JEE Main 2025 Apr 2 Shift 2, Chemistry Q12: Types Concentration And Solubility of Solutions

Water shows anomalous expansion: its density is highest (volume lowest) at $\sim 4{}^{\circ }$C. From 1${}^{\circ }$C to 4${}^{\circ }$C the volume of the solution decreases, while from 4${}^{\circ }$C to 25${}^{\circ }$C the volume increases due to normal thermal expansion.

Since the amount of dissolved NaCl is constant and $M=\frac{n_{\text{solute}}}{V_{\text{solution}}}$:

From 1${}^{\circ }$C $\to$ 4${}^{\circ }$C: $V\downarrow \Rightarrow M\uparrow$.

From 4${}^{\circ }$C $\to$ 25${}^{\circ }$C: $V\uparrow \Rightarrow M\downarrow$.

Molarity therefore peaks near 4${}^{\circ }$C and then falls, matching plot (2).