Among the statements (a)-(d) the correct ones are :
(a) Lithium has the highest hydration enthalpy among the alkali metals.
(b) Lithium chloride is insoluble in pyridine.
(c) Lithium cannot form ethynide upon its reaction with ethyne.
(d) Both lithium and magnesium react slowly with H₂O.

Solution

Chemistry ma’am · 2 steps

Tiny Li⁺ polarises strongly, so lithium copies magnesium and its chloride turns covalent.

1. One root cause. Li⁺ is the smallest alkali metal ion, so its charge density is very high. This explains its strong hydration, the covalent character of its compounds and its diagonal likeness to Mg.

   Statement	Verdict	Reason
   (a) Highest hydration enthalpy	True	The smallest ion pulls water molecules in most strongly. Hydration enthalpy falls from Li⁺ down to Cs⁺.
   (b) LiCl insoluble in pyridine	False	Li⁺ polarises the chloride ion, so LiCl is partly covalent. It dissolves in organic solvents like pyridine and ethanol.
   (c) No ethynide with ethyne	True	Na and K give ethynides (acetylides) with ethyne, but lithium does not. This is one of its anomalous properties.
   (d) Li and Mg react slowly with water	True	This is a diagonal relationship property. Both react only slowly with water, unlike Na and K, which react vigorously.

2. So. Statements (a), (c) and (d) are correct and (b) is wrong, which is option (d).

The idea to keep. Small Li⁺ means high charge density. That gives it the strongest hydration, covalent compounds that dissolve in organic solvents, and a likeness to Mg. Test every lithium statement this way.

Answer from the NTA final answer key. Solution by PrepWiser.