1. Condition for an ideal solution

For an ideal solution, the intermolecular interactions satisfy: $A-A \, \approx \, B-B \, \approx \, A-B$ Because of this, mixing does not involve any heat effect.

Hence, $\Delta H_{\text{mix}} = 0$

2. Entropy of mixing

When two different liquids are mixed, randomness increases. So for ideal solutions, $\Delta S_{\text{mix}} > 0$ not zero.

In fact, for mixing of liquids, $\Delta S_{\text{mix}} = -nR(x_A \ln x_A + x_B \ln x_B)$ which is positive.

Therefore, option A is false.

3. Free energy of mixing

Free energy change on mixing is: $\Delta G_{\text{mix}} = \Delta H_{\text{mix}} - T\Delta S_{\text{mix}}$ For an ideal solution, $\Delta H_{\text{mix}} = 0$ so, $\Delta G_{\text{mix}} = -T\Delta S_{\text{mix}} < 0$ Thus, free energy of mixing is not zero.

Therefore, option B is false.

4. Checking option C

Option C says both free energy and entropy of mixing are zero. But for ideal solutions:

• $\Delta S_{\text{mix}} > 0$
• $\Delta G_{\text{mix}} < 0$ So option C is false.

5. Checking option D

For ideal solution, $\Delta H_{\text{mix}} = 0$ So option D is correct.

Final Answer

The correct option is D: $\text{enthalpy of mixing is zero}$