Two compounds A and B form a low-boiling azeotrope: When the mixture with composition b is distilled,

Question

BlackTom AI · Accepted Answer

A low‑boiling (minimum‑boiling) azeotrope is a composition at which the liquid and its vapor have identical compositions and the mixture boils at a lower temperature than either pure component. This arises from a positive deviation from Raoult’s law (an elevated total vapor pressure) so the mixture has a minimum on the T–x (or T–x–y) diagram.

Relevant concepts:
- Raoult’s law and deviations (positive deviation → higher vapor pressure → minimum boiling azeotrope).
- At an azeotropic composition the vapor and liquid compositions are the same; the azeotrope behaves like a single substance during boiling (constant‑boiling mixture).
- Simple distillation moves the liquid and vapor compositions along tie‑lines toward the azeotropic point; repeated or fractional distillation cannot separate past the azeotrope.

Step‑by‑step reasoning:
1. Start with liquid of composition b and heat to boiling. The vapor in equilibrium with that liquid has a composition given by the vapor–liquid equilibrium curve (y vs x).
2. For systems that form a low‑boiling azeotrope, as boiling proceeds the vapor is enriched toward the azeotropic composition because that composition has the highest vapor pressure (most volatile).
3. When the azeotropic composition is reached, the vapor and liquid compositions are identical and the mixture boils at the lowest temperature (minimum).
4. Therefore the condensate (distillate) collected will be at the azeotropic composition and will boil at the lowest temperature of any composition in the system.

Why the correct option is correct:
- Option d states that the vapour (condensate) forms an azeotrope which has the lowest boiling point — this matches the definition and behavior of a minimum‑boiling azeotrope.

Why the other options are incorrect:
- a: A minimum‑boiling azeotrope does not give the highest boiling point; it gives the lowest.
- b: The liquid in the flask doesn’t necessarily “form an azeotrope” unless its composition is exactly the azeotropic composition; also the azeotrope is not described as “most volatile of all compositions” in that phrasing.
- c: The azeotrope is the most volatile (lowest boiling), not the least volatile.

Conclusion: The correct answer is d. The vapour (condensate) forms an azeotrope, which has the lowest boiling point.

Two compounds A and B form a low-boiling azeotrope: When the mixture with composition b is distilled,单项选择题

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