Question

Raoult’s law is applicable to the__________

• A. Ideal solutions
• B. Real solutions
• C. Mixture of water and alcohol
• D. Non‐ideal gases

Hint:

In the real world, most mixtures show "positive" or "negative" deviations from Raoult's law. Ethanol-water mixtures (Option C) show a significant positive deviation and form an azeotrope.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the specific physical state or system to which Raoult's Law applies most accurately. Raoult's law is a fundamental principle in thermodynamics and food chemistry (relevant to water activity and vapor pressure).
Key Formula or Approach:
Raoult's law states: $P_{solution} = X_{solvent} \cdot P^{\circ}_{solvent}$.
Where $P$ is the vapor pressure and $X$ is the mole fraction.

Step 2: Detailed Explanation:

• Ideal Solution Concept: An ideal solution is one where the intermolecular forces between unlike molecules ($A-B$) are identical to those between like molecules ($A-A$ and $B-B$). There is no heat change on mixing, and no volume change.

• Applicability: Raoult's law is strictly applicable only to ideal solutions. In such systems, the vapor pressure of a solvent above a solution is directly proportional to the mole fraction of the solvent in that solution.

• Real Systems: Most food systems are non-ideal. While very dilute solutions may follow Raoult's law approximately, they are technically "real solutions" that deviate from the law.

• Significance in Food Tech: It is used to estimate the reduction in water activity ($a_w$) when solutes like salt or sugar are added to water.

Step 3: Final Answer:
Raoult’s law is a law of ideal solution behavior. Option (A) is the correct answer.