Raoult’s law and ideality:\nRaoult’s law is exactly obeyed by which type of liquid solution under the same temperature and pressure conditions?

Introduction / Context:
Raoult’s law states that the partial vapor pressure of each component in a liquid solution equals the product of its mole fraction in the liquid and the vapor pressure of the pure component at the same temperature. This principle is foundational for vapor–liquid equilibrium (VLE) and for designing separations such as distillation. However, not all real mixtures obey Raoult’s law exactly; understanding the special case in which it holds is essential.

Given Data / Assumptions:

• Constant temperature and pressure are considered.
• Liquid solution of volatile components A and B.
• Negligible association/dissociation in the liquid phase.

Concept / Approach:
Raoult’s law is exactly valid for an ideal solution. Ideal solutions exhibit similar intermolecular forces between unlike and like molecules (A–B interactions are comparable to A–A and B–B). As a result, activity coefficients approach unity across the composition range, and the escaping tendency of each species is proportional to its mole fraction. Real solutions often deviate (positive or negative) due to differences in molecular size, polarity, or specific interactions like hydrogen bonding.

Step-by-Step Solution:

Verification / Alternative check:
Activity coefficient models (e.g., Margules, NRTL, Wilson) reduce to γ_i = 1 under ideal conditions, reproducing Raoult’s law exactly. Experimental VLE data for nearly ideal systems (e.g., benzene–toluene) align closely with Raoult’s predictions.

Why Other Options Are Wrong:

• Saturated/supersaturated: describe concentration relative to solubility, not VLE ideality.
• “Molar” or “normal” solutions: these are concentration conventions; they do not guarantee ideal behavior.

Common Pitfalls:
Assuming any dilute solution obeys Raoult’s law; in very dilute solutions of a solute, Henry’s law is often more appropriate for the solute, with Raoult’s applying to the solvent.

Final Answer:
An ideal solution.