Chemistry MCQs

Question #121

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Hard water does not lather well with soap because it forms:

A. Foam

B. Soluble sodium salts

C. Alkaline solution

D. Insoluble calcium and magnesium soap (scum)

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Correct Answer: Option D

Explanation:
Soap (sodium/potassium salts of long-chain fatty acids) reacts with Ca²⁺ and Mg²⁺ ions in hard water to form insoluble calcium/magnesium salts (scum), which precipitates and does not lather. This wastes soap. Reaction: 2RCOONa + Ca²⁺ → (RCOO)₂Ca↓ + 2Na⁺. Detergents do not form scum because their calcium salts are soluble. Hardness is measured in ppm of CaCO₃.

This question belongs to: Science chemistry

Question #122

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Chlorine is used in water treatment primarily to:

A. Kill pathogenic microorganisms

B. Remove suspended particles

C. Improve taste

D. Add minerals

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Correct Answer: Option A

Explanation:
Chlorine acts as a disinfectant, killing bacteria, viruses, and other pathogens, making water safe for drinking. It forms hypochlorous acid (HOCl) in water which is a strong oxidizing agent. Sedimentation and filtration remove suspended particles. Taste may be affected if excess chlorine or by-products form. Chlorine residual protects water in distribution pipes.

This question belongs to: Science chemistry

Question #123

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Which of the following is NOT a method to purify water?

A. Reverse osmosis

B. UV irradiation

C. Distillation

D. Electrolysis

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Correct Answer: Option D

Explanation:
Electrolysis of water splits it into hydrogen and oxygen, it is not a purification method but a decomposition. Reverse osmosis uses semipermeable membrane to remove ions and impurities. UV irradiation kills microorganisms. Distillation boils water and condenses steam, removing non-volatile impurities. Other methods: filtration, chlorination, ozonation, boiling.

This question belongs to: Science chemistry

Question #124

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The main greenhouse gas emitted from agricultural activities (livestock and rice paddies) is:

A. Carbon dioxide

B. Nitrous oxide

C. Methane

D. Chlorofluorocarbon

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Correct Answer: Option C

Explanation:
Methane (CH₄) is released from enteric fermentation in livestock (cattle) and from anaerobic decomposition in flooded rice paddies. It has a high global warming potential (25× CO₂ over 100 years). CO₂ mainly from fossil fuels. N₂O from fertilizers. CFCs from refrigerants. Agriculture contributes significantly to anthropogenic GHG emissions. Reducing methane can have rapid climate benefits.

This question belongs to: Science chemistry

Question #125

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Alpha particles are least penetrating because they:

A. Have high mass and charge

B. Are neutral

C. Travel at speed of light

D. Are electromagnetic waves

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Correct Answer: Option A

Explanation:
Alpha particles are relatively heavy (4 amu) and doubly charged (+2), so they interact strongly with matter, losing energy rapidly via ionization. They can be stopped by a sheet of paper or a few centimeters of air. Beta particles (lighter, faster electrons) penetrate further. Gamma rays (uncharged EM waves) are most penetrating. Alpha emitters are hazardous if ingested or inhaled.

This question belongs to: Science chemistry

Question #126

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The atomic number of an element is determined by the number of:

A. Nucleons

B. Electrons

C. Protons

D. Neutrons

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Correct Answer: Option C

Explanation:
Atomic number (Z) is the number of protons in the nucleus, which defines the identity of the element. In a neutral atom, the number of electrons equals Z. Neutrons contribute to mass number but not atomic number. Isotopes have same Z but different number of neutrons. Nucleons are total protons + neutrons = mass number. The modern periodic table is based on atomic number.

This question belongs to: Science chemistry

Question #127

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Which of the following bonds is present in diamond?

A. Hydrogen bond

B. Metallic

C. Ionic

D. Covalent network

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Correct Answer: Option D

Explanation:
Diamond has a three-dimensional network of strong covalent bonds between carbon atoms (sp³ hybridization). This giant covalent structure gives diamond its extreme hardness, high melting point, and electrical insulating property. Graphite also has covalent bonds within layers but conducts due to delocalized electrons. Ionic bonds are in NaCl, metallic bonds in metals, hydrogen bonds in water.

This question belongs to: Science chemistry

Question #128

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The valency of nitrogen in ammonia (NH₃) is:

A. 1

B. 5

C. 3

D. 2

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Correct Answer: Option C

Explanation:
Nitrogen has 5 valence electrons; it needs 3 more to complete the octet. It shares three electrons with three hydrogen atoms, forming three covalent bonds. Therefore, valency is 3. In the case of nitrogen, covalency is 3, while its electrovalency could be 5 (losing all 5). For covalent molecules, valency = number of shared electron pairs. In N₂, valency is also 3 (triple bond).

This question belongs to: Science chemistry

Question #129

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The horizontal rows in the modern periodic table are called:

A. Periods

B. Families

C. Blocks

D. Groups

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Correct Answer: Option A

Explanation:
The periodic table has 7 horizontal rows called periods. Elements in the same period have the same number of electron shells. Properties change gradually across a period. Groups are vertical columns (18), where elements have similar valence electron configurations and thus similar chemical properties. Families are informal group names (e.g., halogens). Blocks (s, p, d, f) are based on the orbital in which the last electron enters.

This question belongs to: Science chemistry

Question #130

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Which of the following is a metal?

A. Sulfur

B. Phosphorus

C. Chlorine

D. Sodium

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Correct Answer: Option D

Explanation:
Sodium (Na) is a soft, silvery-white alkali metal (Group 1). It is highly reactive, conducts electricity, and is malleable. Sulfur and phosphorus are non-metals; chlorine is a non-metal halogen gas. Sodium is never found free in nature; it is stored under kerosene to prevent reaction with air/moisture. It reacts vigorously with water, producing NaOH and H₂.

This question belongs to: Science chemistry

Question #131

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The conversion of iron into steel involves the removal of impurities and the addition of:

A. Oxygen and hydrogen

B. Nitrogen and sulfur

C. Phosphorus only

D. Carbon and other alloying elements

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Correct Answer: Option D

Explanation:
Steel is an alloy of iron with controlled amounts of carbon (0.02-2.1%) and often other elements like manganese, chromium, nickel, etc. Cast iron contains 2-4% carbon. In steelmaking (Bessemer, LD process), impurities like excess carbon, silicon, manganese, sulfur, phosphorus are oxidized and removed, then desired carbon and alloying elements are added to achieve specific properties.

This question belongs to: Science chemistry

Question #132

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CNG is considered a cleaner fuel than petrol because it produces:

A. Less CO₂ and negligible particulate matter

B. Sulfur dioxide

C. More oxides of nitrogen

D. More CO₂

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Correct Answer: Option A

Explanation:
CNG (methane) has a higher H/C ratio, so combustion produces less CO₂ per unit energy compared to petrol/diesel. It also burns completely without generating smoke or particulate matter, and contains no sulfur, thus no SO₂. It reduces NOx and unburned hydrocarbons. It is used in vehicles and is safer in spills because it is lighter than air and disperses quickly.

This question belongs to: Science chemistry

Question #133

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Which of the following is NOT a fossil fuel?

A. Natural gas

B. Coal

C. Biogas

D. Petroleum

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Correct Answer: Option C

Explanation:
Biogas is produced from anaerobic digestion of organic waste (cow dung, agricultural residue) and is renewable. Mainly methane and CO₂. Coal, petroleum, and natural gas are fossil fuels formed over millions of years, non-renewable. Biogas plants generate clean fuel and also produce enriched manure. Charcoal is also not a fossil fuel; it's from wood pyrolysis.

This question belongs to: Science chemistry

Question #134

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The standard reduction potential of a half-cell measures:

A. Tendency to gain electrons

B. Current produced

C. Resistance of electrolyte

D. Tendency to lose electrons

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Correct Answer: Option A

Explanation:
Standard reduction potential (E°) measures the tendency of a species to gain electrons and be reduced. The more positive the E°, the greater the tendency to undergo reduction. Metals with negative E° are good reducing agents (e.g., Zn, -0.76 V). E°cell = E°cathode - E°anode. It is measured under standard conditions (1 M, 1 atm, 25°C) relative to standard hydrogen electrode (SHE) which is assigned 0 V.

This question belongs to: Science chemistry

Question #135

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In an electrolytic cell, the anode is connected to the:

A. Cathode of another cell

B. Ground

C. Negative terminal of the battery

D. Positive terminal of the battery

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Correct Answer: Option D

Explanation:
In electrolysis, the external power source forces non-spontaneous reactions. Electrons are drawn away from the anode, making it positive. Thus anode is connected to the positive terminal. In a galvanic (voltaic) cell, anode is negative. Remember: electrolytic cell – anode positive; oxidation still occurs at anode. Electroplating uses this setup: object to be plated is cathode, source metal is anode.

This question belongs to: Science chemistry

Question #136

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A gas at constant temperature occupies 5 L at 2 atm. What is the volume at 1 atm?

A. 1 L

B. 10 L

C. 2.5 L

D. 5 L

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Correct Answer: Option B

Explanation:
Using Boyle's law: P₁V₁ = P₂V₂ => 2 atm × 5 L = 1 atm × V₂ => V₂ = 10 L. As pressure decreases, volume increases. The same mass of gas expands. This is a direct application of inverse proportionality. If temperature and amount of gas are constant, doubling pressure halves volume, etc.

This question belongs to: Science chemistry

Question #137

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The rate of effusion of two gases A and B are in the ratio 1:2. If molar mass of A is 64 g/mol, molar mass of B is:

A. 16 g/mol

B. 128 g/mol

C. 32 g/mol

D. 256 g/mol

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Correct Answer: Option A

Explanation:
Graham's law: r₁/r₂ = √(M₂/M₁). Given rA/rB = 1/2, so √(MB/MA) = 1/2. Squaring: MB/MA = 1/4 => MB = MA/4 = 64/4 = 16 g/mol. B is lighter and diffuses faster. Logical check: rate inversely proportional to square root of molar mass; if rate is double, mass is quarter. Answer is 16.

This question belongs to: Science chemistry

Question #138

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Which of the following increases the rate of a chemical reaction?

A. Decreasing concentration of reactants

B. Using larger pieces of solid reactants

C. Decreasing temperature

D. Adding a positive catalyst

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Correct Answer: Option D

Explanation:
A positive catalyst increases reaction rate by lowering activation energy. Decreasing temperature generally slows reactions. Decreasing concentration reduces collision frequency, lowering rate. Larger pieces have less surface area, slower reaction. Other rate-increasing factors: increasing temperature, increasing pressure (for gases), using finely powdered solids, and light for photochemical reactions.

This question belongs to: Science chemistry

Question #139

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In the extraction of iron in a blast furnace, limestone is added to:

A. Lower the melting point of iron

B. Remove silica as slag

C. Reduce iron oxide

D. Increase carbon content

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Correct Answer: Option B

Explanation:
Limestone (CaCO₃) decomposes to CaO, which is a basic flux. It reacts with the acidic impurity silica (SiO₂) to form calcium silicate (CaSiO₃), a fusible slag: CaO + SiO₂ → CaSiO₃. Slag floats on molten iron and is tapped separately. Reduction is done by CO/coke. Flux makes gangue removal easier. Slag is used in cement, road building.

This question belongs to: Science chemistry

Question #140

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Which of the following is a phosphate fertilizer?

A. Potassium chloride

B. Urea

C. Ammonium nitrate

D. Single superphosphate

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Correct Answer: Option D

Explanation:
Single superphosphate (SSP) is a phosphorus fertilizer, a mixture of monocalcium phosphate and gypsum, produced by treating rock phosphate with sulfuric acid: Ca₃(PO₄)₂ + 2H₂SO₄ → Ca(H₂PO₄)₂ + 2CaSO₄. Urea and ammonium nitrate are nitrogenous fertilizers. Potassium chloride is a potash fertilizer. Phosphorus promotes root development and flowering.

This question belongs to: Science chemistry

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