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Which of the following conditions is NOT satisfied by an ideal solution?

1. Volume change on mixing is zero (delta_mix V = 0)
2. Entropy change on mixing is zero (delta_mix S = 0)
3. Obeys Raoult's law over the entire composition range
4. Enthalpy change on mixing is zero (delta_mix H = 0)

Correct answer: Entropy change on mixing is zero (delta_mix S = 0)

Solution

An ideal solution is characterized by: (1) delta_mix H = 0 (no heat of mixing), (2) delta_mix V = 0 (no volume change on mixing), (3) Obedience to Raoult's law (p_A = x_A * p_A⁰), and (4) delta_mix S > 0 (positive entropy of mixing, since mixing increases disorder/randomness even without intermolecular interactions). Therefore, delta_mix S = 0 is NOT a property of ideal solutions — ideal solutions actually have positive entropy of mixing. The condition delta_mix S = 0 is NOT satisfied (the actual value is positive, not zero).

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