Chemistry MCQs

Question #1

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Which of the following is an example of a physical change?

A. Rusting of iron

B. Digestion of food

C. Melting of wax

D. Burning of magnesium ribbon

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Correct Answer: Option C

Explanation:
Melting of wax is a physical change because it involves only a change in state from solid to liquid without altering the chemical composition. The wax remains the same hydrocarbon mixture. In contrast, rusting of iron (Fe₂O₃·xH₂O formation), burning of magnesium (MgO formation), and digestion of food (enzymatic breakdown into simpler molecules) are chemical changes where new substances are formed. Physical changes are reversible, while most chemical changes are irreversible. Exam tip: Remember, changes of state (melting, boiling, condensation) are physical changes unless a chemical reaction accompanies them.

This question belongs to: Science chemistry

Question #2

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Which property of matter depends on the amount of substance?

A. Mass

B. Refractive index

C. Density

D. Boiling point

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Correct Answer: Option A

Explanation:
Mass is an extensive property, meaning it depends on the quantity of matter. Density, boiling point, and refractive index are intensive properties that remain constant regardless of the amount of substance. Intensive properties are useful for identifying substances. For example, pure water boils at 100°C at 1 atm irrespective of whether you have 10 mL or 10 L. Exam trick: Extensive properties like mass, volume, and length add up when you combine samples; intensive properties do not.

This question belongs to: Science chemistry

Question #3

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Which separation technique is best suited to separate a mixture of ammonium chloride and common salt?

A. Chromatography

B. Filtration

C. Sublimation

D. Distillation

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Correct Answer: Option C

Explanation:
Ammonium chloride (NH₄Cl) sublimes on heating—it directly converts from solid to vapor without passing through the liquid phase—while common salt (NaCl) remains as a solid residue. This property allows separation by sublimation. Filtration would not work because both are soluble in water, distillation is for liquids, and chromatography separates dissolved substances based on differential adsorption. Lab tip: NH₄Cl vapors condense on the cool funnel to give solid sublimate.

This question belongs to: Science chemistry

Question #4

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Who discovered the electron and determined its charge-to-mass ratio?

A. James Chadwick

B. J. J. Thomson

C. Niels Bohr

D. Ernest Rutherford

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Correct Answer: Option B

Explanation:
J.J. Thomson discovered the electron through cathode ray tube experiments in 1897 and measured the e/m ratio by applying electric and magnetic fields. Rutherford discovered the nucleus via the gold foil experiment. Chadwick discovered the neutron (1932). Bohr proposed the planetary model of the atom with quantized orbits. Thomson's model (plum pudding) was later superseded. Exam memory: Thomson = electron (e/m); Rutherford = nucleus; Chadwick = neutron; Bohr = energy levels.

This question belongs to: Science chemistry

Question #5

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The valency of aluminium in AlCl₃ is:

A. 4

B. 3

C. 1

D. 2

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Correct Answer: Option B

Explanation:
In AlCl₃, aluminium combines with three chlorine atoms. Chlorine has a valency of 1 (needs one electron to complete its octet). By the criss-cross method, Al has valency 3 because it loses three electrons to achieve a stable octet (Al³⁺). Thus, the formula is AlCl₃. Aluminium's electronic configuration is 2,8,3, so it loses three valence electrons. Exam tip: Valency equals the number of electrons lost, gained, or shared to complete octet.

This question belongs to: Science chemistry

Question #6

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Which of the following contains a coordinate covalent bond?

A. NH₃

B. NH₄⁺

C. Cl₂

D. NaCl

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Correct Answer: Option B

Explanation:
Ammonium ion (NH₄⁺) contains a coordinate bond: NH₃ donates its lone pair of electrons to H⁺ to form a covalent bond where both electrons come from nitrogen. NaCl is ionic, NH₃ has ordinary covalent bonds, Cl₂ has a nonpolar covalent bond. In NH₄⁺, all four N–H bonds are equivalent after formation. Coordinate bond is also called dative bond and is common in complex ions like [Cu(NH₃)₄]²⁺.

This question belongs to: Science chemistry

Question #7

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Which bond has the highest ionic character?

A. HF

B. HBr

C. HI

D. HCl

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Correct Answer: Option A

Explanation:
Ionic character increases with the difference in electronegativity between bonding atoms. In hydrogen halides, electronegativity decreases from F to I (F: 4.0, Cl: 3.0, Br: 2.8, I: 2.5). Thus HF has the largest electronegativity difference (1.9) and highest ionic character. The trend: HF > HCl > HBr > HI. However, HF is still a covalent polar molecule. Exam tip: Electronegativity difference > 1.7 usually indicates ionic bond.

This question belongs to: Science chemistry

Question #8

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In the modern periodic table, the element with atomic number 117 belongs to:

A. Group 18, Period 6

B. Group 17, Period 6

C. Group 1, Period 7

D. Group 17, Period 7

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Correct Answer: Option D

Explanation:
Element 117 (tennessine, Ts) is a halogen in Group 17, Period 7. The modern periodic table has 7 periods and 18 groups. Halogens are in Group 17. Period 7 starts with Fr (87) and ends with Og (118). Ts is two places before Og. Exam trick: For atomic numbers after 86 (Rn), use period 7. Group 17 elements are F, Cl, Br, I, At, Ts.

This question belongs to: Science chemistry

Question #9

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Which trend is correct across a period from left to right?

A. Metallic character increases

B. Atomic radius increases

C. Ionization energy generally increases

D. Electron affinity decreases

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Correct Answer: Option C

Explanation:
Across a period, nuclear charge increases while electrons are added to the same shell, leading to stronger attraction, so atomic radius decreases and ionization energy generally increases (with some exceptions like N to O due to half-filled stability). Metallic character decreases, non-metallic character increases. Electron affinity becomes more negative (increases in magnitude). Exam note: Ionization energy trend: Increases across period, decreases down group.

This question belongs to: Science chemistry

Question #10

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Mendeleev's periodic table was based on:

A. Electron configuration

B. Atomic number

C. Density

D. Atomic mass and chemical properties

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Correct Answer: Option D

Explanation:
Mendeleev arranged elements in order of increasing atomic mass and grouped elements with similar chemical properties. He left gaps for undiscovered elements like eka-aluminium (gallium) and eka-silicon (germanium), predicting their properties. The modern table uses atomic number as the basis. Mendeleev's table had anomalies like placing Ar (40) before K (39) due to mass order, later resolved by atomic number concept.

This question belongs to: Science chemistry

Question #11

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Assertion (A): Magnesium oxide is a basic oxide. Reason (R): Metals on reaction with oxygen form acidic oxides.

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 C

Explanation:
Magnesium oxide (MgO) is a basic oxide because it reacts with acids to form salt and water, e.g., MgO + 2HCl → MgCl₂ + H₂O. However, Reason is false: Metals generally form basic oxides, not acidic oxides. Non-metals form acidic oxides (e.g., SO₂, CO₂). Some metals like Al, Zn form amphoteric oxides. So A is true, R false. Exam tip: Metal oxides are usually basic; non-metal oxides are acidic.

This question belongs to: Science chemistry

Question #12

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Which reaction type is: 2AgBr(s) → 2Ag(s) + Br₂(g)?

A. Decomposition reaction

B. Neutralization reaction

C. Combination reaction

D. Displacement reaction

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Correct Answer: Option A

Explanation:
Silver bromide decomposes into silver and bromine on exposure to light. This is a photochemical decomposition reaction, where a single compound breaks down into two or more simpler substances. It is also a redox reaction (Ag⁺ reduced, Br⁻ oxidized). Photographic films use this property. Combination would be A + B → AB. Displacement is A + BC → AC + B. Neutralization is acid + base → salt + water.

This question belongs to: Science chemistry

Question #13

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In the reaction CuO + H₂ → Cu + H₂O, which substance is reduced?

A. H₂O

B. CuO

C. Cu

D. H₂

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Correct Answer: Option B

Explanation:
Reduction is gain of electrons or loss of oxygen. Here, CuO loses oxygen to form Cu, so CuO is reduced. H₂ gains oxygen to form H₂O, so H₂ is oxidized. This is a redox reaction. Oxidation and reduction occur simultaneously. Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain of electrons). In terms of oxygen, loss of O = reduction, gain of O = oxidation.

This question belongs to: Science chemistry

Question #14

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A solution turns red litmus blue. Its pH is likely:

A. 9

B. 5

C. 7

D. 2

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Correct Answer: Option A

Explanation:
A solution turning red litmus blue is basic, so pH > 7. Among options, pH 9 is basic. pH 2 and 5 are acidic (blue litmus turns red). pH 7 is neutral. Litmus is a natural indicator extracted from lichens. Red litmus in base remains blue, blue litmus in acid turns red. Practical: Phenolphthalein gives pink in basic solution (pH > 8.3).

This question belongs to: Science chemistry

Question #15

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Which of the following is used as an antacid?

A. Calcium sulfate

B. Ammonium chloride

C. Sodium chloride

D. Sodium bicarbonate

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Correct Answer: Option D

Explanation:
Sodium bicarbonate (NaHCO₃), baking soda, is a mild base that neutralizes excess stomach acid (HCl) providing relief from acidity. Reaction: NaHCO₃ + HCl → NaCl + H₂O + CO₂. It is a component of many antacid formulations. Sodium chloride is table salt, ammonium chloride is not safe, and calcium sulfate is plaster of Paris ingredient. Milk of magnesia (Mg(OH)₂) is also an antacid.

This question belongs to: Science chemistry

Question #16

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Plaster of Paris has the chemical formula:

A. CaSO₄·½H₂O

B. CaSO₄·2H₂O

C. 2CaSO₄·H₂O

D. CaSO₄

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Correct Answer: Option A

Explanation:
Plaster of Paris is calcium sulfate hemihydrate, CaSO₄·½H₂O. It is obtained by heating gypsum (CaSO₄·2H₂O) at 373 K. It sets to a hard mass on mixing with water reforming gypsum. Option B (2CaSO₄·H₂O) is equivalent to CaSO₄·½H₂O but the conventional representation is CaSO₄·½H₂O. Option A is gypsum, D is anhydrous calcium sulfate. POP is used for making casts, statues, and as a building material.

This question belongs to: Science chemistry

Question #17

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Which metal is most reactive according to the reactivity series?

A. Fe

B. Cu

C. K

D. Zn

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Correct Answer: Option C

Explanation:
Potassium (K) is at the top of the reactivity series, meaning it is the most reactive among the given metals. The order of reactivity (decreasing): K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au. Potassium reacts violently with water and oxygen. Copper (Cu) is relatively unreactive and is found native in nature. Exam trick: Remember 'Please Stop Calling Me A Cute Zebra...' mnemonic.

This question belongs to: Science chemistry

Question #18

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Galvanization is the process of coating iron with:

A. Copper

B. Chromium

C. Zinc

D. Tin

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Correct Answer: Option C

Explanation:
Galvanization is coating iron or steel with a thin layer of zinc to prevent rusting. Zinc acts as a sacrificial anode—it corrodes preferentially (being more reactive) even if the coating is scratched, protecting the underlying iron. Tin coating (tin plating) protects only if the layer is intact. Chromium plating provides a shiny surface and corrosion resistance but works differently. Galvanized iron is used for roofing sheets, buckets, etc.

This question belongs to: Science chemistry

Question #19

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Which allotrope of carbon is the hardest natural substance?

A. Lamp black

B. Diamond

C. Coal

D. Graphite

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Correct Answer: Option B

Explanation:
Diamond is the hardest known natural material due to its rigid three-dimensional network of covalent bonds with sp³ hybridization, forming a tetrahedral structure. Graphite is soft and slippery because of its layered structure with weak van der Waals forces between layers. Coal is an amorphous form of carbon. Diamond is used in cutting, drilling, and jewelry. Mohs hardness of diamond is 10.

This question belongs to: Science chemistry

Question #20

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The main constituent of natural gas is:

A. Ethane

B. Butane

C. Propane

D. Methane

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Correct Answer: Option D

Explanation:
Natural gas is primarily methane (CH₄), typically 70-90%, along with small amounts of ethane, propane, butane, and other hydrocarbons. It is a fossil fuel formed from the remains of ancient marine organisms. CNG (compressed natural gas) is methane stored at high pressure. LPG (liquefied petroleum gas) mainly contains propane and butane. Methane has the highest calorific value among the given options per unit mass.

This question belongs to: Science chemistry

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