Chemistry MCQs

Question #641

Assertion (A): Methanol is highly toxic and can cause blindness if ingested. Reason (R): Methanol is metabolized in the body to formaldehyde and formic acid.

A. A is true but R is false.

B. Both A and R are true and R is the correct explanation of A.

C. Both A and R are true but R is NOT the correct explanation of A.

D. A is false but R is true.

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Correct Answer: Option B

Explanation:
Methanol (wood alcohol) is highly toxic. When ingested, it is metabolized by the enzyme alcohol dehydrogenase into formaldehyde and then formic acid. Formic acid interferes with mitochondrial function, leading to metabolic acidosis, optic nerve damage, and blindness. Both statements are correct and R explains A.

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Question #642

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Sodium hydroxide is manufactured by the electrolysis of aqueous sodium chloride in the:

A. Haber process

B. Solvay process

C. Chlor-alkali process

D. Contact process

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Correct Answer: Option C

Explanation:
The Chlor-alkali process involves the electrolysis of aqueous NaCl (brine). It produces sodium hydroxide (NaOH) at the cathode, chlorine gas (Cl₂) at the anode, and hydrogen gas (H₂) at the cathode. The Solvay process produces sodium carbonate, not sodium hydroxide.

This question belongs to: Science chemistry

Question #643

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The process of saponification involves the hydrolysis of fats/oils with:

A. Alcohol

B. Strong base

C. Dilute acid

D. Water alone

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Correct Answer: Option B

Explanation:
Saponification is the alkaline hydrolysis of fats or oils (which are esters of fatty acids) using a strong base like NaOH or KOH. This reaction produces glycerol and the sodium or potassium salts of fatty acids, which are soaps. It is an irreversible reaction.

This question belongs to: Science chemistry

Question #644

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Detergents are chemically:

A. Sodium salts of higher fatty acids

B. Sodium salts of sulphonic acids

C. Potassium salts of higher fatty acids

D. Calcium salts of fatty acids

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Correct Answer: Option B

Explanation:
Synthetic detergents are primarily sodium salts of long-chain alkyl benzene sulphonic acids or alkyl hydrogen sulphates. Unlike soaps (which are carboxylate salts), detergents do not form insoluble precipitates with calcium and magnesium ions in hard water, making them effective cleansers.

This question belongs to: Science chemistry

Question #645

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The main raw material for the manufacture of cement is:

A. Gypsum and sand

B. Sand and clay

C. Limestone and clay

D. Limestone and gypsum

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Correct Answer: Option C

Explanation:
Portland cement is primarily made by heating a mixture of limestone (calcium carbonate) and clay (aluminosilicates) in a kiln at about 1450°C to form clinker. A small amount of gypsum is then added to the clinker to control the setting time, but limestone and clay are the main raw materials.

This question belongs to: Science chemistry

Question #646

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The addition of gypsum to cement clinker is done to:

A. Slow down the initial setting time.

B. Increase the strength of cement.

C. Change the color of cement.

D. Make it waterproof.

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Correct Answer: Option A

Explanation:
Gypsum (CaSO₄·2H₂O) is added to cement clinker (about 3-5%) to retard the setting time. Without gypsum, cement would set almost immediately upon adding water (flash set) due to the rapid reaction of tricalcium aluminate. Gypsum allows sufficient time for mixing and placing.

This question belongs to: Science chemistry

Question #647

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Which plastic is used for making non-stick cookware?

A. Polythene

B. Teflon

C. PVC

D. Bakelite

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Correct Answer: Option B

Explanation:
Teflon (Polytetrafluoroethylene or PTFE) is used as a non-stick coating for cookware because of its extreme chemical inertness, high heat resistance, and low friction coefficient. Polythene and PVC melt at lower temperatures, and Bakelite is a thermosetting plastic used for electrical switches.

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Question #648

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Assertion (A): Detergents are more effective than soaps in hard water. Reason (R): Detergents do not form insoluble precipitates with calcium and magnesium ions.

A. Both A and R are true but R is NOT the correct explanation of A.

B. Both A and R are true and R is the correct explanation of A.

C. A is false but R is true.

D. A is true but R is false.

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Correct Answer: Option B

Explanation:
Soaps react with Ca²⁺ and Mg²⁺ ions in hard water to form insoluble scum (calcium/magnesium stearate), wasting soap and reducing cleaning efficiency. Detergents form soluble salts with these ions, so they lather well and clean effectively in hard water. Both statements are correct and R explains A.

This question belongs to: Science chemistry

Question #649

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Vulcanization of rubber involves heating it with:

A. Nitrogen

B. Oxygen

C. Carbon

D. Sulphur

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Correct Answer: Option D

Explanation:
Vulcanization is the process of heating natural rubber with sulphur (usually at 100-140°C) to improve its properties. Sulphur forms cross-links (disulfide bridges) between the polymer chains, making the rubber harder, more elastic, and less sensitive to temperature changes.

This question belongs to: Science chemistry

Question #650

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Hardness of water is mainly caused by the presence of dissolved salts of:

A. Calcium and Magnesium

B. Iron and Zinc

C. Sodium and Potassium

D. Copper and Lead

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Correct Answer: Option A

Explanation:
Water hardness is primarily caused by the presence of dissolved multivalent cations, most commonly calcium (Ca²⁺) and magnesium (Mg²⁺) ions, usually in the form of bicarbonates, chlorides, or sulfates. Sodium and potassium salts do not cause hardness.

This question belongs to: Science chemistry

Question #651

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The process of passing water through a bed of zeolite removes:

A. Dissolved gases

B. Hardness

C. Bacteria

D. Suspended impurities

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Correct Answer: Option B

Explanation:
The zeolite (permutit) process is used to remove water hardness. Zeolite (hydrated sodium aluminum silicate) exchanges its Na⁺ ions for the Ca²⁺ and Mg²⁺ ions in hard water, thus softening it. It does not remove bacteria, dissolved gases, or suspended solids.

This question belongs to: Science chemistry

Question #652

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Chlorination of water is done to:

A. Remove suspended impurities

B. Remove hardness

C. Remove bad odor

D. Kill pathogenic bacteria

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Correct Answer: Option D

Explanation:
Chlorination is the addition of chlorine gas or bleaching powder to water to disinfect it. Chlorine is a strong oxidizing agent that kills pathogenic (disease-causing) bacteria and viruses, making the water safe for drinking. It does not remove hardness or suspended impurities.

This question belongs to: Science chemistry

Question #653

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Which of the following is the purest form of water?

A. Distilled water

B. River water

C. Well water

D. Rainwater

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Correct Answer: Option A

Explanation:
Distilled water is considered the purest form of water because it is obtained by boiling water and condensing the steam, leaving almost all impurities, minerals, and salts behind. Rainwater is relatively pure but dissolves atmospheric gases and dust as it falls.

This question belongs to: Science chemistry

Question #654

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Ozonation of water is preferred over chlorination because:

A. It removes hardness.

B. It is cheaper.

C. It does not leave any harmful residual taste or toxic byproducts.

D. It is easier to transport.

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Correct Answer: Option C

Explanation:
Ozonation is a powerful disinfection method that kills bacteria and viruses faster than chlorine. Unlike chlorine, ozone decomposes back into oxygen and does not leave a residual taste or form harmful chlorinated organic byproducts (like trihalomethanes), making it safer for drinking water.

This question belongs to: Science chemistry

Question #655

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Which method is used to obtain completely demineralized water?

A. Ion-exchange process

B. Distillation

C. Filtration

D. Sedimentation

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Correct Answer: Option A

Explanation:
The ion-exchange process (using cation and anion exchange resins) removes all dissolved cations and anions from water, producing completely demineralized (deionized) water. Distillation removes most impurities but may still contain trace dissolved gases or volatile organics.

This question belongs to: Science chemistry

Question #656

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Assertion (A): Boiling water removes temporary hardness. Reason (R): Boiling converts soluble bicarbonates into insoluble carbonates.

A. Both A and R are true but R is NOT the correct explanation of A.

B. A is false but R is true.

C. A is true but R is false.

D. Both A and R are true and R is the correct explanation of A.

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Correct Answer: Option D

Explanation:
Temporary hardness is due to Ca(HCO₃)₂ and Mg(HCO₃)₂. Upon boiling, these decompose into insoluble CaCO₃ and Mg(OH)₂, which precipitate out and can be filtered. Thus, boiling effectively removes temporary hardness, and R correctly explains the chemical mechanism involved.

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Question #657

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The concentration of dissolved oxygen in water is a measure of its:

A. Biological Oxygen Demand (BOD)

B. Purity

C. Acidity

D. Hardness

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Correct Answer: Option A

Explanation:
The amount of dissolved oxygen is crucial for aquatic life. The Biological Oxygen Demand (BOD) is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material. High BOD indicates high organic pollution and low dissolved oxygen levels.

This question belongs to: Science chemistry

Question #658

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Eutrophication in water bodies is primarily caused by an excess of:

A. Nutrients like nitrogen and phosphorus

B. Radioactive isotopes

C. Heavy metals

D. Dissolved oxygen

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Correct Answer: Option A

Explanation:
Eutrophication is the excessive richness of nutrients (mainly nitrates and phosphates from fertilizers and sewage) in a lake or other body of water. This causes a dense growth of plant life (algal blooms) and depletion of oxygen, killing aquatic animals and disrupting the ecosystem.

This question belongs to: Science chemistry

Question #659

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The depletion of the ozone layer is mainly caused by:

A. Sulphur dioxide

B. Chlorofluorocarbons (CFCs)

C. Carbon dioxide

D. Methane

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Correct Answer: Option B

Explanation:
Chlorofluorocarbons (CFCs), used in refrigerants and aerosols, reach the stratosphere where UV radiation breaks them down, releasing chlorine atoms. These chlorine atoms catalytically destroy ozone (O₃) molecules, leading to the depletion of the ozone layer, particularly over Antarctica.

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Question #660

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Acid rain is primarily caused by the emission of:

A. Sulphur dioxide and nitrogen oxides

B. Carbon monoxide and carbon dioxide

C. Methane and nitrous oxide

D. CFCs and ozone

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Correct Answer: Option A

Explanation:
Acid rain is formed when sulfur dioxide (SO₂) and nitrogen oxides (NOx) emitted into the atmosphere (from burning fossil fuels) react with water, oxygen, and other chemicals to form sulfuric and nitric acids. These acids then fall to the ground as rain, damaging ecosystems.

This question belongs to: Science chemistry

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