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Chemistry · Class 12

Simulation · Chemistry · Class 12

When does a mixture stop obeying Raoult's law?

"When does a mixture stop obeying Raoult's law?" is a free, interactive Class 12 Chemistry simulation for NEET and JEE, built on the NCERT section "Ideal and non-ideal solutions" in Solutions. Total and partial vapour pressures bow above or below the straight Raoult lines as the A–B attraction is tuned, with ΔmixH and ΔmixV signs and an azeotrope appearing when the bow is big enough. NEET has asked about this idea 6 times in 10 years of papers. JEE Main asked 53 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).

From the lesson Ideal and non-ideal solutions in Solutions. Change the values and watch what happens.

When does a mixture stop obeying Raoult's law?Chemistry · Class 12

What it shows

NCERT §1.5

Total and partial vapour pressures bow above or below the straight Raoult lines as the A–B attraction is tuned, with ΔmixH and ΔmixV signs and an azeotrope appearing when the bow is big enough.

Controls

4 to try

  • SliderA–B attraction relative to A–A/B–B, −1 (much stronger) to +1 (much weaker) (Margules parameter)
  • Slidermole fraction of A in liquid 0–1
  • Togglepreset pair — benzene+toluene / ethanol+acetone / chloroform+acetone
  • Readoutsp_A, p_B, p_total, vapour y_A, ΔmixH sign, ΔmixV sign

Try this

Tune the A–B attraction until the total-pressure curve gets a maximum (a minimum-boiling azeotrope), then read the azeotrope composition where y_A = x_A.

Common mistake

Weaker A–B attraction is negative deviation / every mixture can be separated by fractional distillation.

In NEET

10 years of papers

Asked 6 times in 10 years of NEET.

See the past questions: Solutions and Colligative Properties, Solutions (chapter)

In JEE Main

42 shifts analysed

53 questions from Solutions across the 42 JEE Main shifts Lumi has analysed (2017–2026).

See the past questions

The idea behind it

NCERT §1.5

  • An ideal solution obeys Raoult's law over the whole range of composition. It also has ΔmixH = 0 (no heat taken in or given out on mixing) and ΔmixV = 0 (the volumes simply add).
  • Ideal behaviour needs A–B attractions nearly equal to the A–A and B–B attractions. No solution is perfectly ideal, but n-hexane with n-heptane, bromoethane with chloroethane, and benzene with toluene come close.
  • A non-ideal solution does not obey Raoult's law over the whole range; its vapour pressure is either higher than predicted (positive deviation) or lower (negative deviation).
  • Positive deviation: A–B attractions are weaker than A–A and B–B, so molecules escape more easily. Ethanol with acetone (acetone breaks some of ethanol's hydrogen bonds) and carbon disulphide with acetone behave this way; mixing is endothermic and the volume increases (ΔmixH > 0, ΔmixV > 0).
  • Negative deviation: A–B attractions are stronger, so escape is harder. Phenol with aniline (hydrogen bond between the phenolic H and the lone pair on N) and chloroform with acetone (chloroform's H bonds to acetone's O) behave this way; mixing is exothermic and the volume shrinks (ΔmixH < 0, ΔmixV < 0).
  • Azeotropes are binary mixtures whose liquid and vapour have the same composition, so they boil at a constant temperature and cannot be separated by fractional distillation.
  • A large positive deviation gives a minimum boiling azeotrope: fractional distillation of fermented ethanol-water stops at about 95% ethanol by volume.
  • A large negative deviation gives a maximum boiling azeotrope: nitric acid and water, about 68% nitric acid and 32% water by mass, boiling at 393.5 K.

Questions

5 answers

What does the "When does a mixture stop obeying Raoult's law?" simulation show?
Total and partial vapour pressures bow above or below the straight Raoult lines as the A–B attraction is tuned, with ΔmixH and ΔmixV signs and an azeotrope appearing when the bow is big enough.
What mistake does the "When does a mixture stop obeying Raoult's law?" simulation fix?
A common mistake it clears up: weaker A–B attraction is negative deviation / every mixture can be separated by fractional distillation.
Has NEET asked about this idea?
Yes. NEET has asked about this idea 6 times in 10 years of papers.
Does JEE Main ask questions from Solutions?
Yes. JEE Main asked 53 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
Is this simulation free?
Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Ideal and non-ideal solutions" in Solutions (Class 12 Chemistry).

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