CBSE Class 9 Science Is matter around us pure Sure Shot Questions Set 02

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ASSIGNMENT QUESTIONS SET - 2

CHAPTER - 2

IS MATTER AROUND US PURE

1. Salt can be recovered from its solution by evaporation. Suggest some other technique for the same? [1 Mark]

Answer:
1. Crystallisation can be used as an alternative technique to recover salt from its solution.
2. Crystallisation is preferred over evaporation because solids decompose or some, like sugar, may get charred on heating to dryness during evaporation.

Teacher's Note:
a) Crystallisation is a pure separation method used to obtain pure crystals from a solution.
b) It prevents thermal decomposition of heat-sensitive solutes.

2. While diluting a solution of salt in water, a student by mistake added acetone (boiling point 56°C). What technique can be employed to get back the acetone? Justify your choice. [3 Marks]

Answer:
1. Simple distillation can be employed to recover acetone from the mixture.
2. Acetone has a much lower boiling point (56°C) compared to water (100°C) and salt.
3. When the mixture is heated, acetone vaporizes first, gets condensed in the condenser, and can be collected separately, leaving water and salt behind.

Teacher's Note:
a) Simple distillation is used for separating miscible liquids with a boiling point difference of more than 25°C.
b) Ensure the temperature is monitored closely around 56°C to prevent water from vaporizing.
c) Mentioning the difference in boiling points is essential for full credit.

3. Classify the following as element, compound or mixture. i) Zinc amalgam (ii) Sea water (iii) Iodine vapour (iv) Gold coin (v) Water. [3 Marks]

Answer:
1. (i) Zinc amalgam - Mixture (alloy of zinc and mercury).
2. (ii) Sea water - Mixture (contains water, salts, and dissolved gases).
3. (iii) Iodine vapour - Element (consists of I₂ molecules made of a single type of atoms).
4. (iv) Gold coin - Mixture (homogeneous mixture of gold and copper/silver).
5. (v) Water - Compound (hydrogen and oxygen combined chemically in a fixed ratio).

Teacher's Note:
a) Alloys and solutions are always classified as homogeneous mixtures.
b) Elements consist of only one kind of atom, whereas compounds have fixed chemical proportions.
c) Watch out for amalgams and coins, which students often misclassify as compounds.

4. How would you separate a mixture of ammonia and hydrogen? [2 Marks]

Answer:
1. Pass the gaseous mixture through water.
2. Ammonia (NH₃) is highly soluble in water and dissolves to form ammonium hydroxide, whereas hydrogen gas is sparingly soluble and passes through unabsorbed.

Teacher's Note:
a) Solubility differences are effectively used here as a physical separation technique.
b) Hydrogen can be recovered separately by drying the escaping gas.

5. Give one example for each of the following mixtures: i) Solid/solid (homogeneous) ii) Solid/solid (heterogeneous) iii) Liquid/liquid (homogeneous) iv) Liquid/liquid (heterogeneous) v) Gas/liquid (homogeneous). [3 Marks]

Answer:
1. (i) Solid/solid (homogeneous) - Brass (alloy of copper and zinc).
2. (ii) Solid/solid (heterogeneous) - Mixture of iron filings and sulphur powder.
3. (iii) Liquid/liquid (homogeneous) - Alcohol in water.
4. (iv) Liquid/liquid (heterogeneous) - Oil and water.
5. (v) Gas/liquid (homogeneous) - Aerated drinks (carbon dioxide dissolved in water).

Teacher's Note:
a) Homogeneous mixtures have a uniform composition throughout.
b) Heterogeneous mixtures have distinct physical boundaries and non-uniform composition.

c) Providing standard textbook examples ensures full marks.

6. Explain why particles of a colloidal solution do not settle down when left undisturbed, while in the case of a suspension they do. [2 Marks]

Answer:
1. Colloidal particles are extremely small (1 nm to 1000 nm) and experience continuous Brownian motion, counterbalancing gravity.
2. Suspension particles are much larger (greater than 1000 nm), heavy, and gravitational force causes them to settle down over time.

Teacher's Note:
a) Brownian movement and electrical charges on colloidal particles prevent aggregation.
b) Suspensions are unstable mixtures due to large particle size.

7. Smoke and fog both are aerosols. In what way are they different? [2 Marks]

Answer:
1. In smoke, the dispersed phase is solid (carbon particles) and the dispersing medium is gas (air).
2. In fog, the dispersed phase is liquid (water droplets) and the dispersing medium is gas (air).

Teacher's Note:
a) Both are types of colloidal systems called aerosols where gas is the dispersion medium.
b) Clear distinction between solid-in-gas and liquid-in-gas types is required.

8. Name the process associated with the following
(a) Dry ice is kept at room temperature and at one atmospheric pressure.
(b) A drop of ink placed on the surface of water contained in a glass spreads throughout the water.
(c) A potassium permanganate crystal is in a beaker and water is poured into the beaker with stirring.
(d) A acetone bottle is left open and the bottle becomes empty.
(e) Milk is churned to separate cream from it.
(e) Settling of sand when a mixture of sand and water is left undisturbed for some time.
(f) Fine beam of light entering through a small hole in a dark room, illuminates the particles in its paths. [4 Marks]

Answer:
1. (a) Sublimation
2. (b) Diffusion
3. (c) Dissolution (and Diffusion)
4. (d) Evaporation
5. (e) Centrifugation
6. (e) Sedimentation
7. (f) Tyndall effect

Teacher's Note:
a) Note that the question paper has duplicate label (e), which should be addressed sequentially as printed.
b) These physical processes form the foundational concepts of matter and separation techniques.
c) Check for exact scientific terminology.
d) Mentioning both centrifugation for milk and sedimentation for sand covers all sub-cases accurately.

9. The teacher instructed three students ‘A’, ‘B’ and ‘C’ respectively to prepare a 50% (mass by volume) solution of sodium hydroxide (NaOH). ‘A’ dissolved 50g of NaOH in 100 mL of water, ‘B’ dissolved 50g of NaOH in 100g of water while ‘C’ dissolved 50g of NaOH in water to make 100 mL of solution. Which one of them has made the desired solution and why? [3 Marks]

Answer:
1. Student 'C' has made the desired solution.
2. Mass by volume percentage is defined as the mass of solute in grams dissolved in 100 mL of the solution.
3. Student 'C' dissolved 50g of NaOH in water to make a total volume of 100 mL of solution, whereas 'A' and 'B' used 100 mL and 100g of water as solvent, resulting in a total volume greater than 100 mL.

Teacher's Note:
a) Formula: \(\text{Mass by volume percentage} = \left(\frac{\text{Mass of solute}}{\text{Volume of solution}}\right) \times 100\).
b) Students often confuse volume of solvent with volume of solution.
c) Clear identification of the total solution volume is the key scoring point.

10. Why is gold alloyed with copper or silver for the purpose of making ornaments? [2 Marks]

Answer:
1. Pure gold (24 carat) is extremely soft and flexible, making it unsuitable for designing durable ornaments.
2. Alloying gold with a small amount of copper or silver makes it hard and strong, so that it retains its shape and withstands daily wear and tear.

Teacher's Note:
a) Pure 24-carat gold bends easily under slight pressure.
b) 22-carat gold means 22 parts pure gold and 2 parts copper or silver.

11. Give some examples of Tyndall effect observed in your surroundings? [2 Marks]

Answer:
1. Sunlight passing through a canopy of a dense forest (where tiny water droplets in mist scatter light).
2. A beam of sunlight entering a dark, dusty room through a small slit or hole.

Teacher's Note:
a) Tyndall effect is the scattering of a beam of light by colloidal particles.
b) Everyday natural occurrences provide the best illustrations of this phenomenon.

12. Calculate the mass of sodium sulphate required to prepare its 20% (mass percent) solution in 100g of water? [3 Marks]

Answer:
1. Let the mass of sodium sulphate required be \(x\) g.
2. Mass of solvent (water) = 100 g, so total mass of solution = \((x + 100)\) g.
3. Mass percentage formula is given by Mass percent = \(\left(\frac{\text{Mass of solute}}{\text{Mass of solution}}\right) \times 100\).
4. Substituting the values: \(20 = \left(\frac{x}{x + 100}\right) \times 100\).
5. Solving for \(x\): \(20(x + 100) = 100x\) implies \(20x + 2000 = 100x\) implies \(80x = 2000\).
6. \(x = \frac{2000}{80} = 25\) g of sodium sulphate.

Teacher's Note:
a) The mass percentage formula relates solute mass to total solution mass, not just solvent mass.
b) Students often mistakenly divide by the mass of water instead of total mass of solution.
c) Always ensure proper unit cancellation during algebraic transposition.

13. How would you separate a mixture of ammonia and hydrogen? [2 Marks]

Answer:
1. Pass the gaseous mixture through water.
2. Ammonia is highly soluble in water and dissolves to form ammonium hydroxide, whereas hydrogen is sparingly soluble and passes through.
3. The dissolved ammonia can later be recovered by boiling the aqueous solution if required.

Teacher's Note:
a) This method uses the significant difference in solubility of the two gases.
b) Ensure proper trapping equipment is used to prevent gas leakage.

14. Action of heat on blue vitriol is a physical as well as chemical change. Justify. [3 Marks]

Answer:
1. Blue vitriol is hydrated copper sulphate (CuSO₄ · 5H₂O).
2. Upon heating, it loses water of crystallization to form white anhydrous copper sulphate (CuSO₄), which is a chemical change because a new substance with different properties is formed.
3. When water is added back to the anhydrous copper sulphate, it turns blue again, and this rehydration combined with the initial loss of water demonstrates a reversible physical-chemical process.

Teacher's Note:
a) Loss of water of hydration involves breaking coordinate bonds, altering chemical composition.
b) Color change from blue to white is the key indicator of this transformation.
c) Reversibility makes it an excellent example to test conceptual clarity.

15. How would you separate a mixture of NH₄Cl and I₂? [2 Marks]

Answer:
1. Both ammonium chloride and iodine sublime on heating.
2. Dissolve the mixture in water or alcohol (like ethanol), in which iodine is soluble while ammonium chloride is less soluble or can be separated chemically, or use solvent extraction where iodine dissolves in organic solvents like carbon tetrachloride.
3. Alternatively, treat with a solvent that selectively dissolves one component.

Teacher's Note:
a) Direct sublimation fails here because both components sublime.
b) Selective solvent extraction is the appropriate alternative technique.

16. Describe a method for separation of the constituents of gunpowder. [3 Marks]

Answer:
1. Gunpowder is a mixture of sulphur, charcoal, and potassium nitrate.
2. Dissolve the powdered mixture in carbon disulphide, which dissolves sulphur while leaving potassium nitrate and charcoal insoluble; filter to obtain sulphur by evaporating the filtrate.
3. Treat the remaining solid residue with water to dissolve potassium nitrate, filter out the insoluble charcoal, and evaporate water to recover potassium nitrate crystals.

Teacher's Note:
a) Solvent extraction is the principal technique applied here based on differing solubilities.
b) Carbon disulphide is extremely flammable and toxic; safety precautions must be emphasized.

17. Describe how you would obtain the substances mentioned below, from the given mixtures. [2 Marks]
a) Iodine from tincture of iodine. b) Lead chloride from a mixture of lead chloride and silver chloride

Answer:
1. (a) Distillation or evaporation can be used; alcohol evaporates leaving solid iodine behind, or distillation recovers both alcohol and iodine.
2. (b) Add water to the mixture and heat it; lead chloride is soluble in hot water whereas silver chloride is insoluble, then filter hot to separate silver chloride as residue and cool the filtrate to crystallize lead chloride.

Teacher's Note:
a) Tincture of iodine is a solution of iodine in alcohol.
b) Differential solubility in hot versus cold water is the key principle for separating lead chloride and silver chloride.

18. Write the characteristics of Suspension method of separation of mixture. [2 Marks]

Answer:
1. It is used to separate heterogeneous mixtures where solid particles are suspended in a liquid.
2. The suspended particles are large enough to be seen by the naked eye and settle down over time due to gravity.

Teacher's Note:
a) Suspensions are unstable mixtures where particles scatter a beam of light (Tyndall effect).
b) Filtration can easily separate the suspended particles from the liquid medium.

19. Briefly describe how to separate, i) Sulphur from a mixture of sulphur and sand. ii) Black CuO from a mixture of CuO and ZnO. [3 Marks]

Answer:
1. (i) Add carbon disulphide to the mixture of sulphur and sand; sulphur dissolves in carbon disulphide while sand remains insoluble.
2. Filter the mixture to get sand as residue and evaporate the filtrate to obtain solid sulphur.
3. (ii) Add dilute sodium hydroxide (NaOH) solution to the mixture of CuO and ZnO; zinc oxide is amphoteric and dissolves in NaOH to form soluble sodium zincate, whereas copper oxide (CuO) remains insoluble and can be filtered out.

Teacher's Note:
a) Chemical properties like amphoteric nature are effectively used in differential separation.
b) Always highlight the distinction between physical extraction (using CS₂) and chemical extraction (using NaOH).

20. Fill in the blanks [5 Marks]
(a) A colloid is a - - - mixture and its components can be separated by the technique known as - - - .
(b) Ice, water and water vapour look different and display different - - properties but they are - - - the same.
(c) A mixture of chloroform and water taken in a separating funnel is mixed and left undisturbed for some time. The upper layer in the separating funnel will be of - - - and the lower layer will be that of - - - .
(d) A mixture of two or more miscible liquids, for which the difference in the boiling points is less than 25 K can be separated by the process called - - - .
(e) When light is passed through water containing a few drops of milk, it shows a bluish tinge. This is due to the - - - of light by milk and the phenomenon is called - - - . This indicates that milk is a - - - solution.

Answer:
1. (a) heterogeneous, centrifugation
2. (b) chemical / physical (physical), chemically
3. (c) water, chloroform
4. (d) fractional distillation
5. (e) scattering, Tyndall effect, colloidal

Teacher's Note:
a) Colloids appear homogeneous but are heterogeneous under microscopic examination.
b) Chloroform is denser than water, which explains why it forms the lower layer.
c) Fractional distillation requires a fractionating column to separate liquids with close boiling points.

21. Which of the following is NOT true about colloids? [1 Mark]
(A) Particles in a colloid can pass through filter paper.
(B) A colloid is a homogeneous solution.
(C) Colloidal particles exhibit Brownian motion.
(D) Colloidal particles exhibit electrophoresis.

Answer: (B) A colloid is a homogeneous solution.

1. Colloids appear homogeneous to the naked eye but are actually heterogeneous in nature.
2. Hence, the statement that a colloid is a homogeneous solution is incorrect.
3. Therefore, the correct option is (B).

Teacher's Note:
a) A colloid is a heterogeneous system because of dispersed phase and dispersion medium.
b) Brownian motion and passing through filter paper are key true properties of colloids.

22. Which of the following is a heterogeneous mixture? [1 Mark]
(A) air
(B) brass
(C) sugar dissolved in water
(D) lime water

Answer: (D) lime water

1. Air and brass are homogeneous mixtures (solutions and alloys respectively).
2. Sugar dissolved in water forms a true homogeneous solution.
3. Lime water contains suspended calcium hydroxide particles making it a heterogeneous suspension/mixture.
4. Hence, the correct option is (D).

Teacher's Note:
a) Lime water is a suspension of calcium hydroxide in water.
b) Solutions and true alloys are always homogeneous.

23. How can we separate cream from milk? [1 Mark]
(A) Centrifugation

Answer: (A) Centrifugation

1. Milk is a colloidal suspension of fat in water.
2. Centrifugation works on the principle that denser particles are forced to the bottom and lighter particles stay at the top when spun rapidly.
3. Hence, the correct option is (A).

Teacher's Note:
a) Centrifugation is widely used in dairy farms and diagnostic laboratories.
b) The denser skimmed milk settles while lighter cream separates out.

24. Removal of clear layer of the liquid without disturbing the settled solid is known as __________. [1 Mark]
(A) Sedimentation
(B) Decantation
(C) Filtration
(D) Evaporation

Answer: (B) Decantation

1. Sedimentation allows heavier solid particles to settle down at the bottom.
2. Decantation is the process of pouring out the clear upper liquid carefully without disturbing the settled solid.
3. Hence, the correct option is (B).

Teacher's Note:
a) Decantation separates insoluble solids from liquids.
b) It is less effective than filtration for very fine particles.

25. A mixture of water and silver chloride can be separated by [1 Mark]
(A) Centrifugation
(B) Sedimentation
(C) Filtration
(D) Sublimation

Answer: (C) Filtration

1. Silver chloride is insoluble in water, forming a solid-liquid heterogeneous mixture.
2. Insoluble solid particles can be easily trapped and separated from the liquid using filter paper during filtration.
3. Hence, the correct option is (C).

Teacher's Note:
a) Filtration is ideal for separating undissolved solid precipitates from solutions.
b) Silver chloride does not dissolve to form a true solution.

26. When two liquids in a mixture differ by their boiling points, which of the following is the best method to separate these liquids? [1 Mark]
(A) Evaporation
(B) Distillation
(C) Chromatography
(D) Filtration

Answer: (B) Distillation

1. Miscible liquids with sufficient difference in their boiling points are separated by distillation.
2. The liquid with the lower boiling point vaporizes first, gets condensed, and is collected separately.
3. Hence, the correct option is (B).

Teacher's Note:
a) Simple distillation applies when boiling point differences are large.
b) Fractional distillation is used if the boiling point difference is less than 25 Kelvin.

27. Solution which has uniform composition throughout is called ___________. [1 Mark]
(A) homogeneous solution
(B) heterogeneous solution
(C) Colloidal solution
(D) none of these

Answer: (A) homogeneous solution

1. A homogeneous mixture has the same uniform composition and properties throughout its mass.
2. Solutions like salt dissolved in water are examples of homogeneous mixtures.
3. Hence, the correct option is (A).

Teacher's Note:
a) Components of a homogeneous solution cannot be distinguished visually.
b) Heterogeneous mixtures have non-uniform compositions.

28. The particles of a suspension will be of size ________ [1 Mark]
(A) less than 1nm (nm = nano metre)
(B) between 1nm to 100 nm
(C) greater than 100nm
(D) less than 0.1 nm

Answer: (C) greater than 100nm

1. True solutions have particle sizes less than 1 nm.
2. Colloids have particle sizes between 1 nm and 100 nm.
3. Suspensions contain much larger particles, typically greater than 100 nm (or 1000 nm), visible to the naked eye.
4. Hence, the correct option is (C).

Teacher's Note:
a) Suspension particles scatter a beam of light and settle down over time.
b) Note that standard textbook ranges sometimes cite 100 nm as the upper colloidal limit, making suspension particles greater than 100 nm.

29. Which of the following is a chemical change? [1 Mark]
(A) Melting of ice
(B) Dissolving salt in water
(C) Rusting of iron
(D) Boiling of water into steam

Answer: (C) Rusting of iron

1. Melting of ice, dissolving salt, and boiling water are physical changes where no new chemical substances are formed.
2. Rusting of iron involves a chemical reaction between iron, oxygen, and moisture to form hydrated iron oxide (rust), creating a completely new substance.
3. Hence, the correct option is (C).

Teacher's Note:
a) Chemical changes are generally permanent and irreversible.
b) Physical changes alter only the state or appearance without changing chemical identity.

30. Which one of the following is TRUE for compounds? [1 Mark]
(A) It shows properties of its constituents.
(B) It may be homogeneous or heterogeneous.
(C) It can be separated by physical methods.
(D) It has fixed melting and boiling point.

Answer: (D) It has fixed melting and boiling point.

1. Compounds are pure substances with fixed compositions and definite physical properties, including fixed melting and boiling points.
2. They do not show the properties of their constituent elements and cannot be separated into constituents by physical methods.
3. Hence, the correct option is (D).

Teacher's Note:
a) Mixtures show properties of constituents, whereas compounds have entirely new properties.
b) Compounds are always homogeneous in nature.