Question

Which of the following liquid mixtures forms an ideal solution that strictly obeys Raoult's Law across the entire concentration range at a constant temperature?

• A. Ethanol + Acetone
• B. Chloroform + Acetone
• C. n-Hexane + n-Heptane
• D. Water + Nitric acid

Hint:

Look for homologous pairs on the exam to identify ideal solutions quickly: n-Hexane + n-Heptane, Bromoethane + Chloroethane, and Benzene + Toluene are the three primary NCERT examples.

Answer 1

The Correct Option is C

Concept: An ideal solution is formed when the solute-solute (\(A-A\)) and solvent-solvent (\(B-B\)) intermolecular attractive forces are nearly identical in magnitude and nature to the newly formed solute-solvent (\(A-B\)) interactions. For ideal solutions, the enthalpy of mixing is zero (\(\Delta H_{\text{mix}} = 0\)) and the volume change on mixing is zero (\(\Delta V_{\text{mix}} = 0\)).

Step 1: Analyze the chemical nature of the components in each mixture.
Ideal solutions are typically formed by mixing two chemical compounds that share highly similar molecular structures, polarities, and functional groups.

• n-Hexane and n-Heptane are both straight-chain non-polar alkanes that interact purely through weak London dispersion forces. Because their structural environments are nearly identical, their mixture forms a near-perfect Ideal Solution.

Step 2: Contrast with non-ideal solution types.

• Ethanol + Acetone: Shows positive deviation from Raoult's law because acetone breaks the strong hydrogen-bonded networks of ethanol.
• Chloroform + Acetone: Shows negative deviation because a new, strong intermolecular hydrogen bond forms between the chloroform hydrogen and the acetone carbonyl oxygen.
• Water + Nitric acid: Forms a maximum boiling azeotrope due to strong negative deviations from Raoult's law.