how-much-solute-dissolves MCQs for UPSC Prelims

30 practice questions on how-much-solute-dissolves from the The Amazing World of Solutes, Solvents, and Solutions section of the UPSC Prelims syllabus. 30 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

9 Easy 15 Medium 6 Hard

Sample questions

Q1
hard

Sea water is first passed through a cloth to remove sand and bits of weed, and then left in wide shallow pans in the Sun. After some days workers collect a white solid from the pans. Which statement about this solid is correct?

  1. A It was dissolved in the sea water and stayed behind when the water evaporated
  2. B It is the sand that slipped through the cloth
  3. C It came off the walls and floor of the pan
  4. D It was created by sunlight acting on the water
Show answer and explanation

Correct answer: A - It was dissolved in the sea water and stayed behind when the water evaporated

The salt was already dissolved in the sea water, and evaporation removes only the water, so the salt is left in the pan. Sunlight supplies heat but cannot create a new substance. The pans give the same yield whatever they are lined with, and the sand was removed before the water reached the pans.

Q2
medium

Consider these statements. Statement I: Powdering a solid raises the amount of it that a fixed quantity of water can hold. Statement II: Powdering a solid raises the rate at which it dissolves.

  1. A Both statements are wrong
  2. B Both statements are correct
  3. C Only Statement II is correct
  4. D Only Statement I is correct
Show answer and explanation

Correct answer: C - Only Statement II is correct

Powdering exposes more surface, so the solid dissolves faster and Statement II is correct. The greatest amount a solvent can hold depends on the substances and the temperature, not on the size of the pieces, so Statement I is wrong.

Q3
hard

Assertion (A): Warming a solution makes a solid dissolve in it faster. Reason (R): Powdering a solid increases the surface that is in contact with the solvent.

  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

Warming does speed up dissolving, so A is true, and powdering does increase the exposed surface, so R is true as well. But R describes a completely different way of speeding up dissolving and says nothing about temperature, so it cannot be the explanation of A.

Q4
easy

Powdered salt dissolves faster than the same mass of coarse rock salt in water at the same temperature, because the powder

  1. A tastes less salty than rock salt
  2. B has far more surface in contact with the water
  3. C is a different substance from rock salt
  4. D weighs less than the rock salt
Show answer and explanation

Correct answer: B - has far more surface in contact with the water

Breaking a solid into fine grains exposes far more of its surface to the solvent, so water can attack it at many more places at once. Powdered salt and rock salt are the same substance, and the two samples have equal mass. Taste has nothing to do with how fast a solid dissolves.

Q5
easy

A solution in which no more of the solute will dissolve at that temperature is said to be

  1. A pure
  2. B saturated
  3. C dilute
  4. D unsaturated
Show answer and explanation

Correct answer: B - saturated

A saturated solution already holds as much solute as it can at that temperature, so any further solute stays undissolved. An unsaturated solution can still take in more. Dilute only means a small amount of solute is present, and pure describes a single substance, not a solution.

Q6
medium

At room temperature, $30\ \text{g}$ of a solid is the most that will dissolve in $100\ \text{g}$ of water. If only $20\ \text{g}$ of the solid is stirred into $100\ \text{g}$ of water at the same temperature, the solution formed is

  1. A saturated
  2. B unsaturated
  3. C not a solution at all
  4. D saturated only while it is being stirred
Show answer and explanation

Correct answer: B - unsaturated

The water can hold $30\ \text{g}$, and only $20\ \text{g}$ has been added, so it can still dissolve more and is unsaturated. It is not saturated, because more solute would still dissolve. The solid does dissolve fully, so a solution certainly forms, and stirring changes only the speed, not whether the solution is saturated.

Q7
medium

Two glasses hold the same amount of water at the same temperature. In glass P, $10\ \text{g}$ of sugar dissolves completely. In glass Q, $40\ \text{g}$ of sugar is added and a little stays at the bottom even after long stirring. Which statement is correct?

  1. A Both are unsaturated
  2. B P is saturated and Q is unsaturated
  3. C P is unsaturated and Q is saturated
  4. D Both are saturated
Show answer and explanation

Correct answer: C - P is unsaturated and Q is saturated

In P all the sugar dissolved and more could still go in, so P is unsaturated. In Q some sugar refuses to dissolve, which shows the water is holding all it can and Q is saturated. The remaining choices swap the two labels or force both glasses into the same state, which the observations do not support.

Q8
medium

When a salt solution is evaporated to dryness, the water

  1. A escapes into the air as vapour, leaving the salt behind
  2. B changes into salt
  3. C stays behind in the dish along with the salt
  4. D joins with the salt to form a new substance
Show answer and explanation

Correct answer: A - escapes into the air as vapour, leaving the salt behind

Evaporation turns liquid water into vapour that leaves the dish, while the salt has no such tendency and stays as a solid. If the water stayed behind, the dish would not be dry. Water cannot turn into salt, and no new substance forms, because dissolving is not a chemical change.

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