For two liquids A and B to form an ideal solution

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70th BPSC Exam Official Paper (Re-exam held on 4th January 2025) Test
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  1. Entropy of mixing must be zero
  2. Free energy of mixing should be zero
  3. Enthalpy of mixing should be zero
  4. None of the above

Answer (Detailed Solution Below)

Option 3 : Enthalpy of mixing should be zero
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Detailed Solution

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The correct answer is option 3. 

Key Points

  • For two liquids (A and B) to form an ideal solution, they must follow Raoult’s Law and exhibit ideal mixing behavior. This requires:
  • ΔH_mix (Enthalpy of Mixing) = 0
    • In an ideal solution, the intermolecular forces between A-A, B-B, and A-B are nearly identical.
    • No heat is absorbed or released upon mixing, meaning the enthalpy change (ΔH_mix) is zero. Hence option 3 is the correct answer. 
  • ΔS_mix (Entropy of Mixing) > 0
    • Entropy increases due to greater randomness when two liquids mix.
  • ΔG_mix (Free Energy of Mixing) < 0
    • A spontaneous mixing process requires negative free energy change (ΔG_mix).
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