Chapter 2: Is Matter Around Us Pure? Exercise Questions & Answers
1Which separation techniques will you apply for the separation of the following? (a) Sodium chloride from its solution in water (b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride (c) Small pieces of metal in the engine oil of a car (d) Different pigments from an extract of flower petals (e) Butter from curd (f) Oil from water (g) Tea leaves from tea (h) Iron pins from sand (i) Wheat grains from husk (j) Fine mud particles suspended in water
Answer & Reference:
(a) Evaporation – Used to recover solid solute from a solution. (Page 15, Activity 2.1)
(b) Sublimation – Ammonium chloride sublimes on heating, while NaCl does not. (Page 23, Exercise 1b)
(c) Filtration – Insoluble solids (metal pieces) can be separated from liquids by filtration. (Page 16, Fig. 2.2)
(d) Chromatography – Used to separate colored components (pigments) based on differential adsorption. (Page 23, Exercise 1d)
(e) Centrifugation – Butter (fat) is separated from curd by spinning, using density difference. (Page 23, Exercise 1e)
(f) Separating funnel – Immiscible liquids (oil and water) form layers and are separated using a separating funnel. (Page 23, Exercise 1f)
(g) Filtration – Insoluble tea leaves are separated as residue. (Page 16, filtration principle)
(h) Magnetic separation – Iron is magnetic; sand is not. (Page 23, Exercise 1h)
(i) Winnowing – Light husk is blown away by wind; heavier grains fall down. (Page 23, Exercise 1i)
(j) Sedimentation, decantation, or filtration – Mud particles settle (sedimentation) or are filtered out. (Page 17, Suspension properties)
2Write the steps you would use for making tea. Use the words: solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Answer & Reference:
1. Take water (solvent) in a pan.
2. Add sugar (solute), which is soluble, and let it dissolve to form a solution.
3. Add tea leaves (insoluble) and boil.
4. Filter the mixture.
5. The liquid collected is the filtrate.
6. The tea leaves left on the filter paper are the residue.
(Reference: Page 16, Section 2.2; Page 17 – filtration and residue)
3Pragya tested the solubility of three different substances... (a) Mass of KNO₃ for saturated solution in 50 g water at 313 K? (b) Observation when saturated KCl solution at 353 K is cooled? (c) Which salt has highest solubility at 293 K? (d) Effect of temperature on solubility of a salt?
Answer & Reference:
(a) 31 g – Solubility of KNO₃ at 313 K = 62 g / 100 g water → For 50 g water: 31 g. (Page 23, Table 2.2)
(b) Crystals of KCl will form because solubility decreases with temperature. (Page 17, Activity 2.3)
(c) Ammonium chloride (37 g/100 g water at 293 K). (Page 23, Table 2.2)
(d) Solubility generally increases with temperature for most salts. (Page 17, Activity 2.3 conclusion)
4Explain the following giving examples: (a) Saturated solution (b) Pure substance (c) Colloid (d) Suspension
Answer & Reference:
(a) Saturated solution: A solution that cannot dissolve more solute at a given temperature. Example: 36 g NaCl in 100 g water at 293 K. (Page 17)
(b) Pure substance: A substance with fixed composition and same chemical properties throughout. Example: Iron, oxygen, distilled water. (Page 14)
(c) Colloid: Heterogeneous mixture with particle size 1–100 nm; shows Tyndall effect. Example: Milk, fog. (Page 18, Section 2.2.3)
(d) Suspension: Heterogeneous mixture with large particles (>100 nm) that settle down. Example: Muddy water, chalk in water. (Page 17, Section 2.2.2)
5Classify as homogeneous or heterogeneous mixture: soda water, wood, air, soil, vinegar, filtered tea
Answer & Reference: Homogeneous: soda water, air, vinegar, filtered tea Heterogeneous: wood, soil
(Reference: Page 15, Activity 2.1; Page 16 – air and soda water as solutions)
6How would you confirm that a colourless liquid is pure water?
Answer & Reference:
Check its boiling point (373 K / 100°C) and melting point (273 K / 0°C). Pure substances have fixed melting and boiling points. (Page 19, Section 2.3; Page 24, Exercise 6)
7Which are “pure substances”? (a) Ice (b) Milk (c) Iron (d) Hydrochloric acid (e) Calcium oxide (f) Mercury (g) Brick (h) Wood (i) Air
Answer & Reference: Pure substances: (a) Ice, (c) Iron, (d) Hydrochloric acid, (e) Calcium oxide, (f) Mercury
(Reference: Page 14 – pure substance definition; Page 20–21 – elements and compounds are pure)
8Identify the solutions: (a) Soil (b) Sea water (c) Air (d) Coal (e) Soda water
Answer & Reference: Solutions: (b) Sea water, (c) Air, (e) Soda water
(Reference: Page 16 – “Air is a homogeneous mixture”; soda water and sea water are solutions)
9Which show “Tyndall effect”? (a) Salt solution (b) Milk (c) Copper sulphate solution (d) Starch solution
Answer & Reference: (b) Milk and (d) Starch solution – both are colloids.
(Reference: Page 18, Section 2.2.3; Fig. 2.3 shows milk shows Tyndall effect, CuSO₄ does not)
Answer & Reference: Elements: (a) Sodium, (d) Silver, (f) Tin, (g) Silicon Compounds: (e) CaCO₃, (k) Methane, (l) CO₂
To understand the fundamental structure and calculate the mass of these
Elements and Compounds (Atomic Mass and Molar Mass Calculations)
, check out our Chapter 3 Q&A.
Mixtures: (b) Soil, (c) Sugar solution, (h) Coal, (i) Air, (j) Soap, (m) Blood
(Reference: Page 19–21, Section 2.4)
11Which are chemical changes? (a) Growth of a plant (b) Rusting of iron (c) Mixing iron filings and sand (d) Cooking of food (e) Digestion of food (f) Freezing of water (g) Burning of a candle
Answer & Reference: Chemical changes: (a), (b), (d), (e), (g)
(Reference: Page 19, Section 2.3 – “Chemical change brings change in chemical properties... new substances”)