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 #761
Which of the following is used as a chemical in fire extinguishers?
A. Calcium oxide
B. Sodium chloride
C. Magnesium sulfate
D. Sodium bicarbonate
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Correct Answer: Option D

Explanation:
NaHCO₃ decomposes on heating, releasing CO₂ which smothers fire. Also used in soda-acid extinguishers with H₂SO₄ to produce CO₂. NaCl is table salt, CaO is quicklime, MgSO₄ is Epsom salt.
This question belongs to: Science chemistry
Question #762
Ethanol reacts with sodium metal to produce:
A. Oxygen
B. Ethane
C. Carbon dioxide
D. Hydrogen gas
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Correct Answer: Option D

Explanation:
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑. Alcohols react with alkali metals to form alkoxides and hydrogen, similar to water but slower.
This question belongs to: Science chemistry
Question #763
Soap is prepared from fats and oils by heating with:
A. NaCl solution
B. Ethanol
C. NaOH solution
D. HCl
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Correct Answer: Option C

Explanation:
Saponification: Fat/oil + NaOH → glycerol + sodium salt of fatty acid (soap). NaCl is used to precipitate soap from solution (salting out).
This question belongs to: Science chemistry
Question #764
Cement sets under water because of:
A. Evaporation
B. Oxidation
C. Hydration and hydrolysis of calcium silicates
D. Polymerization
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Correct Answer: Option C

Explanation:
Cement sets by hydration reactions forming C-S-H gel and Ca(OH)₂, even underwater. It's a chemical process, not drying. Gypsum controls the setting rate.
This question belongs to: Science chemistry
Question #765
Polyethylene is a:
A. Elastomer
B. Thermoplastic
C. Natural polymer
D. Thermosetting plastic
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Correct Answer: Option B

Explanation:
Polyethylene (polythene) softens on heating and hardens on cooling, so it is thermoplastic. Bakelite is thermosetting; rubber is elastomer; silk is natural polymer.
This question belongs to: Science chemistry
Question #766
Vulcanization of rubber involves heating with:
A. Carbon
B. Sulfur
C. Nitrogen
D. Oxygen
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Correct Answer: Option B

Explanation:
Vulcanization cross-links rubber polymer chains with sulfur bridges, improving elasticity, strength, and durability. Invented by Charles Goodyear. It converts raw rubber into useful form.
This question belongs to: Science chemistry
Question #767
Boiling water removes:
A. Temporary hardness
B. All dissolved salts
C. Permanent hardness
D. Microorganisms only
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Correct Answer: Option A

Explanation:
Boiling precipitates Ca/Mg bicarbonates as carbonates, removing temporary hardness. It does not remove permanent hardness (sulfates/chlorides). It kills microbes but does not remove dissolved salts.
This question belongs to: Science chemistry
Question #768
Which disinfectant is used in water treatment that does not leave any residual taste or odor?
A. Chlorine
B. Iodine
C. Bromine
D. Ozone
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Correct Answer: Option D

Explanation:
Ozone (O₃) is a powerful disinfectant that decomposes to O₂, leaving no residual taste or odor. However, it must be generated on-site and provides no lasting protection. Chlorine leaves residual.
This question belongs to: Science chemistry
Question #769
Which of the following gases is a major contributor to the greenhouse effect?
A. Oxygen
B. Carbon dioxide
C. Ammonia
D. Nitrogen
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Correct Answer: Option B

Explanation:
CO₂ is the primary anthropogenic greenhouse gas from fossil fuel combustion. N₂, O₂ not IR active. Ammonia is not a major GHG. Methane, N₂O also contribute.
This question belongs to: Science chemistry
Question #770
Radioactive decay follows:
A. Pseudo-first order
B. First-order kinetics
C. Zero-order kinetics
D. Second-order kinetics
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Correct Answer: Option B

Explanation:
Radioactive decay is a first-order process because the rate of decay is proportional to the number of radioactive nuclei present. Half-life is constant, independent of initial amount.
This question belongs to: Science chemistry
Question #771
J.J. Thomson is credited with the discovery of:
A. Nucleus
B. Proton
C. Electron
D. Neutron
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Correct Answer: Option C

Explanation:
Thomson used cathode ray tubes to discover the electron and measure its charge-to-mass ratio (e/m). He proposed the 'plum pudding' model. Proton identified by Rutherford, neutron by Chadwick.
This question belongs to: Science chemistry
Question #772
Dmitri Mendeleev's periodic table was based on:
A. Atomic number
B. Number of neutrons
C. Atomic mass and chemical properties
D. Mass number
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Correct Answer: Option C

Explanation:
Mendeleev arranged elements in order of increasing atomic mass, grouping those with similar properties. He left gaps for undiscovered elements. Modern table uses atomic number.
This question belongs to: Science chemistry
Question #773
Which of the following is a pure substance?
A. Sea water
B. Oxygen gas
C. Brass
D. Milk
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Correct Answer: Option B

Explanation:
Oxygen (O₂) is an element, a pure substance. Milk is a colloid; brass is an alloy (mixture); sea water is a solution (mixture). Pure substances have fixed composition.
This question belongs to: Science chemistry
Question #774
Crystallization is used to purify:
A. Solids
B. Gases
C. Liquids
D. Plasma
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Correct Answer: Option A

Explanation:
Crystallization separates a pure solid from a solution by cooling or evaporating the solvent. Impurities remain in the mother liquor. Example: purifying copper sulfate. Distillation for liquids; filtration for insoluble solids.
This question belongs to: Science chemistry
Question #775
The neutron was discovered by:
A. Thomson
B. Chadwick
C. Rutherford
D. Bohr
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Correct Answer: Option B

Explanation:
James Chadwick discovered the neutron in 1932. It is a neutral particle with mass almost equal to proton. Its discovery explained isotopes' mass differences.
This question belongs to: Science chemistry
Question #776
Atoms of different elements having same mass number are called:
A. Isotopes
B. Isomers
C. Isotones
D. Isobars
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Correct Answer: Option D

Explanation:
Isobars: same mass number, different atomic number. Example: ⁴⁰Ar, ⁴⁰Ca. Isotopes: same atomic number, different mass number. Isotones: same number of neutrons. Isomers: same molecular formula, different structure.
This question belongs to: Science chemistry
Question #777
The valency of phosphorus in PH₃ is:
A. 2
B. 1
C. 5
D. 3
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Correct Answer: Option D

Explanation:
In PH₃, phosphorus forms three covalent bonds with three hydrogen atoms, so its valency is 3. It has one lone pair. Its electronic configuration is 2,8,5; shares three electrons.
This question belongs to: Science chemistry
Question #778
The bond in HF is:
A. Non-polar covalent
B. Polar covalent
C. Coordinate
D. Ionic
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Correct Answer: Option B

Explanation:
HF has a polar covalent bond due to large electronegativity difference between H (2.1) and F (4.0), causing unequal sharing. It is not ionic because the difference is not extreme enough (>1.7 is typical but borderline). The bond is polar.
This question belongs to: Science chemistry
Question #779
Which group of elements is known as halogens?
A. Group 18
B. Group 2
C. Group 17
D. Group 1
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Correct Answer: Option C

Explanation:
Group 17 (F, Cl, Br, I, At) are halogens (salt formers). They have 7 valence electrons, high electronegativity, and are highly reactive non-metals. Group 1 are alkali metals, 18 noble gases, 2 alkaline earth metals.
This question belongs to: Science chemistry
Question #780
Ionization energy is maximum for:
A. Mg
B. Cl
C. Al
D. Na
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Correct Answer: Option B

Explanation:
Across period 3, ionization energy increases from Na to Ar. Chlorine has high ionization energy due to high effective nuclear charge and small atomic radius, though Argon is highest. Among these, Cl has highest IE.
This question belongs to: Science chemistry

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