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Exploring Mixtures and their Separation: CBSE Class 9 Chemistry practice questions

Practise 50 questions on Exploring Mixtures and their Separation, free, with an explanation for each answer.

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Sample questions

Riya dissolves 15 g15\ \text{g} of sugar in 60 g60\ \text{g} of water. What is the concentration of the solution as a mass by mass percentage?

Sugar being spooned into a beaker of water on a digital balance
  1. 20%20\%
  2. 25%25\%
  3. 400%400\%
  4. 15%15\%
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A. 20%20\%

Mass of solution =15 g+60 g=75 g= 15\ \text{g} + 60\ \text{g} = 75\ \text{g}. % m/m=mass of solutemass of solution×100=1575×100=20%\%\ \text{m/m} = \dfrac{\text{mass of solute}}{\text{mass of solution}} \times 100 = \dfrac{15}{75} \times 100 = 20\% Dividing by the water alone (60 g) gives the trap answer 25%, and dividing the water by the sugar (6015×100\dfrac{60}{15} \times 100) gives 400%. 15% just repeats the mass of sugar, which would only be right if the solution weighed exactly 100 g.

Which list marks every mixture correctly as homogeneous (Hm) or heterogeneous (Ht)?

Smoke from a chimney, a glass of milk, a bottle of vinegar and a brass bell
  1. Air — Hm, Milk — Ht, Sugar solution — Hm, Smoke — Hm
  2. Brass — Ht, Fog — Ht, Vinegar — Ht, Muddy water — Hm
  3. Copper sulfate solution — Hm, Salt solution — Hm, Milk — Hm, Bronze — Hm
  4. Muddy water — Ht, Milk — Ht, Blood — Ht, Brass — Hm
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D. Muddy water — Ht, Milk — Ht, Blood — Ht, Brass — Hm

Muddy water is a suspension, and milk and blood are colloids. All three are heterogeneous, even though milk and blood look uniform: their particles scatter light. Brass is an alloy, which is a homogeneous mixture of copper and zinc. List A wrongly calls smoke (solid particles in air) homogeneous. List B gets brass, vinegar and muddy water wrong. List C calls milk homogeneous.

Vinegar is 5% v/v acetic acid. Glacial acetic acid is pure (100%) acetic acid. How should you make 500 mL500\ \text{mL} of vinegar from glacial acetic acid?

A bottle of vinegar next to a measuring cylinder
  1. Add 5 mL of glacial acetic acid to 500 mL of water
  2. Measure 25 mL of glacial acetic acid and add water until the total volume is 500 mL
  3. Mix 100 mL of glacial acetic acid with 400 mL of water
  4. Add 25 mL of glacial acetic acid to 500 mL of water
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B. Measure 25 mL of glacial acetic acid and add water until the total volume is 500 mL

Volume of acid needed =5100×500 mL=25 mL= \dfrac{5}{100} \times 500\ \text{mL} = 25\ \text{mL}. The water must bring the total to 500 mL. Adding 25 mL of acid to 500 mL of water makes about 525 mL, which is only about 4.8% v/v. 5 mL wrongly reads "5%" as 5 mL, and 100 mL in 500 mL would make a 20% solution.

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