Chemistry — Practice

Question 1

Which of the following is an example of a chemical change?
(a) Melting of ice  (b) Dissolving sugar in water  (c) Rusting of iron  (d) Cutting a sheet of paper

Accepted Answer

Rusting of iron forms a new substance — hydrated iron oxide — with properties different from iron, and is not simply reversed. This makes it a chemical change.

Answer: (c) Rusting of iron. Traps (a), (b) and (d) are all physical changes — no new substance is formed, and each is reversible by simple physical means (re-freezing, evaporating the water, taping the paper back together).

Question 2

An atom of an element has atomic number 11 and mass number 23. The number of neutrons in this atom is:
(a) 11  (b) 12  (c) 23  (d) 34

Accepted Answer

Number of neutrons = Mass number − Atomic number = 23 − 11 = 12.

Answer: (b) 12. Trap (a) 11 wrongly equates neutron count with the atomic number (which is actually the proton count); trap (c) 23 uses the mass number directly without subtracting; trap (d) 34 adds the two numbers instead of subtracting them.

Question 3

Calcium (valency 2) combines with chlorine (valency 1) to form calcium chloride. The correct chemical formula is:
(a) CaCl  (b) Ca₂Cl  (c) CaCl₂  (d) Ca₂Cl₂

Accepted Answer

Crisscrossing the valencies: Ca has valency 2, Cl has valency 1, so one Ca combines with two Cl atoms, giving CaCl₂.

Answer: (c) CaCl₂. Trap (a) CaCl assumes a 1:1 ratio, ignoring the crisscross rule entirely; trap (b) Ca₂Cl reverses which element gets the higher subscript; trap (d) Ca₂Cl₂ doubles both subscripts instead of reducing to the simplest ratio (which would actually simplify back to CaCl, not CaCl₂ — a genuine arithmetic trap).

Question 4

Elements in the same group of the modern periodic table have the same number of:
(a) Protons  (b) Neutrons  (c) Electron shells  (d) Valence electrons

Accepted Answer

Elements in a group share the same number of valence (outermost-shell) electrons, which is why they show similar chemical properties — this is the entire basis of grouping elements together.

Answer: (d) Valence electrons. Trap (a) Protons is the atomic number, unique to each element (never shared even within a group); trap (b) Neutrons varies even between isotopes of the same element; trap (c) Electron shells is what elements in the same PERIOD share, not the same group — a classic group/period swap.

Question 5

Which of the following elements is classified as a metalloid, showing properties of both metals and non-metals?
(a) Sodium  (b) Silicon  (c) Chlorine  (d) Calcium

Accepted Answer

Silicon lies along the 'staircase' border between metals and non-metals in the periodic table, alongside boron, germanium, arsenic, antimony and tellurium, and shows intermediate properties (it behaves as a semiconductor).

Answer: (b) Silicon. Trap (a) Sodium (Group 1) and (d) Calcium (Group 2) are both typical reactive metals; trap (c) Chlorine (Group 17) is a typical reactive non-metal — none of the three sit on the metalloid border.

Question 6

Brass, commonly used for utensils and decorative fittings, is an alloy of:
(a) Copper and Tin  (b) Copper and Zinc  (c) Iron and Carbon  (d) Lead and Tin

Accepted Answer

Brass is an alloy of copper and zinc.

Answer: (b) Copper and Zinc. Trap (a) Copper and Tin is bronze, not brass — a classic brass/bronze mix-up; trap (c) Iron and Carbon is steel; trap (d) Lead and Tin is solder — all genuine alloys, but none is brass.

Question 7

Sodium chloride (NaCl) is formed by:
(a) Sharing of electrons between Na and Cl  (b) Transfer of an electron from Na to Cl  (c) Transfer of an electron from Cl to Na  (d) Equal sharing of a pair of electrons

Accepted Answer

Sodium (2,8,1) loses its single outer electron to attain a stable (2,8) configuration, becoming Na⁺; chlorine (2,8,7) gains that electron to attain a stable (2,8,8) configuration, becoming Cl⁻. The oppositely charged ions then attract electrostatically — an ionic bond.

Answer: (b) Transfer of an electron from Na to Cl. Traps (a) and (d) both describe covalent bonding (the wrong bond type for a metal-nonmetal pair); trap (c) reverses the direction of transfer — the metal (Na) always loses the electron, not the non-metal.

Question 8

The reaction CaCO₃ →(heat) CaO + CO₂ is an example of a:
(a) Combination reaction  (b) Decomposition reaction  (c) Displacement reaction  (d) Double displacement reaction

Accepted Answer

A single reactant (CaCO₃) breaks down into two simpler products (CaO and CO₂) under heat — the defining pattern of a decomposition reaction.

Answer: (b) Decomposition reaction. Trap (a) Combination is the opposite direction (two reactants merging into one product); traps (c) and (d) both require two separate reactants exchanging elements or ions, which doesn't apply here since there is only one reactant.

Question 9

Which of the following is the correctly balanced equation for the reaction of magnesium with hydrochloric acid?
(a) Mg + HCl → MgCl₂ + H₂  (b) Mg + 2HCl → MgCl₂ + H₂  (c) Mg + 2HCl → MgCl + H₂  (d) 2Mg + HCl → MgCl₂ + H₂

Accepted Answer

Magnesium has valency 2, so its chloride is MgCl₂. Balancing atom-for-atom: Mg (1=1), Cl (2=2 only if 2HCl is used), H (2=2). So Mg + 2HCl → MgCl₂ + H₂ is fully balanced.

Answer: (b) Mg + 2HCl → MgCl₂ + H₂. Trap (a) uses only 1 HCl, leaving Cl and H unbalanced (1 vs 2 needed on each side); trap (c) uses the wrong formula MgCl instead of MgCl₂ (ignoring magnesium's valency of 2); trap (d) doubles Mg instead of HCl, leaving Cl and H unbalanced.

Question 10

An iron nail placed in a copper sulphate (CuSO₄) solution turns the blue solution pale green and gets coated with a reddish-brown deposit. This happens because:
(a) Iron is less reactive than copper and gets displaced  (b) Iron is more reactive than copper and displaces it from the solution  (c) Copper reacts with iron to form iron sulphate only, with no metal deposit  (d) This is a double displacement reaction between Fe and CuSO₄

Accepted Answer

Iron sits above copper in the reactivity series, so it displaces copper from copper sulphate solution: Fe + CuSO₄ → FeSO₄ (pale green) + Cu (reddish-brown deposit) — a single displacement reaction.

Answer: (b) Iron is more reactive than copper and displaces it. Trap (a) reverses the reactivity comparison; trap (c) denies the copper metal deposit that is actually observed; trap (d) misclassifies this as a double displacement, when only one element (Cu) is actually displaced, not two ionic partners exchanged.

Question 11

Which of the following pH values represents the most strongly acidic solution?
(a) pH 9  (b) pH 7  (c) pH 4  (d) pH 1

Accepted Answer

On the pH scale, the LOWER the value (closer to 0), the more strongly acidic the solution; pH 7 is neutral, and values above 7 are basic.

Answer: (d) pH 1. Trap (a) pH 9 is basic, not acidic at all; trap (b) pH 7 is neutral; trap (c) pH 4 is acidic but far less strongly than pH 1 — a common trap for students who read 'higher number = stronger' without checking which side of 7 they're on.

Question 12

Phenolphthalein indicator turns pink when added to:
(a) An acidic solution  (b) A neutral solution  (c) A basic solution  (d) Distilled water

Accepted Answer

Phenolphthalein is colourless in acidic and neutral solutions and turns pink (magenta) only in basic (alkaline) solutions.

Answer: (c) A basic solution. Trap (a) reverses the actual colour-change condition (it stays colourless in acid); traps (b) and (d) — neutral solutions and distilled water — also leave phenolphthalein colourless, not pink.

Question 13

Baking soda, commonly used in antacids and as a leavening agent, has the chemical formula:
(a) Na₂CO₃  (b) NaHCO₃  (c) CaOCl₂  (d) NaOH

Accepted Answer

Baking soda is sodium bicarbonate, NaHCO₃.

Answer: (b) NaHCO₃. Trap (a) Na₂CO₃ is washing soda (sodium carbonate) — a related but distinctly different compound; trap (c) CaOCl₂ is bleaching powder; trap (d) NaOH is caustic soda, a strong base, not baking soda.

Question 14

CNG (Compressed Natural Gas), widely used as a cleaner automobile fuel, consists mainly of:
(a) Butane  (b) Methane  (c) Ethanol  (d) Propane

Accepted Answer

CNG is compressed natural gas, and natural gas is composed mainly of methane (CH₄).

Answer: (b) Methane. Traps (a) Butane and (d) Propane are the main constituents of LPG, not CNG — a classic LPG/CNG mix-up; trap (c) Ethanol is a biofuel/alcohol, unrelated to CNG's composition.

Practice — Chemistry

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