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 23 of 55
Question #441
Ozone layer depletion is mainly caused by:
A. Sulphur dioxide
B. Chlorofluorocarbons (CFCs)
C. Ammonia
D. Carbon monoxide
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Correct Answer: Option B

Explanation:
CFCs release chlorine atoms under UV radiation in the stratosphere, which catalytically destroy ozone molecules. CO causes smog and is poisonous, SO₂ causes acid rain, ammonia is not a major ozone depleter. The Montreal Protocol phased out CFCs to protect the ozone layer.
This question belongs to: Science chemistry
Question #442
Acid rain is primarily caused by the oxides of:
A. Carbon and hydrogen
B. Sodium and potassium
C. Calcium and magnesium
D. Nitrogen and sulfur
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Correct Answer: Option D

Explanation:
Acid rain results from SO₂ and NOₓ (oxides of nitrogen) dissolving in water vapor to form sulfuric and nitric acids. These are released from burning fossil fuels and industrial processes. Carbon dioxide forms weak carbonic acid (normal rain pH ~5.6), but SO₂ and NOₓ cause pH below 5.0.
This question belongs to: Science chemistry
Question #443
Which radiation is the most penetrating?
A. Alpha
B. Gamma
C. Beta
D. Positron
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Correct Answer: Option B

Explanation:
Gamma rays are electromagnetic waves with no mass and no charge, thus they interact least with matter and have the highest penetrating power. Alpha particles (He nuclei) are stopped by paper. Beta particles (electrons) can be stopped by aluminum. Gamma requires thick lead or concrete.
This question belongs to: Science chemistry
Question #444
Half-life of a radioactive substance is 5 years. After 10 years, how much of the original amount remains?
A. 1/8
B. 1/16
C. 1/2
D. 1/4
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Correct Answer: Option D

Explanation:
Number of half-lives = total time / half-life = 10/5 = 2. Fraction remaining = (1/2)ⁿ = (1/2)² = 1/4. Thus 25% remains. After 3 half-lives (15 years) it would be 1/8.
This question belongs to: Science chemistry
Question #445
The scientist who proposed the 'plum pudding' model of the atom was:
A. Niels Bohr
B. John Dalton
C. J. J. Thomson
D. Ernest Rutherford
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Correct Answer: Option C

Explanation:
J.J. Thomson discovered the electron and proposed the 'plum pudding' model (positive sphere with embedded electrons). Rutherford proposed the nuclear model after the gold foil experiment. Bohr gave quantized orbits. Dalton gave the solid sphere model.
This question belongs to: Science chemistry
Question #446
John Dalton is known for:
A. Neutron
B. Periodic table
C. Discovery of electron
D. Atomic theory
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Correct Answer: Option D

Explanation:
John Dalton put forward the atomic theory in 1808, proposing that matter is made of indivisible atoms, atoms of same element are identical, and compounds are combinations of atoms. Electron was discovered by J.J. Thomson, periodic table by Mendeleev, neutron by Chadwick.
This question belongs to: Science chemistry
Question #447
Which of the following is a homogeneous mixture?
A. Iron filings and sulfur
B. Oil and water
C. Salt in water
D. Sand in water
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Correct Answer: Option C

Explanation:
A homogeneous mixture has uniform composition throughout. Salt completely dissolves in water, forming a single phase (solution). Oil and water are immiscible (heterogeneous). Sand in water is suspension (heterogeneous). Iron filings and sulfur can be heterogeneous unless alloyed.
This question belongs to: Science chemistry
Question #448
The process of separating cream from milk uses:
A. Distillation
B. Sublimation
C. Centrifugation
D. Filtration
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Correct Answer: Option C

Explanation:
Centrifugation spins the mixture at high speed, causing denser particles (skimmed milk) to settle at the bottom while lighter cream floats on top. This is based on density differences. Distillation is for liquids, filtration for insoluble solids, sublimation for sublimable solids.
This question belongs to: Science chemistry
Question #449
The number of protons in an atom is equal to its:
A. Valency
B. Number of neutrons
C. Atomic number
D. Mass number
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Correct Answer: Option C

Explanation:
Atomic number (Z) is defined as the number of protons in the nucleus of an atom. It determines the identity of the element. In a neutral atom, the number of electrons equals Z. Mass number = protons + neutrons.
This question belongs to: Science chemistry
Question #450
Which of the following compounds has an electrovalent bond?
A. CH₄
B. CaO
C. NH₃
D. CCl₄
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Correct Answer: Option B

Explanation:
CaO is an ionic compound: Ca loses 2 electrons to form Ca²⁺, O gains 2 electrons to form O²⁻. CH₄, CCl₄, NH₃ are covalent molecules where electrons are shared. Electrovalent compounds are formed between metals and non-metals with a large electronegativity difference.
This question belongs to: Science chemistry
Question #451
The bond in nitrogen molecule (N₂) is:
A. Triple bond
B. Single bond
C. Ionic bond
D. Double bond
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Correct Answer: Option A

Explanation:
N₂ has a triple bond (N≡N), consisting of one sigma and two pi bonds. This is the strongest type of covalent bond, giving N₂ high stability and low reactivity. Bond order is 3. The molecule is diatomic and nonpolar.
This question belongs to: Science chemistry
Question #452
In the molecule CO, there is a coordinate covalent bond. Which atom donates the electron pair?
A. Neither; it's ionic
B. Carbon
C. Both share equally
D. Oxygen
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Correct Answer: Option D

Explanation:
Carbon monoxide (CO) has a triple bond. In the formation, oxygen, with two lone pairs, donates one lone pair to form a coordinate bond with carbon after normal double bond formation, resulting in a structure :C≡O:. Thus oxygen donates the pair. This satisfies the octet for both.
This question belongs to: Science chemistry
Question #453
Which element is in Group 2, Period 4 of the periodic table?
A. Ca
B. Be
C. Sr
D. Mg
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Correct Answer: Option A

Explanation:
Calcium (Ca, Z=20) has configuration 2,8,8,2. 4 shells → Period 4; 2 valence electrons → Group 2. Mg is Group 2, Period 3. Sr is Group 2, Period 5. Be is Group 2, Period 2.
This question belongs to: Science chemistry
Question #454
Ionization energy generally increases across a period because:
A. Effective nuclear charge increases
B. Atomic radius increases
C. Nuclear charge decreases
D. Number of shells increases
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Correct Answer: Option A

Explanation:
Across a period, electrons are added to the same shell, but the number of protons increases, leading to a greater effective nuclear charge. This pulls electrons closer, decreasing atomic radius and making it harder to remove an electron, so ionization energy increases.
This question belongs to: Science chemistry
Question #455
Which of the following is a displacement reaction?
A. NaOH + HCl → NaCl + H₂O
B. 2H₂O → 2H₂ + O₂
C. 2Na + Cl₂ → 2NaCl
D. CuSO₄ + Fe → FeSO₄ + Cu
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Correct Answer: Option D

Explanation:
In displacement reactions, a more reactive element displaces a less reactive one from its compound. Here, iron (more reactive than copper) displaces copper from copper sulfate. A is combination; B is decomposition; D is neutralization (double displacement).
This question belongs to: Science chemistry
Question #456
Which of the following is an example of a redox reaction?
A. NaCl + AgNO₃ → AgCl + NaNO₃
B. NH₃ + HCl → NH₄Cl
C. Zn + 2HCl → ZnCl₂ + H₂
D. CaO + H₂O → Ca(OH)₂
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Correct Answer: Option C

Explanation:
In Zn + 2HCl, Zn is oxidized (0 to +2) and H⁺ is reduced (+1 to 0). So it is a redox reaction. The other reactions are double displacement (A), combination without oxidation state change (B, D). Redox requires change in oxidation states.
This question belongs to: Science chemistry
Question #457
Which indicator remains colorless in acidic solution and turns pink in basic solution?
A. Phenolphthalein
B. Litmus
C. Turmeric
D. Methyl orange
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Correct Answer: Option A

Explanation:
Phenolphthalein is colorless in acidic and neutral solutions (pH < 8.3) and turns pink to deep red in basic solutions (pH > 10). Methyl orange is red in acid, yellow in base. Litmus is red in acid, blue in base. Turmeric is yellow in acid, reddish-brown in base.
This question belongs to: Science chemistry
Question #458
Common salt (NaCl) is a neutral salt because it is formed from:
A. Strong acid and strong base
B. Weak acid and strong base
C. Strong acid and weak base
D. Weak acid and weak base
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Correct Answer: Option A

Explanation:
NaCl is the product of neutralization of HCl (strong acid) and NaOH (strong base). Neither ion hydrolyses appreciably, so the solution remains neutral (pH=7). Salts of strong acid + weak base are acidic (NH₄Cl), weak acid + strong base are basic (CH₃COONa).
This question belongs to: Science chemistry
Question #459
Which of the following metals reacts vigorously with cold water?
A. Copper
B. Sodium
C. Zinc
D. Iron
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Correct Answer: Option B

Explanation:
Sodium (alkali metal) reacts violently with cold water: 2Na + 2H₂O → 2NaOH + H₂ + heat. Copper and iron do not react with cold water; iron reacts with steam. Zinc also reacts with steam. Reactivity with water increases down Group 1.
This question belongs to: Science chemistry
Question #460
An alloy of mercury is called:
A. Amalgam
B. Brass
C. Solder
D. Bronze
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Correct Answer: Option A

Explanation:
An amalgam is an alloy of mercury with another metal. For example, sodium amalgam is used as a reducing agent; dental amalgam contains mercury, silver, tin, and copper. Bronze is Cu+Sn, brass is Cu+Zn, solder is Pb+Sn.
This question belongs to: Science chemistry

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