separating-homogeneous-mixtures MCQs for UPSC Prelims

25 practice questions on separating-homogeneous-mixtures from the Exploring Mixtures and their Separation section of the UPSC Prelims syllabus. 25 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

8 Easy 12 Medium 5 Hard

Sample questions

Q1
hard
Pair P is two liquids that mix completely and boil at $78^\circ\text{C}$ and $82^\circ\text{C}$. Pair Q is two liquids that mix completely and boil at $35^\circ\text{C}$ and $110^\circ\text{C}$. Which pair really needs a fractionating column for a clean separation, and why?
  1. A Pair P, because their boiling points are so close that simple distillation would bring both liquids over together
  2. B Neither pair, because a column is needed only when the liquids refuse to mix
  3. C Pair Q, because the wider gap between the boiling points makes separation harder
  4. D Both pairs equally, because both mix completely
Show answer and explanation

Correct answer: A - Pair P, because their boiling points are so close that simple distillation would bring both liquids over together

A gap of only $4^\circ\text{C}$ means that as soon as the first liquid boils the second is close behind, so the vapour is a mixture and a column is needed to purify it step by step. A gap of $75^\circ\text{C}$ is wide enough for simple distillation to do the job. Mixing completely is what makes distillation the right family of method, not what decides whether a column is needed, and liquids that do not mix are separated in a funnel instead.
Q2
medium
A student wants to recover the dissolved solid from a coloured solution, but this solid is spoiled when it is heated strongly, and boiling the solution over a flame ruins it. What should she do instead?
  1. A Boil the solution harder so that the water leaves more quickly
  2. B Pour the solution through filter paper to trap the solid
  3. C Spread the solution in a wide shallow dish and let the water evaporate slowly at room temperature
  4. D Add more water so that there is less solid to be damaged
Show answer and explanation

Correct answer: C - Spread the solution in a wide shallow dish and let the water evaporate slowly at room temperature

A wide shallow dish gives a large surface, so the water evaporates steadily at room temperature and the solid is never heated. Boiling harder makes the damage worse, and adding water does not change how much solid is present, only how long the drying takes. Filter paper cannot hold back a dissolved solid, since its particles pass through with the liquid.
Q3
medium
A spot of black ink is placed near the bottom of a strip of filter paper, and the lower edge of the paper is dipped into water so that the spot stays just above the water level. After some time, several coloured bands appear at different heights up the paper. What does this show?
  1. A The ink has been split into water and solid carbon
  2. B The black ink is a mixture of several dyes that travel up the damp paper at different speeds
  3. C The paper itself held dyes that the water has washed out of it
  4. D The water has reacted with the ink to make new colours
Show answer and explanation

Correct answer: B - The black ink is a mixture of several dyes that travel up the damp paper at different speeds

Water creeping up the paper carries the dyes with it, and a dye that is held less tightly by the paper moves further, so the components of the ink end up as separate bands. No new substances are made, which is why the bands are the colours the ink was mixed from. Plain filter paper carries no dye of its own, and the bands are dyes rather than carbon.
Q4
medium
Assertion (A): Distillation can be used to obtain pure water from a solution of common salt in water. Reason (R): Common salt dissolves readily in water.
  1. A Both A and R are true, and R is the correct explanation of A
  2. B Both A and R are true, but R is not the correct explanation of A
  3. C A is true, but R is false
  4. D A is false, but R is true
Show answer and explanation

Correct answer: B - Both A and R are true, but R is not the correct explanation of A

Both statements are true, but (R) only explains why salt water is a solution in the first place. Distillation works for a different reason: the water turns to vapour at its boiling point while the salt does not, so the vapour that is condensed is pure water. A substance being easy to dissolve does not by itself make distillation possible.
Q5
hard
To grow crystals, a chemist dissolves as much of a solid as she can in $200\ \text{g}$ of water at $80^\circ\text{C}$ and then leaves the flask to cool slowly to $20^\circ\text{C}$. The solid's solubility is $50\ \text{g}$ per $100\ \text{g}$ of water at $80^\circ\text{C}$ and $15\ \text{g}$ per $100\ \text{g}$ of water at $20^\circ\text{C}$. What mass of crystals can she collect from the flask?
  1. A $30\ \text{g}$
  2. B $35\ \text{g}$
  3. C $70\ \text{g}$
  4. D $100\ \text{g}$
Show answer and explanation

Correct answer: C - $70\ \text{g}$

With $200\ \text{g}$ of water, the hot solution holds $50 \times 2 = 100\ \text{g}$ of salt, and the cooled solution can keep only $15 \times 2 = 30\ \text{g}$, so $100 - 30 = 70\ \text{g}$ crystallises. Subtracting the two solubility figures alone gives $35\ \text{g}$ and forgets that there is twice the standard mass of water. The value $100\ \text{g}$ is all the salt present and $30\ \text{g}$ is what stays dissolved.
Q6
easy
In a distillation set-up, what happens inside the condenser?
  1. A The vapour is heated further so that it rises faster
  2. B The liquid in the flask is heated until it boils
  3. C The dissolved solid is trapped as the vapour passes through
  4. D The vapour is cooled and turns back into a liquid
Show answer and explanation

Correct answer: D - The vapour is cooled and turns back into a liquid

The condenser is surrounded by cold water, so vapour passing through it loses heat and becomes liquid again, which then runs into the receiver. Heating happens in the flask, not the condenser. The dissolved solid never enters the vapour at all, so there is nothing for the condenser to trap, and heating the vapour further would stop it condensing.
Q7
medium
Assertion (A): Evaporating a salt solution to dryness leaves the salt behind but loses the water. Reason (R): During evaporation only the water changes into vapour and escapes, while the salt stays in the dish.
  1. A Both A and R are true, and R is the correct explanation of A
  2. B Both A and R are true, but R is not the correct explanation of A
  3. C A is true, but R is false
  4. D A is false, but R is true
Show answer and explanation

Correct answer: A - Both A and R are true, and R is the correct explanation of A

Both statements are true. Because only the water vaporises and nothing is done to catch that vapour, the salt is left in the dish and the water is lost to the air, so (R) is exactly why (A) happens. Catching and cooling the vapour instead would turn the method into distillation and save the water as well.
Q8
easy
A shallow dish of sea water is left standing in the sun until all the liquid has gone, leaving a white solid in the dish. Which method has been used, and what does it recover?
  1. A Filtration, which recovers the dissolved salt
  2. B Sublimation, which recovers the dissolved salt
  3. C Condensation, which recovers the water
  4. D Evaporation, which recovers the dissolved salt
Show answer and explanation

Correct answer: D - Evaporation, which recovers the dissolved salt

The water has changed to vapour and escaped, leaving the dissolved solid behind, and that is evaporation. Filtration would trap nothing, because dissolved salt passes straight through filter paper. Condensation is vapour turning back to liquid, which is not what happened here, and sublimation is a solid turning straight to vapour, which the salt did not do.

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