The s-Block Elements - MCQs with Explanations

1. Magnesium burns in air to give:

• (a) MgO
• (b) \(Mg_{3}N_{2}\)
• (c) \(MgCO_{3}\)
• (d) MgO and \(Mg_{3}N_{2}\)

Correct Answer: (d)

Explanation: Magnesium burns in air to form both the oxide (MgO) and the nitride (\(Mg_{3}N_{2}\)).

2. The correct arrangement of increasing order of atomic radii among Na, K, Mg, Rb is:

• (a) \(Mg < K < Na < Rb\)
• (b) \(Mg < Na < K < Rb\)
• (c) \(Mg < Na < Rb < K\)
• (d) \(Na < K < Rb < Mg\)

Correct Answer: (b)

Explanation: Atomic radius increases moving down a group and decreases from left to right across a period. Therefore, the order is \(Mg < Na < K < Rb\).

3. Which of the following salts does not impart colour to the flame?

• (a) \(MgCl_{2}\)
• (b) \(SrCl_{2}\)
• (c) \(BaCl_{2}\)
• (d) \(LiCl\)

Correct Answer: (a)

Explanation: Due to the high ionization energy of Magnesium, the energy required for excitation falls in the ultraviolet region rather than the visible spectrum, so it does not impart colour to the flame.

4. Which among the following has the tendency to form covalent compounds?

• (a) Calcium
• (b) Beryllium
• (c) Strontium
• (d) Magnesium

Correct Answer: (b)

Explanation: Beryllium has a tendency to form covalent salts because of its small size and high ionization enthalpy.

5. \(MgSO_{4}\) on reaction with \(NH_{4}OH\) and \(Na_{2}HPO_{4}\) forms a white crystalline precipitate. What is its formula?

• (a) \(Mg(NH_{4})PO_{4}\)
• (b) \(Mg_{3}(PO_{4})_{2}\)
• (c) \(MgCl_{2} \cdot MgSO_{4}\)
• (d) \(MgSO_{4}\)

Correct Answer: (a)

Explanation: The reaction produces magnesium ammonium phosphate, \(Mg(NH_{4})PO_{4}\).

6. In all oxides, peroxides and superoxides, the oxidation state of alkali metals is:

• (a) +1 and -1
• (b) +1 and +2
• (c) +1 only
• (d) +1, -1 and +2

Correct Answer: (c)

Explanation: Alkali metals always exhibit a +1 oxidation state in their compounds.

7. The first ionisation potential of Na, Mg, Al and Si are in the order:

• (a) \(Na < Mg > Al < Si\)
• (b) \(Na > Mg > Al > Si\)
• (c) \(Na < Mg < Al < Si\)
• (d) \(Na > Mg > Al < Si\)

Correct Answer: (a)

Explanation: \(Mg\) has a higher IE than \(Al\) because \(Mg\) has a stable fully filled \(3s^{2}\) configuration, while \(Si\) is higher than both due to increased nuclear charge.

8. Which of the following metal carbonate is decomposed on heating?

• (a) \(Na_{2}CO_{3}\)
• (b) \(MgCO_{3}\)
• (c) \(K_{2}CO_{3}\)
• (d) \(Rb_{2}CO_{3}\)

Correct Answer: (b)

Explanation: Alkaline earth metal carbonates like \(MgCO_{3}\) decompose on heating to form the oxide and \(CO_{2}\).

9. Which of the following sulphates have the highest solubility?

• (a) \(BeSO_{4}\)
• (b) \(MgSO_{4}\)
• (c) \(BaSO_{4}\)
• (d) \(CaSO_{4}\)

Correct Answer: (a)

Explanation: \(BeSO_{4}\) is the most soluble because its hydration energy is greater than its lattice energy.

10. Which of the following can be extracted from sea water?

• (a) Li
• (b) K
• (c) Mg
• (d) Ca

Correct Answer: (c)

Explanation: Sea water contains approximately 0.13% \(Mg^{2+}\) ions, from which Magnesium is extracted.

11. Several blocks of magnesium are fixed to the bottom of a ship to:

• (a) prevent puncturing by under sea rocks
• (b) keep away the sharks
• (c) prevent action of water and salt
• (d) make the ship lighter

Correct Answer: (c)

Explanation: Magnesium acts as a sacrificial anode (cathodic protection) to prevent the corrosion of the ship's steel hull by sea water.

12. \(Be + 2NH_{3} + 4HF \rightarrow A \rightarrow BeF_{2} + 2NH_{4}F\). The compound (A) is:

• (a) \(NF_{3}\)
• (b) \(NH_{3}-BeF_{2}\)
• (c) \((NH_{4})_{2}[BeF_{4}]\)
• (d) \(NF_{3}NH_{3}\)

Correct Answer: (c)

Explanation: The intermediate compound (A) in the synthesis is \((NH_{4})_{2}[BeF_{4}]\).

13. The decomposition temperature is maximum for:

• (a) \(MgCO_{3}\)
• (b) \(SrCO_{3}\)
• (c) \(CaCO_{3}\)
• (d) \(BaCO_{3}\)

Correct Answer: (d)

Explanation: Thermal stability of Group 2 carbonates increases down the group as the size of the cation increases.

14. For two ionic solids CaO and KI, identify the wrong statement:

• (a) Lattice energy of CaO is much higher than that of KI.
• (b) KI is soluble in benzene.
• (c) CaO is sparingly soluble in water.
• (d) KI has high melting point.

Correct Answer: (d)

Explanation: Because KI has a much lower lattice energy compared to CaO, its melting point is significantly lower than that of CaO.

15. Which one of the following pairs do not impart colour to the flame?

• (a) \(BeCl_{2}\) and \(SrCl_{2}\)
• (b) \(BeCl_{2}\) and \(MgCl_{2}\)
• (c) \(CaCl_{2}\) and \(BaCl_{2}\)
• (d) \(BaCl_{2}\) and \(SrCl_{2}\)

Correct Answer: (b)

Explanation: Both Be and Mg have high ionization enthalpies, meaning they do not impart colour to the flame.

16. Sodium on heating with moist air produces:

• (a) NaO
• (b) \(NaO_{2}\)
• (c) \(Na_{2}O\)
• (d) \(Na_{2}CO_{3}\)

Correct Answer: (d)

Explanation: In moist air, sodium eventually reacts with \(CO_{2}\) and moisture to form sodium carbonate (\(Na_{2}CO_{3}\)).

17. Which is used in the treatment of manic depression disorders?

• (a) \(Na_{2}CO_{3}\)
• (b) \(Li_{2}CO_{3}\)
• (c) \(K_{2}CO_{3}\)
• (d) \(MgCO_{3}\)

Correct Answer: (b)

Explanation: Lithium carbonate (\(Li_{2}CO_{3}\)) is used in psychiatric medicine for treating manic depression.

18. A compound X on heating gives a colourless gas which is not supporter of combustion. The residue is dissolved in water and excess of \(CO_{2}\) is passed through it. Compound Y is formed, which is recovered in solid form. Y on gentle heating gives back X. The compound X is:

• (a) \(CaCO_{3}\)
• (b) \(NaNO_{3}\)
• (c) \(CaSO_{4} \cdot 2H_{2}O\)
• (d) \(K_{2}CO_{3}\)

Correct Answer: (a)

Explanation: \(CaCO_{3}\) (X) decomposes to \(CaO\) and \(CO_{2}\). Dissolving \(CaO\) in water gives \(Ca(OH)_{2}\), which reacts with excess \(CO_{2}\) to form \(Ca(HCO_{3})_{2}\) (Y). Gentle heating of (Y) returns (X).

19. Which oxide is formed when potassium is heated in excess of oxygen?

• (a) \(K_{2}O\)
• (b) KO
• (c) \(K_{2}O_{2}\)
• (d) \(KO_{2}\)

Correct Answer: (d)

Explanation: Potassium reacts with excess oxygen to form the superoxide (\(KO_{2}\)).

20. Oxone is:

• (a) \(KO_{2}\)
• (b) \(Na_{2}O_{2}\)
• (c) \(Li_{2}O\)
• (d) CaO

Correct Answer: (b)

Explanation: Sodium peroxide (\(Na_{2}O_{2}\)) is commercially known as Oxone.

21. \(Na_{2}O_{2}\) has light yellow colour. This is due to:

• (a) presence of unpaired electron in the molecule
• (b) presence of trace of \(NaO_{2}\)
• (c) presence of \(KO_{2}\) as an impurity
• (d) none of these

Correct Answer: (b)

Explanation: Pure \(Na_{2}O_{2}\) is white, but the commercial product is often yellowish due to traces of the superoxide \(NaO_{2}\).

22. Substance A reacts with water to produce combustible gas B and solution C. D reacts with C to produce B on warming, but D can produce B with dil. \(H_{2}SO_{4}\) at room temperature. A imparts golden yellow colour to flame. A, B, C, D are:

• (a) Na, \(H_{2}\), NaOH and Zn
• (b) K, \(H_{2}\), KOH and Al
• (c) Ca, \(H_{2}\), \(Ca(OH)_{2}\) and Sn
• (d) \(CaC_{2}\), \(C_{2}H_{2}\), \(Ca(OH)_{2}\) and Sn

Correct Answer: (a)

Explanation: Na (A) reacts with water to give \(H_{2}\) (B) and NaOH (C). Zn (D) reacts with NaOH to give \(H_{2}\), and also reacts with dilute acid to give \(H_{2}\).

23. Match (X) with (Y): P. Sorel cement; Q. Anhydrone; R. Hydrolith; S. Lithopone.

• (a) 1-P, 2-Q, 3-R, 4-S
• (b) 2-P, 3-Q, 4-R, 1-S
• (c) 3-P, 4-Q, 1-R, 2-S
• (d) 4-P, 1-Q, 2-R, 3-S

Correct Answer: (c)

Explanation: Sorel cement is \(MgCl_{2} \cdot 5MgO \cdot xH_{2}O\). Anhydrone is \(Mg(ClO_{4})_{2}\). Hydrolith is \(CaH_{2}\). Lithopone is \(BaSO_{4} + ZnS\).

24. Among \(KO_{2}\), \(AlO_{2}^{-}\), \(BaO_{2}\) and \(NO_{2}^{+}\), unpaired electron is present in:

• (a) \(NO_{2}^{+}\) and \(BaO_{2}\)
• (b) \(KO_{2}\) and \(AlO_{2}^{-}\)
• (c) \(KO_{2}\) only
• (d) \(BaO_{2}\) only

Correct Answer: (c)

Explanation: \(KO_{2}\) is a superoxide (\(O_{2}^{-}\)), which contains one unpaired electron in its \(\pi^{*}\) molecular orbital and is paramagnetic.

25. The name and formula of the compound of magnesium, chlorine and oxygen used as a drying agent is:

• (a) magnesium oxychlorite, \(Mg(OCl)_{2}\)
• (b) magnesium chlorate, \(Mg(ClO_{3})_{2}\)
• (c) magnesium perchlorate, \(Mg(ClO_{4})_{2}\)
• (d) none of the above

Correct Answer: (c)

Explanation: Magnesium perchlorate (\(Mg(ClO_{4})_{2}\)), also known as Anhydrone, is used as a powerful drying agent.

26. \(Be_{2}C, CaC_{2}, Mg_{2}C_{3}\) on hydrolysis give X, Y, Z respectively:

• (a) \(CH_{4}, C_{2}H_{2}, C_{3}H_{8}\)
• (b) \(CH_{4}, C_{2}H_{6}, C_{3}H_{8}\)
• (c) \(CH_{4}, C_{2}H_{2}, C_{3}H_{4}\)
• (d) \(C_{2}H_{2}, C_{2}H_{6}, C_{3}H_{4}\)

Correct Answer: (c)

Explanation: Beryllium carbide gives methane, calcium carbide gives acetylene, and magnesium carbide (\(Mg_{2}C_{3}\)) gives propyne (\(C_{3}H_{4}\)).

27. Calculate the heat of solution of sodium chloride given hydration energy of \(Na^{+} = -389.4\), \(Cl^{-} = -382.3\) and lattice energy of NaCl = -776 kJ/mol.

• (a) +8.6 kJ/mol
• (b) +4.3 kJ/mol
• (c) -4.3 kJ/mol
• (d) -8.6 kJ/mol

Correct Answer: (b)

Explanation: \(\Delta H_{sol} = \text{Hydration energy} - \text{Lattice energy} = (-389.4 - 382.3) - (-776) = -771.7 + 776 = +4.3\) kJ/mol.

28. The carbide of which metal on hydrolysis gives propyne?

• (a) Be
• (b) Ca
• (c) Al
• (d) Mg

Correct Answer: (d)

Explanation: Magnesium carbide (\(Mg_{2}C_{3}\)) reacts with water to produce propyne.

29. Property of all alkaline earth metals that increases with atomic number is:

• (a) ionisation energy
• (b) solubility of their hydroxides
• (c) solubility of their sulphates
• (d) electronegativity

Correct Answer: (b)

Explanation: As the size of the cation increases down the group, lattice enthalpy decreases more rapidly than hydration enthalpy, resulting in increased hydroxide solubility.

30. Carnallite is the mineral of:

• (a) Mg
• (b) Na
• (c) Zn
• (d) Ca

Correct Answer: (a)

Explanation: Carnallite has the formula \(KCl \cdot MgCl_{2} \cdot 6H_{2}O\).

31. The radius of which ion (hydrated) is lowest?

• (a) \([Li_{(aq)}]^{+}\)
• (b) \([Na_{(aq)}]^{+}\)
• (c) \([K_{(aq)}]^{+}\)
• (d) \([Cs_{(aq)}]^{+}\)

Correct Answer: (d)

Explanation: Larger ions have a lower charge density and attract fewer water molecules, so \(Cs^{+}\) has the smallest hydrated radius.

32. An ingredient of baking powder is:

• (a) \(NaHCO_{3}\)
• (b) \(Na_{2}CO_{3}\)
• (c) \(Na_{2}SO_{4}\)
• (d) NaCl

Correct Answer: (a)

Explanation: Sodium bicarbonate (\(NaHCO_{3}\)) is a primary component of baking powder.

33. Which of the bicarbonates does not exist in solid state?

• (a) \(NaHCO_{3}\)
• (b) \(KHCO_{3}\)
• (c) \(Ca(HCO_{3})_{2}\)
• (d) \(RbHCO_{3}\)

Correct Answer: (c)

Explanation: Bicarbonates of alkaline earth metals are known only in aqueous solution and do not exist in the solid state.

34. Strongest reducing agent amongst alkali metals in solution is:

• (a) Li
• (b) Na
• (c) K
• (d) Cs

Correct Answer: (a)

Explanation: Although Lithium has the highest ionization enthalpy, its very high hydration energy makes it the strongest reducing agent in aqueous solution.

35. Sodium carbonate can be manufactured by Solvay's process but potassium carbonate cannot be prepared because:

• (a) \(K_{2}CO_{3}\) is more soluble
• (b) \(K_{2}CO_{3}\) is less soluble
• (c) \(KHCO_{3}\) is more soluble than \(NaHCO_{3}\)
• (d) \(KHCO_{3}\) is less soluble than \(NaHCO_{3}\)

Correct Answer: (c)

Explanation: \(KHCO_{3}\) is too soluble to be precipitated by the addition of ammonium bicarbonate to a saturated solution of KCl.

36. Which of the following statements is/are true for IIA group elements?

• (a) Except beryllium halides, all are ionic.
• (b) All form nitrides in air.
• (c) Hydroxide solubility increases down the group.
• (d) All are correct.

Correct Answer: (d)

Explanation: All three provided statements correctly describe properties of alkaline earth metals.

37. The correct sequence of increasing covalent character is:

• (a) \(BeCl_{2} < NaCl < LiCl\)
• (b) \(NaCl < LiCl < BeCl_{2}\)
• (c) \(BeCl_{2} < LiCl < NaCl\)
• (d) \(LiCl < NaCl < BeCl_{2}\)

Correct Answer: (b)

Explanation: According to Fajan's rule, smaller cations with higher charges (like \(Be^{2+}\)) have higher polarizing power and thus more covalent character.

38. Which is least thermally stable?

• (a) \(Li_{2}CO_{3}\)
• (b) \(MgCO_{3}\)
• (c) \(BaCO_{3}\)
• (d) \(BeCO_{3}\)

Correct Answer: (d)

Explanation: Thermal stability of carbonates increases down the group; thus \(BeCO_{3}\) is the least stable.

39. The element which does not directly combine with carbon on strong heating is:

• (a) Li
• (b) Be
• (c) K
• (d) Ca

Correct Answer: (c)

Explanation: While Li and Group 2 elements form carbides directly, the other alkali metals do not react with carbon directly.

40. Sodium metal reacts with \(Al_{2}O_{3}\) at high temp to give X. X reacts with \(CO_{2}\) in water to form Y. Y is:

• (a) \(Na_{2}O_{2}\)
• (b) \(Na_{2}O\)
• (c) \(Na_{2}CO_{3}\)
• (d) \(NaAlO_{2}\)

Correct Answer: (c)

Explanation: Sodium reduces alumina to give \(Na_{2}O\) (X), which then reacts with \(CO_{2}\) in water to form sodium carbonate (Y).

41. On heating quick lime with coke in an electric furnace, we get:

• (a) Ca and \(CO_{2}\)
• (b) \(CaCO_{3}\)
• (c) CaO
• (d) \(CaC_{2}\)

Correct Answer: (d)

Explanation: The reaction between quick lime (CaO) and coke at high temperature produced calcium carbide (\(CaC_{2}\)).

42. What are the products formed when \(Li_{2}CO_{3}\) undergoes decomposition?

• (a) \(Li_{2}O_{2} + CO\)
• (b) \(Li_{2}O + CO\)
• (c) \(Li_{2}O + CO_{2}\)
• (d) \(LiO_{2} + CO\)

Correct Answer: (c)

Explanation: Lithium carbonate decomposes uniquely among alkali carbonates to form the oxide and \(CO_{2}\).

43. In Solvay ammonia process, sodium bicarbonate is precipitated due to:

• (a) presence of \(NH_{3}\)
• (b) reaction with \(CO_{2}\)
• (c) reaction with brine solution
• (d) reaction with NaOH

Correct Answer: (c)

Explanation: The high concentration of \(Na^{+}\) ions in the brine (concentrated salt solution) causes \(NaHCO_{3}\) to precipitate due to the common ion effect.

44. Select the correct statement:

• (a) Solubility of alkali hydroxides: \(LiOH > NaOH > KOH > RbOH\)
• (b) Solubility of alkali carbonates: \(Li_{2}CO_{3} > Na_{2}CO_{3} > K_{2}CO_{3} > Rb_{2}CO_{3}\)
• (c) Both are correct
• (d) None is correct

Correct Answer: (b)

Explanation: According to the source, the solubility of alkali metal carbonates follows the order: \(Li_{2}CO_{3} > Na_{2}CO_{3} > K_{2}CO_{3} > Rb_{2}CO_{3}\).

45. The pair of compounds which cannot exist together in solution is:

• (a) \(NaHCO_{3}\) and NaOH
• (b) \(Na_{2}CO_{3}\) and \(NaHCO_{3}\)
• (c) \(Na_{2}CO_{3}\) and NaOH
• (d) \(NaHCO_{3}\) and NaCl

Correct Answer: (a)

Explanation: Sodium bicarbonate is an acidic salt and reacts with the strong base NaOH to form the neutral carbonate, so they cannot coexist.