9th Class Chapter 2 Is Matter Around Us Pure or Not

1. Which Separation Technique?

For each mixture, choose the best physical method to separate the components:

(a) Sodium chloride from its solution in water
(b) Ammonium chloride from a mixture containing sodium chloride & ammonium chloride
(c) Small metal pieces in car engine oil
(d) Different pigments from flower-petal extract
(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

Answers:

(a) Evaporation / Crystallization
(b) Sublimation of ammonium chloride
(c) Filtration (plus magnetic separation if needed)
(d) Paper (or column) chromatography
(e) Churning (centrifugation) to split butter (fat) from buttermilk
(f) Separating funnel (density difference)
(g) Filtration (tea strainer or filter paper)
(h) Magnetic separation
(i) Winnowing (air‐flow removes lighter husk)
(j) Filtration or sedimentation + decantation

2. Steps to Make Tea

Describe the process using these terms: solution, solvent, solute, dissolve, soluble, insoluble, filtrate, residue.

Boil water (the solvent) in a pan.
Add tea leaves and let the soluble flavors dissolve, forming a tea solution.
The insoluble leaves remain intact as the residue.
Pour the mixture through a strainer (filter). The liquid collected is the filtrate – your tea.
Optionally, add sugar (solute) and stir until it dissolves.

3. Solubility Data & Questions

Solubility (g solute per 100 g water) at various temperatures:

Substance	283 K	293 K	313 K	333 K	353 K
Potassium nitrate (KNO₃)	21	32	62	106	167
Sodium chloride (NaCl)	36	36	37	38	39
Potassium chloride (KCl)	34	35	40	46	54
Ammonium chloride (NH₄Cl)	24	27	41	55	66

Mass of KNO₃ for 50 g water at 313 K?
Solubility = 62 g per 100 g water ⇒ for 50 g water ⇒ 62×0.5 = 31 g.
Cooling KCl solution made at 353 K down to room T:
At 353 K it dissolves 54 g per 100 g water. At 293 K its solubility drops to 35 g. So 19 g of KCl will crystallize out as the solution cools.
Solubility at 293 K & highest:
- KNO₃: 32 g
- NaCl: 36 g
- KCl: 35 g
- NH₄Cl: 27 g
⇒ NaCl has the highest solubility at 293 K.
Effect of temperature on solubility:
In general, the solubility of most salts (endothermic dissolution) increases with temperature. Some (like NaCl) change only slightly.

4. Definitions & Examples

Saturated solution: contains the maximum solute at a given T. e.g. a sugar–water solution that cannot dissolve more sugar at 25 °C.
Pure substance: single chemical composition throughout. e.g. distilled water, crystalline sodium chloride.
Colloid: intermediate particle size (1–100 nm), Tyndall effect visible. e.g. milk, fog, gelatin sol.
Suspension: large particles that settle on standing. e.g. muddy water, chalk in water.

5. Homogeneous vs Heterogeneous Mixtures

Soda water – homogeneous
Soil – heterogeneous
Vinegar – homogeneous
Filtered tea – homogeneous

6. Confirming Pure Water

Tests you can perform:

Measure boiling point ⇒ should be 100 °C at 1 atm
Measure freezing point ⇒ 0 °C
Check electrical conductivity ⇒ pure water conducts very poorly
Sensory test (taste/smell) ⇒ should be neutral

7. Which are Pure Substances?

Identify which of the following qualify as pure substances:

(a) Ice
(b) Milk
(c) Iron
(d) Hydrochloric acid
(e) Calcium oxide
(f) Mercury
(g) Brick
(h) Wood
(i) Air

Answers:
Pure substances: (a) Ice, (c) Iron, (d) Hydrochloric acid, (e) Calcium oxide, (f) Mercury

8. Identify the Solutions

Choose which of the following are solutions (homogeneous mixtures):

(a) Soil
(b) Sea water
(c) Air
(d) Coal
(e) Soda water

Answers:
Solutions: (b) Sea water, (c) Air, (e) Soda water

9. Tyndall Effect

Which of the following mixtures show the Tyndall effect?

(a) Salt solution
(b) Milk
(c) Copper sulphate solution
(d) Starch solution

Answers:
Tyndall effect visible in: (b) Milk, (d) Starch solution

10. Classify the Samples

Label each of the following as an element, compound or mixture:

(a) Sodium
(b) Soil
(c) Sugar solution
(d) Silver
(e) Calcium carbonate
(f) Tin
(g) Silicon
(h) Coal
(i) Air
(j) Soap
(k) Methane
(l) Carbon dioxide
(m) Blood

Answers:

Elements: (a) Sodium, (d) Silver, (f) Tin, (g) Silicon
Compounds: (e) Calcium carbonate, (k) Methane, (l) Carbon dioxide
Mixtures: (b) Soil, (c) Sugar solution, (h) Coal, (i) Air, (j) Soap, (m) Blood

11. Chemical Changes

Identify which of the following are chemical changes:

(a) Growth of a plant
(b) Rusting of iron
(c) Mixing of iron filings and sand
(d) Cooking of food
(e) Digestion of food
(f) Freezing of water
(g) Burning of a candle

Answers:
Chemical changes: (a) Growth of a plant, (b) Rusting of iron, (d) Cooking of food, (e) Digestion of food, (g) Burning of a candle

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