Real solutions do not always obey Raoult's law. Positive deviation means the new A-B forces between the two liquids are WEAKER than the forces in the pure liquids, so molecules escape more easily and the vapour pressure goes ABOVE the ideal value. Negative deviation means A-B forces are STRONGER (often new hydrogen bonds), so molecules escape less and vapour pressure goes BELOW the ideal value. Memory hook: "Weaker glue = higher escape (Positive, P for Pop-out); Stronger glue = lower escape (Negative)."
Left: positive deviation, the real curve rises ABOVE the ideal Raoult's law line because A-B forces are weaker (weaker glue, more escape). Right: negative deviation, the real curve falls BELOW the ideal line because A-B forces are stronger, such as a new hydrogen bond (stronger glue, less escape).
Your doubts, answered
What is the simplest way to tell positive from negative deviation?
Look at the forces between the two different molecules (A-B) compared to the forces inside each pure liquid (A-A and B-B). If the A-B force is WEAKER than the pure-liquid forces, molecules escape more easily, vapour pressure rises above the ideal line = POSITIVE deviation. If the A-B force is STRONGER (like a new hydrogen bond), molecules are held tighter, vapour pressure drops below the ideal line = NEGATIVE deviation. Weaker glue means positive, stronger glue means negative.
Why does acetone + chloroform show negative deviation?
When you mix them, the acidic hydrogen of chloroform (CHCl3) forms a NEW hydrogen bond with the oxygen of acetone's C=O group. This new A-B attraction is stronger than the attractions in pure acetone and pure chloroform. Stronger attraction means molecules escape less, so the observed vapour pressure is LOWER than Raoult's law predicts. That is negative deviation. This exact pair is a NEET favourite.
Why does ethanol + water (or ethanol + acetone) show positive deviation?
Pure ethanol and pure water have strong hydrogen bonds. When you add the other liquid, its molecules get between the ethanol molecules and BREAK some of those hydrogen bonds. The new A-B attraction is weaker than the broken A-A and B-B bonds. So molecules escape more easily and vapour pressure goes ABOVE the ideal value = positive deviation.
What are the signs of delta H(mix) and delta V(mix) for each?
For POSITIVE deviation: mixing needs energy to break the stronger pure-liquid bonds, so delta H(mix) is positive (endothermic) and delta V(mix) is positive (volume expands). For NEGATIVE deviation: new stronger bonds release energy, so delta H(mix) is negative (exothermic) and delta V(mix) is negative (volume shrinks). Only IDEAL solutions have delta H = 0 and delta V = 0.
How do I check deviation using numbers in a NEET question?
First find the IDEAL total pressure with Raoult's law: P(ideal) = P_A x_A + P_B x_B. Then compare it to the OBSERVED pressure given in the question. If observed is GREATER than ideal, it is positive deviation. If observed is LESS than ideal, it is negative deviation. If they are equal, it is an ideal solution.
Which deviation gives which type of azeotrope?
Positive deviation gives a MINIMUM boiling azeotrope (high vapour pressure = boils at a lower temperature), for example ethanol + water. Negative deviation gives a MAXIMUM boiling azeotrope (low vapour pressure = boils at a higher temperature), for example water + nitric acid or acetone + chloroform. This link is tested directly in NEET.
⚠️ The NEET trap ✗ Acetone + chloroform shows positive deviation because two different molecules are mixed. ✓ Acetone + chloroform shows NEGATIVE deviation. A new hydrogen bond forms between chloroform's H and acetone's C=O oxygen, so A-B attraction is stronger than the pure-liquid forces, and vapour pressure drops below the ideal value. 🧠 See 'acetone + chloroform' in a NEET option? Reflex answer: NEGATIVE deviation, new H-bond, minimum vapour pressure, maximum boiling azeotrope.
Real NEET questions
NEET 2020
The mixture which shows positive deviation from Raoult's law is:
A · Acetone + Chloroform
B · Chloroethane + Bromoethane
C · Ethanol + Acetone ✓
D · Benzene + Toluene
Solution: Positive deviation happens when A-B interactions are WEAKER than A-A and B-B interactions, so vapour pressure rises above the ideal value. In ethanol + acetone, acetone molecules break the hydrogen bonds present in pure ethanol, weakening the overall attraction, so it shows positive deviation. Acetone + chloroform is the standard NEGATIVE deviation pair (a new H-bond forms). Chloroethane + bromoethane and benzene + toluene behave nearly ideally. Answer: (C).
NEET 2025
5 moles of liquid X and 10 moles of liquid Y make a solution having a vapour pressure of 70 torr. The vapour pressures of pure X and Y are 63 torr and 78 torr respectively. Which of the following is true regarding the described solution?
A · The solution is ideal.
B · The solution has volume greater than the sum of individual volumes.
C · The solution shows positive deviation.
D · The solution shows negative deviation. ✓
Solution: Mole fractions: x(X) = 5/15 = 1/3 and x(Y) = 10/15 = 2/3. If the solution were ideal, Raoult's law gives P(ideal) = 63 x (1/3) + 78 x (2/3) = 21 + 52 = 73 torr. The observed pressure is 70 torr, which is LESS than 73 torr. Since observed vapour pressure is below the ideal value, the solution shows NEGATIVE deviation. Answer: (D).
NEET 2026
A mixture of chloroform and acetone forms a solution with negative deviation from Raoult's law due to:
A · Increase in escaping tendency of molecules of each component.
B · Formation of hydrogen bonding between acetone and chloroform. ✓
C · Stronger intermolecular forces between chloroform molecules than those between chloroform and acetone molecules.
D · Repulsive forces.
Solution: Chloroform (CHCl3) and acetone ((CH3)2CO) show negative deviation because the acidic hydrogen of chloroform forms a NEW hydrogen bond with the carbonyl oxygen of acetone. This new A-B attraction is stronger than the A-A and B-B attractions, so the escaping tendency and vapour pressure drop below the Raoult's law value. Answer: (B).
Solved Solutions NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Does positive deviation mean high or low vapour pressure?
High. Positive deviation means the observed vapour pressure is HIGHER than the ideal value predicted by Raoult's law, because the A-B forces are weaker and molecules escape more easily.
Is delta H positive or negative for positive deviation?
Positive (endothermic). Mixing must break stronger pure-liquid bonds and only form weaker A-B bonds, so it absorbs heat. delta V is also positive (volume increases).
Give two common examples of each deviation for NEET.
Nearly every year NEET asks you to identify positive or negative deviation, or to link it to azeotropes and the sign of delta H. Knowing the force rule (weaker A-B = positive, stronger A-B = negative) lets you answer these in seconds without calculation.
What happens at the extremes of the composition (pure liquids)?
At both ends, where one liquid is almost pure, real solutions get very close to Raoult's law line. The deviation is largest in the middle composition range and disappears as you approach either pure liquid.
