry
Question #309
The neutron was discovered by James Chadwick in 1932 by bombarding which element with alpha particles?
A. Uranium
B. Gold
C. Beryllium
D. Lead
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Correct Answer: Option C

Explanation:
Chadwick bombarded beryllium (Be) with alpha particles from polonium, producing a highly penetrating neutral radiation, which he identified as neutrons: ⁹Be + ⁴He → ¹²C + ¹n. Gold was used by Rutherford for nucleus discovery. Uranium for fission. Lead for shielding.
This question belongs to: Science chemistry
Question #310
John Dalton's atomic theory stated that atoms are indivisible. This was disproved by the discovery of:
A. Chemical reactions
B. Subatomic particles (electrons, protons, neutrons)
C. Molecules
D. Isotopes
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Correct Answer: Option B

Explanation:
Dalton considered atoms the ultimate, indivisible particles. The discovery of electrons (J.J. Thomson), protons (Rutherford), and neutrons (Chadwick) proved atoms are divisible and have internal structure. Isotopes showed atoms of same element can differ. However, Dalton's theory still formed the foundation of modern chemistry.
This question belongs to: Science chemistry
Question #311
A mixture of salt and sand can be separated by:
A. Fractional distillation
B. Dissolution, filtration, and evaporation
C. Magnetic separation
D. Sublimation
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Correct Answer: Option B

Explanation:
Salt is soluble in water, sand is not. Add water, stir to dissolve salt, filter to separate sand. Then evaporate the filtrate to recover salt. Sublimation separates sublimable solids (NH₄Cl, naphthalene). Magnetic for magnetic materials. Fractional distillation for liquid mixtures. This is a standard separation technique.
This question belongs to: Science chemistry
Question #312
Which of the following is a chemical property of acids?
A. React with metals to produce hydrogen gas
B. Conduct electricity in solution
C. Turn blue litmus red
D. Sour taste
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Correct Answer: Option A

Explanation:
Reacting with metals (e.g., Zn + 2HCl → ZnCl₂ + H₂) to produce hydrogen gas is a chemical property because new substances are formed. Sour taste is physical sensation, color change of litmus is an indicator of chemical reaction but is not itself a chemical property; reactivity is. Conductivity is physical. Chemical properties describe a substance's ability to undergo chemical change.
This question belongs to: Science chemistry
Question #313
The number of atoms in a molecule of phosphorus (P₄) is:
A. 1
B. 4
C. 2
D. 8
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Correct Answer: Option B

Explanation:
White phosphorus exists as P₄ tetrahedral molecules, containing 4 phosphorus atoms. So atomicity is 4. Sulfur is S₈, oxygen is O₂, ozone O₃. Atomicity is the number of atoms in a molecule of an element. Phosphorus is a non-metal. Red phosphorus is polymeric but P₄ is the common molecular form.
This question belongs to: Science chemistry
Question #314
The mass number of an atom is the sum of:
A. Protons and neutrons
B. Protons and electrons
C. Electrons and neutrons
D. Protons, neutrons, and electrons
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Correct Answer: Option A

Explanation:
Mass number (A) = number of protons (Z) + number of neutrons (N). Electrons have negligible mass and are not counted. It is the total number of nucleons. Example: ¹²C has 6 protons + 6 neutrons = 12. Isotopes have same Z but different A due to varying N. Atomic mass (weight) is the weighted average.
This question belongs to: Science chemistry
Question #315
Isobars have different:
A. Atomic numbers
B. Both B and C
C. Numbers of neutrons
D. Mass numbers
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Correct Answer: Option B

Explanation:
Isobars are atoms of different elements, so they have different atomic numbers (protons). Since mass number is same, number of neutrons also differs. Example: ⁴⁰Ar (Z=18, N=22) and ⁴⁰Ca (Z=20, N=20). Mass number = 40 same. So both atomic number and neutrons differ. Option D is correct as both B and C are true.
This question belongs to: Science chemistry
Question #316
The valency of nitrogen in NO₂ is:
A. 2
B. 5
C. 4
D. 3
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Correct Answer: Option C

Explanation:
In NO₂, each oxygen has valency 2; with two oxygens, total combining capacity = 4. Thus nitrogen's covalency is 4. The structure involves a coordinate bond. Oxidation state of N is +4. Valency is number of bonds formed: N forms two single bonds and one coordinate bond with O, total 4 bonds. In NO, valency is 2.
This question belongs to: Science chemistry
Question #317
Which of the following contains ionic, covalent, and coordinate bonds?
A. CH₄
B. CO₂
C. NaCl
D. NH₄Cl
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Correct Answer: Option D

Explanation:
Ammonium chloride (NH₄Cl) consists of NH₄⁺ and Cl⁻ ions. Ionic bond between NH₄⁺ and Cl⁻. Within NH₄⁺, N-H are covalent bonds, with one coordinate bond formed by donation of N's lone pair to H⁺. So all three bond types are present. NaCl purely ionic. CO₂ purely covalent. CH₄ purely covalent.
This question belongs to: Science chemistry
Question #318
Electrovalency is the number of electrons lost or gained by an atom. The electrovalency of magnesium in MgCl₂ is:
A. 4
B. 1
C. 2
D. 6
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Correct Answer: Option C

Explanation:
Magnesium loses two electrons to achieve noble gas configuration (Mg → Mg²⁺), so its electrovalency is +2. In MgCl₂, Mg²⁺ and two Cl⁻. Electrovalency is the charge on the ion. Covalency is number of covalent bonds. For Group 2 metals, electrovalency is always +2 (loss of 2 electrons).
This question belongs to: Science chemistry
Question #319
Modern periodic law states that properties of elements are periodic functions of their:
A. Atomic masses
B. Number of isotopes
C. Atomic numbers
D. Mass numbers
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Correct Answer: Option C

Explanation:
Modern periodic law: The physical and chemical properties of elements are periodic functions of their atomic numbers. This was established by Moseley. Mendeleev's periodic law was based on atomic masses. Atomic number (Z) determines electronic configuration, which determines chemical behavior. Mass numbers vary with isotopes.
This question belongs to: Science chemistry
Question #320
Which of the following elements has the smallest atomic radius in Group 1?
A. Cesium
B. Sodium
C. Potassium
D. Lithium
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Correct Answer: Option D

Explanation:
Atomic radius increases down a group due to addition of shells. Lithium (Z=3, 2 shells) is the first element in Group 1 and has the smallest radius. Cesium has the largest. The effective nuclear charge also plays a role, but shell number dominates. Trend: Li < Na < K < Rb < Cs.
This question belongs to: Science chemistry

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