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 36 of 55
Question #701
The pH of a solution is 4.5. What is the nature of the solution?
A. Weakly acidic
B. Basic
C. Neutral
D. Strongly acidic
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Correct Answer: Option A

Explanation:
pH < 7 indicates acidic. Since 4.5 is less than 7 but not extremely low (like 1-2), it is weakly acidic. Neutral pH=7, basic pH>7. A solution with pH 4 is more acidic than pH 5.
This question belongs to: Science chemistry
Question #702
Which indicator is red in acidic medium and yellow in basic medium?
A. Phenolphthalein
B. Litmus
C. Methyl orange
D. Turmeric
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Correct Answer: Option C

Explanation:
Methyl orange is red at pH < 3.1 and yellow at pH > 4.4. Litmus: red in acid, blue in base. Phenolphthalein: colorless in acid, pink in base. Turmeric: yellow in acid, reddish-brown in base.
This question belongs to: Science chemistry
Question #703
Sodium carbonate is a salt of:
A. Strong acid and strong base
B. Strong acid and weak base
C. Weak acid and weak base
D. Weak acid and strong base
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Correct Answer: Option D

Explanation:
Na₂CO₃ is formed from NaOH (strong base) and H₂CO₃ (weak acid). Its aqueous solution is basic due to hydrolysis: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻. Strong acid + strong base salts are neutral (NaCl).
This question belongs to: Science chemistry
Question #704
Baking soda on heating produces:
A. Na and CO₂
B. Na₂CO₃, CO₂, and H₂O
C. NaHCO₃ only
D. Na₂O and CO₂
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Correct Answer: Option B

Explanation:
2NaHCO₃ (heat) → Na₂CO₃ + CO₂ + H₂O. This is used in baking as CO₂ causes dough to rise. The sodium carbonate formed gives a bitter taste if excess soda is used, hence baking powder (with acid) is preferred.
This question belongs to: Science chemistry
Question #705
Washing soda (Na₂CO₃·10H₂O) is obtained by:
A. Recrystallization of sodium carbonate from its aqueous solution
B. Heating baking soda
C. Reaction of NaCl with CaCO₃
D. Electrolysis of brine
Discuss
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Correct Answer: Option A

Explanation:
Sodium carbonate (soda ash) is dissolved in water and crystallized to obtain washing soda. Heating baking soda gives anhydrous sodium carbonate. Brine electrolysis gives NaOH, Cl₂, H₂. Solvay process makes Na₂CO₃.
This question belongs to: Science chemistry
Question #706
Plaster of Paris should be stored in a moisture-proof container because:
A. It evaporates
B. It sublimes
C. It reacts with moisture to form a hard solid (gypsum)
D. It dissolves in water
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Correct Answer: Option C

Explanation:
CaSO₄·½H₂O readily absorbs moisture to set to CaSO₄·2H₂O (gypsum), becoming hard and useless for molding. So it must be kept dry. Reaction: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
This question belongs to: Science chemistry
Question #707
The reaction of zinc with dilute nitric acid does NOT produce hydrogen because:
A. Nitric acid is a strong oxidizing agent and oxidizes H₂ to water
B. Zinc does not react with acids
C. Nitric acid contains no hydrogen
D. Zinc is less reactive than hydrogen
Discuss
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Correct Answer: Option A

Explanation:
Dilute HNO₃ is an oxidizing agent; it oxidizes the hydrogen produced to water and itself gets reduced to nitrogen oxides (NO, N₂O, etc.). Very dilute HNO₃ with Mg/Mn can produce H₂. Metals above hydrogen normally displace H₂ from non-oxidizing acids like HCl.
This question belongs to: Science chemistry
Question #708
Stainless steel contains iron, chromium, and often nickel. Which element provides the corrosion resistance?
A. Carbon
B. Chromium
C. Nickel
D. Iron
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Correct Answer: Option B

Explanation:
Chromium (minimum 10.5%) forms a thin, adherent, transparent oxide layer (Cr₂O₃) on the surface, preventing further oxidation. Nickel adds ductility and enhances corrosion resistance. Iron is the base; carbon is for hardness but not corrosion resistance.
This question belongs to: Science chemistry
Question #709
Graphite is used as a dry lubricant because:
A. It is a liquid
B. It is soft like diamond
C. It has a layered structure that allows sheets to slide
D. It reacts with the metal surface
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Correct Answer: Option C

Explanation:
Graphite's carbon atoms are arranged in layers held together by weak van der Waals forces. Layers can slide over each other, reducing friction. It is stable at high temperatures, so used where oil would decompose.
This question belongs to: Science chemistry
Question #710
The simplest alkene is:
A. Methane
B. Ethane
C. Ethene
D. Propyne
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Correct Answer: Option C

Explanation:
Alkenes contain at least one carbon-carbon double bond. The simplest alkene is ethene (C₂H₄). Methane is an alkane; ethane is an alkane; propyne is an alkyne. General formula for alkenes: CₙH₂ₙ.
This question belongs to: Science chemistry
Question #711
Which of the following is a product of incomplete combustion of hydrocarbon?
A. NaOH
B. CO₂
C. CO
D. H₂O
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Correct Answer: Option C

Explanation:
Incomplete combustion due to insufficient oxygen produces carbon monoxide (CO) and/or soot (C). Complete combustion gives CO₂ and H₂O. CO is poisonous, binds to hemoglobin.
This question belongs to: Science chemistry
Question #712
The calorific value of a fuel is measured using a:
A. Manometer
B. Barometer
C. Thermometer
D. Bomb calorimeter
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Correct Answer: Option D

Explanation:
A bomb calorimeter burns a known mass of fuel in excess oxygen inside a sealed bomb immersed in water. The temperature rise of water is measured to calculate heat released. Thermometer is part of it but not the device itself.
This question belongs to: Science chemistry
Question #713
During electrolysis of molten NaCl, the product at anode is:
A. Oxygen
B. Hydrogen
C. Sodium
D. Chlorine
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Correct Answer: Option D

Explanation:
At anode (oxidation): 2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas). At cathode (reduction): Na⁺ + e⁻ → Na (liquid sodium). In aqueous NaCl, due to water, cathode produces H₂ and anode Cl₂ (or O₂ depending on concentration).
This question belongs to: Science chemistry
Question #714
Faraday's first law: mass deposited ∝ quantity of electricity. The constant of proportionality (electrochemical equivalent) depends on:
A. Time
B. Temperature
C. Nature of the substance
D. Current
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Correct Answer: Option C

Explanation:
Electrochemical equivalent (Z) is mass deposited per unit charge. It depends on molar mass and number of electrons transferred (valency). Z = M/(nF). It differs for different substances. Current and time affect total charge but not Z.
This question belongs to: Science chemistry
Question #715
Boyle's law is valid only when:
A. Only mass is constant
B. Pressure and volume are constant
C. Only temperature is constant
D. Temperature and mass of gas are constant
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Correct Answer: Option D

Explanation:
Boyle's law: P ∝ 1/V at constant temperature and for a fixed mass (number of moles) of gas. If T or n changes, the simple inverse relationship does not hold.
This question belongs to: Science chemistry
Question #716
According to Charles' law, at constant pressure, volume of a gas is directly proportional to:
A. Pressure
B. Density
C. Absolute temperature (Kelvin)
D. Temperature in Celsius
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Correct Answer: Option C

Explanation:
V ∝ T (in Kelvin). Using Celsius gives a linear relationship but not direct proportionality (line does not pass through origin). Absolute zero (0 K = -273.15°C) is the theoretical temperature of zero volume.
This question belongs to: Science chemistry
Question #717
Which of the following gases will diffuse slowest under identical conditions?
A. H₂
B. He
C. O₂
D. SO₂
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Correct Answer: Option D

Explanation:
Graham's law: rate ∝ 1/√M. Higher molar mass → slower diffusion. Molar masses: H₂=2, He=4, O₂=32, SO₂=64. SO₂ has highest mass, diffuses slowest. H₂ diffuses fastest.
This question belongs to: Science chemistry
Question #718
A catalyst works by:
A. Lowering the activation energy
B. Increasing the energy of reactants
C. Increasing the heat of reaction
D. Consuming itself in the reaction
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Correct Answer: Option A

Explanation:
Catalysts provide an alternative reaction pathway with lower activation energy, increasing the fraction of effective collisions. They are not consumed and do not alter the overall enthalpy change (ΔH) or equilibrium constant.
This question belongs to: Science chemistry
Question #719
Which enzyme converts glucose into ethanol?
A. Urease
B. Maltase
C. Zymase
D. Invertase
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Correct Answer: Option C

Explanation:
Zymase, a complex of enzymes from yeast, catalyzes fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Invertase hydrolyzes sucrose; urease hydrolyzes urea; maltase converts maltose to glucose.
This question belongs to: Science chemistry
Question #720
The ore of mercury is:
A. Galena
B. Magnetite
C. Cinnabar
D. Bauxite
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Correct Answer: Option C

Explanation:
Cinnabar is HgS, the chief ore of mercury. Bauxite (Al), galena (Pb), magnetite (Fe). Mercury is extracted by roasting cinnabar in air: HgS + O₂ → Hg + SO₂.
This question belongs to: Science chemistry

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