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Exploring Mixtures and their Separation: questions with answers

Class 9 Science, Chapter 5 (NCERT). 15 questions by concept. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

Homogeneous and heterogeneous mixtures

Question 1

Salt disappears uniformly in water, while sand remains visible and later settles. How should the mixtures be classified?

  1. (a)Salt-water is homogeneous; sand-water is heterogeneous.
  2. (b)Both are uniform because both were stirred.
  3. (c)The clear salt-water sample must be a pure substance.
  4. (d)Both mixtures must be homogeneous because all mixtures are uniform.
Show answer

(a) Salt-water is homogeneous; sand-water is heterogeneous.

Uniform dissolved salt forms a solution. Visible sand and settling show non-uniform composition.

Question 2

How is clear white vinegar classified?

  1. (a)A colloid
  2. (b)A suspension
  3. (c)A homogeneous mixture
  4. (d)A pure substance
Show answer

(c) A homogeneous mixture

White vinegar is a solution of acetic acid in water, with the same composition throughout. It is a homogeneous mixture; clarity does not establish purity.

Solutions, solute, solvent, and concentration

Question 3

A large amount of sugar dissolves fully in water. In this sugar solution, which substance is the solute and which is the solvent?

  1. (a)Sugar the solute, water the solvent
  2. (b)No solute and no solvent, as this is not a solution
  3. (c)Sugar the solute, the sugar solution the solvent
  4. (d)Water the solute, sugar the solvent
Show answer

(a) Sugar the solute, water the solvent

The solute is the substance that dissolves. The solvent is the substance in which it dissolves. Sugar dissolves in water, so sugar is the solute and water is the solvent. A large amount of dissolved sugar still gives a solution.

Question 4

Three salt solutions each have a volume of 100 mL. They contain 2 g, 5 g and 10 g of salt. Which solution is the most concentrated?

  1. (a)The solution containing 5 g of salt
  2. (b)The solution containing 10 g of salt
  3. (c)The three solutions are equally concentrated.
  4. (d)The solution containing 2 g of salt
Show answer

(b) The solution containing 10 g of salt

All three solutions have the same volume, 100 mL. Concentration compares the amount of solute present in a given amount of solution. The solution holding the most salt, 10 g, is therefore the most concentrated.

Percentage concentration by mass and volume

Question 5

Ten grams of salt are dissolved in 90 g of water. What is the mass-by-mass percentage of the solution?

  1. (a)0.10% m/m
  2. (b)10% m/m
  3. (c)11.1% m/m
  4. (d)10% v/v
Show answer

(b) 10% m/m

Mass of solution = 10 g + 90 g = 100 g. Mass by mass percentage = (10/100) × 100 = 10% m/m.

Question 6

A solution contains 5 g glucose in a final volume of 100 mL. What is its concentration?

  1. (a)The value must use the water volume before dissolving.
  2. (b)5% m/v
  3. (c)5% m/m because every percentage uses masses
  4. (d)0.05% m/v because 5/100 is already a percentage
Show answer

(b) 5% m/v

Mass of solute over final solution volume gives 5/100 × 100 = 5% m/v.

Solubility, saturation, and crystallization

Question 7

A solution is saturated at a stated temperature. How much solute does it hold?

  1. (a)The maximum that dissolves at that temperature
  2. (b)The same amount at every temperature
  3. (c)Only what boiling the solvent away leaves
  4. (d)None
Show answer

(a) The maximum that dissolves at that temperature

A saturated solution holds the maximum solute that can dissolve at that temperature. This limit is fixed only while the temperature stays the same.

Question 8

A saturated solution is prepared by dissolving 287 g of compound B in 100 g of water at 60°C. The solubility of compound B is 241 g per 100 g of water at 40°C. How much compound B separates as crystals when this solution is cooled to 40°C?

  1. (a)Nothing separates, because solubility does not change with temperature.
  2. (b)46 g of compound B separates as crystals.
  3. (c)Crystals appear only after every drop of water evaporates.
  4. (d)All 287 g separates, because saturated means nothing is dissolved.
Show answer

(b) 46 g of compound B separates as crystals.

The solution starts with 287 g of compound B dissolved in 100 g of water. At 40°C the same 100 g of water can keep only 241 g dissolved. So 287 − 241 = 46 g separates as crystals.

Suspensions, centrifugation, and coagulation

Question 9

Which observation is characteristic of a sand-water suspension?

  1. (a)The particles dissolve when the container is centrifuged.
  2. (b)The particles remain dispersed forever like a stable colloid.
  3. (c)The mixture becomes a clear solution after stirring.
  4. (d)Visible undissolved particles can settle on standing.
Show answer

(d) Visible undissolved particles can settle on standing.

Sand does not dissolve in water. The sand particles are large enough to be seen, they scatter light, and they settle at the bottom when the mixture is left standing. Stirring only spreads them again; it does not make a solution, and centrifuging only forces them down faster.

Question 10

What does high-speed centrifugation do to a mixture such as blood?

  1. (a)It drives the heavier components outward and leaves the lighter liquid nearer the centre.
  2. (b)It mixes the blood into one uniform solution.
  3. (c)It dissolves the solid particles in the liquid.
  4. (d)It stops the suspended particles from settling at all.
Show answer

(a) It drives the heavier components outward and leaves the lighter liquid nearer the centre.

Fast spinning throws the denser components outward, so they collect away from the centre while the lighter liquid stays nearer it. Nothing dissolves, and the mixture does not become uniform.

Colloids and the Tyndall effect

Question 11

Which description fits a colloid such as milk?

  1. (a)A true solution with particles below 1 nm
  2. (b)Dispersed particles large enough to scatter light
  3. (c)No scattering of light
  4. (d)Particles that settle quickly, like a suspension
Show answer

(b) Dispersed particles large enough to scatter light

Colloid particles are about 1 nm to 1000 nm across. They are larger than the particles of a true solution, which are below 1 nm, and smaller than suspension particles. So they stay dispersed instead of settling, and they scatter a beam of light.

Question 12

A laser path is invisible through salt solution but visible through diluted milk. Why?

  1. (a)The milk beam is visible only because its particles quickly settle.
  2. (b)Colloid particles in milk scatter the beam; true-solution particles are too small to do so.
  3. (c)Milk must be a true solution because it looks uniform.
  4. (d)The salt solution shows the stronger Tyndall effect because it is transparent.
Show answer

(b) Colloid particles in milk scatter the beam; true-solution particles are too small to do so.

The visible path is the Tyndall effect caused by particles of suitable size.

Separating immiscible liquids and sublimable solids

Question 13

Mustard oil floats above water in a separating funnel. What is collected first through the stopcock?

  1. (a)The lower water layer
  2. (b)The lower mustard oil layer
  3. (c)The upper mustard oil layer
  4. (d)One uniform layer of oil and water
Show answer

(a) The lower water layer

Mustard oil and water do not mix, so they stand as two layers. Oil floats, so water is the lower layer. The stopcock is at the bottom of the funnel, so the lower water layer runs out first.

Question 14

Two immiscible liquids are poured into a jar and left to stand. What decides which liquid forms the upper layer?

  1. (a)The order in which they were poured
  2. (b)Their identities; water is always the upper layer
  3. (c)Their relative densities
  4. (d)Nothing; immiscible liquids merge into one layer
Show answer

(c) Their relative densities

Immiscible liquids do not mix, so they stay as two layers. Gravity sorts them by density, and the less dense liquid forms the upper layer. Which liquid it is, and the order of pouring, do not decide this.

Distillation and paper chromatography

Question 15

During distillation, what sequence produces the collected liquid?

  1. (a)The condenser adds heat so the vapour cannot condense.
  2. (b)Only the residue left in the flask is collected; the vapour is thrown away.
  3. (c)The vapour is collected as distillate first and then heated in the condenser.
  4. (d)The mixture is heated, vapour enters the condenser, and the cooled vapour becomes distillate.
Show answer

(d) The mixture is heated, vapour enters the condenser, and the cooled vapour becomes distillate.

Heating changes one part of the mixture into vapour. The condenser cools this vapour and turns it back into liquid, which is collected as distillate.

Common misconception in Exploring Mixtures and their Separation

Open full misconception guide →

Does clear water have to be pure?

Wrong

A clear liquid must be pure water.

Why it feels right. I use visible particles to recognise mixtures.

Right

Dissolved substances can be invisible.

Worked example

Pure water
Clear sample→Evaporate→No solid residue
Salt solution
Clear sample→Evaporate→Salt remains
The two clear samples look alike at first. Evaporating them reveals the dissolved solid in one.
  1. Compare pure water and a clear salt solution.
  2. Evaporate a sample of each under supervision.
  3. The salt solution leaves a solid residue.