1. What is being asked?

We need the qualitative variation of molarity $M$ of an NaCl solution as temperature increases from $1^\circ\text{C}$ to $25^\circ\text{C}$.

Given:

• $x$ g NaCl is added to water in a closed beaker with lid.
• Temperature is increased.
• Solubility of NaCl is assumed unchanged in this range.

So we must determine how $M = \frac{\text{moles of solute}}{\text{volume of solution in litre}}$ changes with temperature.

2. Effect on number of moles of solute

The amount of NaCl added is fixed: $x$ g. Hence moles of NaCl are constant: $n = \frac{x}{58.5}$ So the numerator of molarity remains constant.

Also, because the vessel has a lid, there is no loss of solvent by evaporation from the system.

3. Effect on volume of solution

As temperature increases, the solution undergoes thermal expansion. Therefore, the volume of the solution increases with temperature.

Thus, in $M = \frac{n}{V}$ with $n$ constant and $V$ increasing, molarity must decrease with temperature.

4. Nature of the decrease

For liquids over a small temperature range, volume generally changes approximately as $V_T = V_0(1 + \alpha \Delta T)$ where $\alpha$ is the coefficient of volume expansion.

Hence $M_T = \frac{n}{V_0(1+\alpha \Delta T)} = \frac{M_0}{1+\alpha \Delta T}$ This is a decreasing curve, not an increasing one.

Over a limited temperature range, it decreases smoothly with temperature.

5. Choosing the correct plot

So the correct graph must show:

• molarity decreases as temperature increases,
• because volume increases while moles remain constant.

Therefore, the best suited plot is Option D.

6. Comparison with stored answer

Stored correct answer: D

My derived answer: D

So they agree.