Chemistry MCQs

Question #781

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Which reaction type is 2Na + 2H₂O → 2NaOH + H₂?

A. Displacement

B. Double displacement

C. Combination

D. Decomposition

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Correct Answer: Option A

Explanation:
Sodium displaces hydrogen from water, a single displacement reaction. It is also a redox reaction (Na oxidized, H⁺ reduced). Not combination (one product), not decomposition (one reactant), not double displacement.

This question belongs to: Science chemistry

Question #782

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In the reaction MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O, HCl is:

A. Catalyst

B. Reducing agent

C. Oxidizing agent

D. Dehydrating agent

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Correct Answer: Option B

Explanation:
HCl is oxidized (Cl⁻ → Cl₂, loss of electrons), so it acts as the reducing agent. MnO₂ is reduced (Mn⁴⁺ → Mn²⁺) and is the oxidizing agent.

This question belongs to: Science chemistry

Question #783

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A solution with [H⁺] = 10⁻⁹ M has pH:

A. 14

B. 9

C. 5

D. 7

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Correct Answer: Option B

Explanation:
pH = -log[H⁺] = -log(10⁻⁹) = 9. The solution is basic (pOH = 5). pH>7 indicates basic nature. Pure water has [H⁺] = 10⁻⁷.

This question belongs to: Science chemistry

Question #784

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Common salt is obtained from sea water by:

A. Filtration

B. Distillation

C. Sublimation

D. Evaporation

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Correct Answer: Option D

Explanation:
Sea water is trapped in shallow pans and allowed to evaporate under sunlight, leaving behind crude salt. This is then purified. Distillation gives pure water, not salt.

This question belongs to: Science chemistry

Question #785

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Which of the following metals is stored under kerosene?

A. Zinc

B. Iron

C. Copper

D. Sodium

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Correct Answer: Option D

Explanation:
Sodium reacts vigorously with moisture and oxygen in air, so it is stored under kerosene to prevent contact. Potassium, lithium are similarly stored. Iron, zinc, copper do not require such storage.

This question belongs to: Science chemistry

Question #786

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The alloy brass is composed of:

A. Copper and tin

B. Copper and zinc

C. Iron and chromium

D. Aluminium and magnesium

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Correct Answer: Option B

Explanation:
Brass: Cu + Zn. Bronze: Cu + Sn. Stainless steel: Fe + Cr + Ni. Duralumin: Al + Cu + Mg. Brass is used for musical instruments and utensils.

This question belongs to: Science chemistry

Question #787

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Carbon forms bonds by:

A. Losing 4 electrons

B. Gaining 4 electrons

C. Ionic transfer

D. Sharing 4 electrons

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Correct Answer: Option D

Explanation:
Carbon has 4 valence electrons; it forms four covalent bonds by sharing electrons to achieve octet. It does not form ionic bonds easily due to high ionization energy and electron affinity.

This question belongs to: Science chemistry

Question #788

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Petroleum refining involves:

A. Sublimation

B. Fractional distillation

C. Electrolysis

D. Crystallization

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Correct Answer: Option B

Explanation:
Crude oil is separated into fractions (petrol, kerosene, diesel, etc.) based on boiling points by fractional distillation. Cracking, reforming are secondary processes.

This question belongs to: Science chemistry

Question #789

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Which fuel has the lowest calorific value?

A. Methane

B. Hydrogen

C. Coal

D. Wood

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Correct Answer: Option D

Explanation:
Wood has lower calorific value (~17 kJ/g) compared to coal (~25-35 kJ/g), methane (~55.5 kJ/g), and hydrogen (~142 kJ/g). It contains moisture and oxygen, reducing energy density.

This question belongs to: Science chemistry

Question #790

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In an electrolytic cell, the cathode is the electrode where:

A. Oxidation occurs

B. Reduction occurs

C. Electrons enter the external circuit

D. Anions are discharged

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Correct Answer: Option B

Explanation:
Cathode is where reduction takes place (gain of electrons). In electrolytic cells, cathode is negative (connected to negative terminal of battery). Cations migrate to cathode and are reduced.

This question belongs to: Science chemistry

Question #791

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The combined gas law is represented by:

A. P₁V₁/T₁ = P₂V₂/T₂

B. PV = constant

C. PV = nRT

D. V/T = constant

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Correct Answer: Option A

Explanation:
Combined gas law combines Boyle's, Charles', and Gay-Lussac's laws for a fixed mass of gas. It relates P, V, T when all change. PV=nRT is ideal gas equation for n moles.

This question belongs to: Science chemistry

Question #792

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Graham's law relates rates of diffusion to:

A. Molar mass

B. Pressure

C. Volume

D. Temperature

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Correct Answer: Option A

Explanation:
Rate ∝ 1/√(molar mass) at constant T and P. Lighter gases diffuse faster. This is Graham's law of diffusion/effusion.

This question belongs to: Science chemistry

Question #793

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An example of a negative catalyst is:

A. V₂O₅ in Contact process

B. Fe in Haber process

C. MnO₂ in decomposition of KClO₃

D. Alcohol in chloroform

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Correct Answer: Option D

Explanation:
Ethanol is added to chloroform to inhibit its oxidation to phosgene (COCl₂) – a negative catalyst. Others are positive catalysts that speed up reactions.

This question belongs to: Science chemistry

Question #794

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Enzymes are made up of:

A. Proteins

B. Fats

C. Carbohydrates

D. Nucleic acids

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Correct Answer: Option A

Explanation:
Enzymes are proteins (except ribozymes which are RNA). They have specific 3D structures with active sites. They catalyze biological reactions. Denatured by heat or extreme pH.

This question belongs to: Science chemistry

Question #795

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The extraction of aluminium is done by:

A. Thermite process

B. Reduction with aluminium

C. Electrolysis of molten alumina in cryolite

D. Reduction with carbon

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Correct Answer: Option C

Explanation:
Aluminium is highly reactive; its oxide cannot be reduced by carbon. Hall-Héroult process electrolyzes alumina dissolved in molten cryolite (Na₃AlF₆) using carbon anodes.

This question belongs to: Science chemistry

Question #796

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Which fertilizer provides both nitrogen and phosphorus?

A. SSP

B. MOP

C. Urea

D. DAP

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Correct Answer: Option D

Explanation:
DAP (diammonium phosphate) contains N (18%) and P₂O₅ (46%). Urea only N; MOP (muriate of potash) only K; SSP only P. NPK mixtures provide all three.

This question belongs to: Science chemistry

Question #797

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Caustic soda is produced along with chlorine and hydrogen by electrolysis of:

A. Molten NaCl

B. NaOH solution

C. HCl

D. Brine (concentrated NaCl solution)

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Correct Answer: Option D

Explanation:
Chlor-alkali process: electrolysis of brine gives NaOH (at cathode), Cl₂ (at anode), and H₂ (at cathode). Molten NaCl gives Na and Cl₂, not NaOH.

This question belongs to: Science chemistry

Question #798

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Which of the following is used as an antiseptic and bleaching agent?

A. Hydrogen peroxide

B. Acetone

C. Methanol

D. Ethanol

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Correct Answer: Option A

Explanation:
H₂O₂ releases oxygen that kills bacteria (antiseptic) and bleaches colored materials. Ethanol is disinfectant but not strong bleach. Methanol is toxic.

This question belongs to: Science chemistry

Question #799

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The alcohol present in alcoholic beverages is:

A. Propanol

B. Methanol

C. Butanol

D. Ethanol

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Correct Answer: Option D

Explanation:
Ethanol (C₂H₅OH) is the intoxicating ingredient in beer, wine, spirits. Methanol is toxic. Propanol and butanol are used as solvents, not beverages.

This question belongs to: Science chemistry

Question #800

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Detergents are preferred over soaps in hard water because they:

A. Contain more carbon

B. Form soluble calcium and magnesium salts

C. Are cheaper

D. Are natural

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Correct Answer: Option B

Explanation:
Detergents (e.g., sulfonates) do not precipitate with Ca²⁺/Mg²⁺, so they lather well in hard water. Soaps form insoluble scum. This is a major advantage.

This question belongs to: Science chemistry

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