Think about the experiment you have performed and consider the following statements: The consecutive collected fractions are increasingly richer in water. The consecutive collected fractions are increasingly richer in ethanol. As the mixture in the distillation flask continues boiling, it boils at progressively higher temperatures. As the mixture in the distillation flask continues boiling, it boils at progressively lower temperatures. Select the combination of CORRECT statements:

Question

BlackTom AI · Accepted Answer

Ethanol is more volatile (bp 78.4°C) than water (bp 100.0°C), so vapour leaving the boiling flask is richer in ethanol than the liquid that remains. As distillation proceeds, the more-volatile ethanol is preferentially removed, leaving the liquid residue increasingly enriched in the less-volatile component (water). This changes the boiling behaviour of the residue.

Relevant concepts:
- Volatility and boiling point: lower bp = more volatile, preferentially enters vapour.
- Fractional/simple distillation principle: early distillate is richer in the more volatile component; the flask residue becomes richer in the less volatile component.
- Composition–temperature relationship: as composition shifts toward the less volatile component, the boiling temperature of the remaining mixture increases (until any azeotrope limit is reached).

Step-by-step reasoning:
1. Start with an ethanol–water mixture. Vapour in equilibrium above the liquid is ethanol-enriched due to ethanol’s higher vapour pressure.
2. Collect the early fractions: these are richer in ethanol relative to the original mixture.
3. After removing ethanol with successive fractions, the liquid left in the flask becomes progressively richer in water (less volatile).
4. Because the remaining mixture contains a higher fraction of the higher-boiling component (water), its boiling point rises. Thus successive boilings of the flask occur at progressively higher temperatures (until an azeotropic composition is approached, which can stabilize the boiling point).
5. Therefore collected fractions taken later will contain relatively more water than earlier ones.

Why the correct answer is correct:
- Statement (1) is true: later fractions are richer in water.
- Statement (3) is true: the residue’s boiling point increases as ethanol is removed.

Why the other statements are incorrect:
- Statement (2) is false because fractions do not become richer in ethanol over time; they become poorer as ethanol is removed.
- Statement (4) is false because the boiling point does not decrease as the mixture becomes enriched in the higher-boiling component.

Conclusion: The correct combination is c. (1) and (3).

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