Class 9 Science: Exploring Mixtures and their Separation MCQs (NCERT)
126 practice questions on Exploring Mixtures and their Separation, a chapter in the NCERT Class 9 Science textbook, organised into 2 topics. Every question includes a written explanation. Pick a topic below, or try the samples first.
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?
APair P, because their boiling points are so close that simple distillation would bring both liquids over together
BNeither pair, because a column is needed only when the two liquids refuse to mix and form separate layers
CPair Q, because the wider gap between the boiling points makes the two liquids much harder to separate by simple distillation
DBoth 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?
ABoil the solution harder so that the water leaves more quickly, giving the solid less time in the flame to be spoiled
BPour the solution through filter paper to trap the dissolved solid
CSpread the solution in a wide shallow dish and let the water evaporate slowly at room temperature
DAdd 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
easy
Two liquids that mix completely are combined, and the concentration is given as a volume by volume percentage. What does that percentage state?
AThe mass of one liquid present in $100\ \text{mL}$ of the mixture
BThe volume of one liquid present in $100\ \text{mL}$ of the mixture
CThe volume of one liquid present in $100\ \text{mL}$ of the other liquid
DThe volume of mixture obtained from $100\ \text{mL}$ of one liquid
Show answer and explanation
Correct answer: B - The volume of one liquid present in $100\ \text{mL}$ of the mixture
Volume by volume percentage divides the volume of one component by the volume of the whole mixture and multiplies by $100$. Using the other liquid as the denominator instead of the whole mixture always gives too large a figure. A mass in the numerator makes it a mass by volume percentage, and starting from one liquid and asking how much mixture results reverses the comparison.
Q4
hard
Consider these statements about using sublimation to separate a mixture. (i) It works only when one component changes straight from solid to vapour on heating. (ii) The vapour must meet a cool surface somewhere if the subliming solid is to be recovered. (iii) It can separate two solids that both sublime at about the same temperature. Which statements are correct?
A(i), (ii) and (iii)
B(i) and (ii) only
C(ii) and (iii) only
D(i) and (iii) only
Show answer and explanation
Correct answer: B - (i) and (ii) only
Statement (i) holds, because a component that melts and boils in the ordinary way would leave the other solid behind as a liquid, not as a clean separation. Statement (ii) holds as well: without a cool surface the vapour simply escapes and is lost. Statement (iii) fails, since two solids that both sublime at about the same temperature would leave the dish together and recondense as the same mixture.
Q5
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?
AThe ink has been split into water and solid carbon
BThe black ink is a mixture of several dyes that travel up the damp paper at different speeds
CThe paper itself held dyes that the water has washed out of it and carried up to different heights
DThe 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.
Q6
medium
Why does a true solution show no Tyndall effect?
AIts dissolved particles are far too small to scatter a beam of light
BIt contains no particles of any kind
CIts particles soak up all the light instead of scattering it
DIts particles settle to the bottom before the light reaches them
Show answer and explanation
Correct answer: A - Its dissolved particles are far too small to scatter a beam of light
Scattering needs particles above a certain size, and dissolved particles fall well below it, so the beam passes straight through unseen. A solution is full of particles, but they are the particles of solute and solvent themselves. A solution does not soak up light, which is why it looks clear, and dissolved particles never settle out.
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