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 #481
Which scientist arranged elements in increasing order of atomic weights and grouped them by similar properties?
A. John Newlands
B. Henry Moseley
C. Antoine Lavoisier
D. Dmitri Mendeleev
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Correct Answer: Option D

Explanation:
Dmitri Mendeleev formulated the Periodic Law based on atomic masses and left gaps for undiscovered elements. Moseley later modified it to atomic number. Newlands gave Law of Octaves. Lavoisier classified elements into metals and non-metals.
This question belongs to: Science chemistry
Question #482
Which of the following is a compound?
A. Steel
B. Air
C. Oxygen
D. Water
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Correct Answer: Option D

Explanation:
Water (H₂O) is a compound – a pure substance composed of two or more elements chemically combined in a fixed ratio. Oxygen is an element. Air is a mixture of gases. Steel is an alloy (mixture). Compounds can be broken down by chemical methods.
This question belongs to: Science chemistry
Question #483
The process of separating ammonium chloride from a mixture of ammonium chloride and salt uses:
A. Sublimation
B. Filtration
C. Distillation
D. Evaporation
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Correct Answer: Option A

Explanation:
Ammonium chloride sublimes on heating; it converts directly from solid to vapor, leaving behind common salt. The vapors can be condensed on a cold surface to obtain solid NH₄Cl. This method works for any mixture where one component sublimes and the other does not.
This question belongs to: Science chemistry
Question #484
The atomic number of an element is 8. Its valency is:
A. 2
B. 6
C. 4
D. 8
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Correct Answer: Option A

Explanation:
Atomic number 8 is oxygen (2,6). It needs 2 more electrons to complete octet, so it gains 2 or shares 2 electrons. Its valency is 2 (typically). It can form two covalent bonds. In H₂O, oxygen forms two bonds.
This question belongs to: Science chemistry
Question #485
Ionic compounds have high melting points because:
A. The electrostatic forces between ions are strong
B. They are soft
C. They are covalent
D. They have weak intermolecular forces
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Correct Answer: Option A

Explanation:
Ionic compounds consist of positive and negative ions held together by strong electrostatic forces of attraction (ionic bonds). A large amount of energy is required to overcome these forces, resulting in high melting and boiling points. They also conduct electricity when molten or in solution.
This question belongs to: Science chemistry
Question #486
In the molecule BF₃, the type of bonding is:
A. Metallic
B. Covalent
C. Coordinate
D. Ionic
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Correct Answer: Option B

Explanation:
Boron trifluoride (BF₃) has covalent bonds where boron shares three electrons with three fluorine atoms. Although BF₃ can accept a lone pair to form coordinate bonds (as in BF₄⁻), its own bonds are polar covalent. Ionic bonding occurs between metal and non-metal.
This question belongs to: Science chemistry
Question #487
Which of the following is an element of Group 18?
A. Helium
B. Oxygen
C. Hydrogen
D. Nitrogen
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Correct Answer: Option A

Explanation:
Group 18 elements are noble gases: helium, neon, argon, krypton, xenon, radon. They have completely filled valence shells, making them chemically inert (though xenon can form compounds). Nitrogen is Group 15, Oxygen Group 16, Hydrogen is unique (Group 1).
This question belongs to: Science chemistry
Question #488
Which of the following has the largest atomic radius?
A. Cl
B. F
C. I
D. Br
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Correct Answer: Option C

Explanation:
Atomic radius increases down a group as the number of shells increases. Iodine (I) is in Period 5, below F (2), Cl (3), Br (4). Thus iodine has the largest atomic radius among these halogens.
This question belongs to: Science chemistry
Question #489
The reaction between an acid and a base to form salt and water is:
A. Reduction
B. Decomposition
C. Oxidation
D. Neutralization
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Correct Answer: Option D

Explanation:
Neutralization: Acid + Base → Salt + Water. Example: HCl + NaOH → NaCl + H₂O. It is a double displacement reaction. The pH of the resulting solution may be neutral, acidic, or basic depending on the strengths of the acid and base.
This question belongs to: Science chemistry
Question #490
In the reaction 2FeCl₃ + SnCl₂ → 2FeCl₂ + SnCl₄, which substance is reduced?
A. SnCl₂
B. FeCl₃
C. FeCl₂
D. SnCl₄
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Correct Answer: Option B

Explanation:
Fe in FeCl₃ goes from +3 to +2 in FeCl₂, gaining an electron – reduction. Sn in SnCl₂ goes from +2 to +4 in SnCl₄, losing electrons – oxidation. FeCl₃ is the oxidizing agent, SnCl₂ is the reducing agent.
This question belongs to: Science chemistry
Question #491
Which of the following salts is used in fire extinguishers?
A. Calcium sulfate
B. Sodium chloride
C. Sodium bicarbonate
D. Potassium nitrate
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Correct Answer: Option C

Explanation:
Sodium bicarbonate (NaHCO₃) is used in soda-acid fire extinguishers. It reacts with sulfuric acid to produce CO₂, which smothers the fire. Also, dry powder extinguishers use NaHCO₃. It is also baking soda.
This question belongs to: Science chemistry
Question #492
Which metal is used in the galvanization of iron?
A. Tin
B. Nickel
C. Chromium
D. Zinc
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Correct Answer: Option D

Explanation:
Galvanization coats iron with zinc. Zinc is more reactive, corrodes first (sacrificial protection). Tin coating protects only if intact. Chromium is used for plating, nickel for alloys, but galvanizing specifically uses zinc.
This question belongs to: Science chemistry
Question #493
Which of the following is an allotropic form of carbon used in lead pencils?
A. Fullerene
B. Graphite
C. Diamond
D. Charcoal
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Correct Answer: Option B

Explanation:
Graphite mixed with clay forms pencil 'lead'. Its layered structure marks paper. Diamond is used in cutting, fullerene in nanotechnology, charcoal in filters and fuel.
This question belongs to: Science chemistry
Question #494
The compound CH₄ is named:
A. Methane
B. Ethane
C. Methanol
D. Propane
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Correct Answer: Option A

Explanation:
One carbon atom with four hydrogens is methane (CH₄). Ethane is C₂H₆, propane C₃H₈. Methanol is CH₃OH (alcohol). Methane is the simplest alkane.
This question belongs to: Science chemistry
Question #495
Fossil fuels are formed from:
A. Direct combination of elements
B. Remains of dead plants and animals buried millions of years ago
C. Volcanic eruptions
D. Meteorites
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Correct Answer: Option B

Explanation:
Fossil fuels (coal, petroleum, natural gas) are formed from organic matter deposited in sedimentary layers, subjected to heat and pressure over geological timescales. They are non-renewable because their formation takes millions of years.
This question belongs to: Science chemistry
Question #496
The fuel with the highest calorific value per gram is:
A. Coal
B. Wood
C. Methane
D. Hydrogen
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Correct Answer: Option D

Explanation:
Hydrogen has about 141.9 kJ/g, the highest of any fuel. Methane ~55.5 kJ/g, coal ~25-35 kJ/g, wood ~17 kJ/g. Hydrogen produces only water on combustion, making it a clean fuel.
This question belongs to: Science chemistry
Question #497
During electrolysis of water, hydrogen is liberated at:
A. Cathode
B. Salt bridge
C. Both electrodes
D. Anode
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Correct Answer: Option A

Explanation:
Cathode (negative electrode): 2H₂O + 2e⁻ → H₂ + 2OH⁻ (or 2H⁺ + 2e⁻ → H₂). Hydrogen gas is collected at the cathode. At the anode, oxygen is produced: 2H₂O → O₂ + 4H⁺ + 4e⁻. The volume of H₂ is double that of O₂.
This question belongs to: Science chemistry
Question #498
The relationship P₁/T₁ = P₂/T₂ for a fixed mass of gas at constant volume is known as:
A. Boyle's law
B. Charles' law
C. Avogadro's law
D. Gay-Lussac's law
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Correct Answer: Option D

Explanation:
Gay-Lussac's law states that at constant volume, the pressure of a fixed mass of gas is directly proportional to its absolute temperature. P ∝ T, so P/T = constant. This is used in pressure cookers and autoclaves.
This question belongs to: Science chemistry
Question #499
A catalyst that slows down a reaction is called:
A. Negative catalyst (inhibitor)
B. Biocatalyst
C. Autocatalyst
D. Positive catalyst
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Correct Answer: Option A

Explanation:
A negative catalyst (inhibitor) decreases the rate of a reaction by increasing activation energy or destroying active intermediates. Example: phosphoric acid inhibits H₂O₂ decomposition. Positive catalysts speed up, enzymes are biocatalysts, autocatalysts are products that catalyze their own formation.
This question belongs to: Science chemistry
Question #500
Which of the following enzymes is present in tears and kills bacteria?
A. Trypsin
B. Lipase
C. Pepsin
D. Lysozyme
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Correct Answer: Option D

Explanation:
Lysozyme is an antibacterial enzyme found in tears, saliva, and egg white. It breaks down bacterial cell walls. Lipase digests fats, trypsin and pepsin digest proteins. Lysozyme is part of the innate immune system.
This question belongs to: Science chemistry

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