Question

Identify the correct statements from the following
I. LiF is less soluble in water than NaF
II. Both LiCl and MgCl\(_2\) are insoluble in ethanol
III. Both Li and Mg form nitrides
IV. Na\(_2\)CO\(_3\) gives CO\(_2\) on heating

• A. I & IV
• B. I & III
• C. I & II
• D. II & III

Hint:

The diagonal relationship between Lithium (Group 1, Period 2) and Magnesium (Group 2, Period 3) is a key concept. They share many similar properties, such as forming nitrides, having covalent halides soluble in organic solvents, and having carbonates that decompose on heating.

Answer 1

The Correct Option is B

Let's analyze each statement.
I. LiF is less soluble in water than NaF: This statement is correct. The solubility of alkali metal fluorides depends on a balance between lattice enthalpy and hydration enthalpy. For LiF, the small size of both Li\(^+\) and F\(^-\) leads to a very high lattice enthalpy, which is not overcome by the hydration enthalpy of the ions. In contrast, NaF has a lower lattice enthalpy, making it more soluble.
II. Both LiCl and MgCl\(_2\) are insoluble in ethanol: This statement is incorrect. LiCl and MgCl\(_2\) are covalent in nature (due to the small size and high polarizing power of Li\(^+\) and Mg\(^{2+}\)). Covalent compounds tend to be soluble in organic solvents like ethanol. Both LiCl and MgCl\(_2\) are indeed soluble in ethanol.
III. Both Li and Mg form nitrides: This statement is correct. Lithium shows a diagonal relationship with Magnesium. One of the similarities is their reaction with nitrogen. Lithium reacts directly with nitrogen gas to form lithium nitride (\(Li_3N\)). Magnesium also reacts directly with nitrogen to form magnesium nitride (\(Mg_3N_2\)). Other alkali metals do not form nitrides directly.
IV. Na\(_2\)CO\(_3\) gives CO\(_2\) on heating: This statement is incorrect. Sodium carbonate (Na\(_2\)CO\(_3\)) is a very stable compound due to the high electropositivity of sodium. It does not decompose upon heating, even at its melting point (851 °C). In contrast, lithium carbonate (\(Li_2CO_3\)) and carbonates of Group 2 metals (like CaCO\(_3\)) do decompose on heating to give the oxide and CO\(_2\).
Therefore, the correct statements are I and III.