RowQ
The Vault
RowQ
The Vault
CBSE Class 9 Chemistry · 12 questions · 29 marks
Almost nothing you handle in a day is chemically pure — the air you breathe, the tea you drink, and the steel in your compass box are all mixtures. This chapter teaches you to tell a pure substance from a mixture, to classify solutions, suspensions and colloids by how their particles behave, and to pick the right separation technique for any pair of substances you are handed in the lab.
A torch beam is shone through four transparent containers. In which one will the path of the beam be clearly visible?
Answer
Very dilute milk in water. Dilute milk is a colloid, and its particles (1 nm to 100 nm) are just large enough to scatter light, so the path of the beam becomes visible — this is the Tyndall effect. Sugar solution, copper sulphate solution and filtered rainwater are true solutions whose particles are far too small to scatter light, so the beam passes through invisibly.
A chemist must separate a liquid mixture of propanone (boiling point 56 °C) and water (boiling point 100 °C). The most suitable technique is:
Answer
Simple distillation. The two liquids are miscible, so a separating funnel is useless, and they are not solid particles, so filtration will not work. Since their boiling points differ by 44 °C — comfortably more than the 25 K guideline — simple distillation is enough: propanone vaporises first, travels through the condenser and is collected, leaving water in the flask. Fractional distillation is only needed when the boiling points are closer than 25 K.
Which of the following is a compound and not a mixture?
Answer
Sodium chloride crystals. In NaCl the sodium and chlorine are chemically combined in a fixed mass ratio, the crystals have a fixed melting point, and the components cannot be separated by any physical method. Brass (copper and zinc), air (nitrogen, oxygen and other gases) and sea water (water with dissolved salts) are all mixtures of variable composition whose components keep their own properties and can be separated physically.
12 g of a salt is completely dissolved in 88 g of water. The concentration of the solution by mass percentage is:
Answer
12%. Mass of solution = mass of solute + mass of solvent = 12 g + 88 g = 100 g. Mass percentage = (12 ÷ 100) × 100 = 12%. The common error is to divide by the mass of the solvent (88 g) instead of the mass of the whole solution, which wrongly gives 13.6%.
Assertion (A): The particles of a colloid do not settle down even when the colloid is left undisturbed for several days. Reason (R): The particles of a colloid are large enough to scatter a beam of light passing through it.
Answer
Both A and R are true but R is not the correct explanation of A. Colloidal particles genuinely stay suspended, and they genuinely scatter light (the Tyndall effect), so both statements are correct. However, the reason they do not settle is that they are small and light and are kept in constant random zig-zag Brownian motion by collisions with the particles of the dispersion medium — light scattering is a separate consequence of their size, not the cause of their stability.
A student is given a solid mixture of camphor and powdered common salt. Name the technique she should use to separate them and explain the property it depends on.
Answer
She should use sublimation. Camphor is a sublimable solid — on gentle heating it changes directly from the solid state to vapour without passing through the liquid state, while common salt does not sublime and stays behind in the china dish. The mixture is heated in a china dish covered with an inverted funnel whose stem is plugged with cotton. The camphor vapour rises, cools on the cold inner surface of the funnel and deposits there as pure solid camphor, leaving the salt as the residue.
A 40 g sample of a glucose solution is found to contain 6 g of glucose. Calculate (i) the mass of the solvent present and (ii) the concentration of the solution by mass percentage.
Answer
(i) Mass of solvent = mass of solution − mass of solute = 40 g − 6 g = 34 g of water. (ii) Mass percentage of solute = (mass of solute ÷ mass of solution) × 100 = (6 ÷ 40) × 100 = 15%. So the solution is 15% glucose by mass, meaning every 100 g of this solution would contain 15 g of glucose dissolved in 85 g of water.
What is centrifugation? State two everyday or laboratory situations where it is preferred over ordinary filtration.
Answer
Centrifugation is a separation technique in which a mixture is spun very rapidly in a centrifuge so that the denser suspended particles are thrown outwards and collect at the bottom of the tube, while the lighter liquid stays above and can be decanted off. It is used when the suspended particles are too fine to be trapped by filter paper. Two situations: (1) separating butter or cream from milk in a dairy, and (2) separating blood cells from plasma in a pathology laboratory. Washing machines also use the same principle to spin water out of wet clothes.
Classify each of the following as a physical change or a chemical change, giving a one-line reason for each: (i) cutting a sheet of aluminium foil, (ii) souring of milk left outside the refrigerator, (iii) dissolving crystals of potassium nitrate in water.
Answer
(i) Cutting aluminium foil is a physical change, because only the size and shape of the foil change while the substance is still aluminium with all its original properties. (ii) Souring of milk is a chemical change, because bacteria convert the sugar in milk into lactic acid — a completely new substance with a new taste and smell is formed, and the change cannot be reversed. (iii) Dissolving potassium nitrate in water is a physical change, because no new substance is formed and the salt can be recovered unchanged by evaporating the water.
You are handed a single solid mixture containing ammonium chloride, fine sand and common salt. Describe a complete step-by-step scheme to obtain all three components separately, naming the technique and the property used at every stage.
Answer
Step 1 — Sublimation to remove ammonium chloride: Place the mixture in a china dish and cover it with an inverted funnel whose stem is plugged with cotton wool. Heat gently. Ammonium chloride sublimes, and its vapour condenses as a white solid on the cool inner walls of the funnel, from where it is scraped off. Property used: only ammonium chloride is sublimable. The residue in the dish is now sand plus common salt. Step 2 — Dissolution and filtration to remove sand: Add water to the residue and stir well. Common salt dissolves; sand does not. Filter the mixture through a filter paper in a funnel. The sand is retained as residue on the filter paper — wash it with a little distilled water and dry it to obtain pure sand. Property used: difference in solubility in water and difference in particle size. Step 3 — Evaporation or crystallisation to recover common salt: Transfer the filtrate (salt solution) to an evaporating dish and heat it gently over a water bath until the water evaporates, leaving solid common salt behind. Property used: the solvent is volatile while the salt is not. If very pure crystals are wanted, the solution should instead be concentrated and then allowed to cool slowly so that pure salt crystallises out, since crystallisation avoids the risk of decomposing the salt by direct strong heating. By the end of the three steps, ammonium chloride, sand and common salt have each been obtained in a separate pure form, and every stage relied only on physical properties — which confirms that the original solid was a mixture and not a compound.
Compare true solutions, colloids and suspensions with respect to particle size, appearance, stability, behaviour towards a beam of light, and the method used to separate their particles. Support your comparison with one example of each.
Answer
Particle size: In a true solution the particles are smaller than about 1 nm; in a colloid they lie between about 1 nm and 100 nm; in a suspension they are larger than about 100 nm and are often visible to the naked eye. Appearance and nature: A true solution is a genuinely homogeneous mixture and is perfectly transparent, for example common salt dissolved in water. A colloid appears homogeneous to the eye but is really heterogeneous, since the dispersed phase and the dispersion medium are two separate phases — milk and fog are familiar colloids. A suspension is clearly heterogeneous and usually looks cloudy or muddy, for example chalk powder stirred into water. Stability: A true solution is completely stable and its solute never settles. A colloid is also stable on standing, because Brownian motion keeps the light particles moving randomly and prevents them from collecting at the bottom. A suspension is unstable — on standing, the heavy particles settle to the bottom under gravity and the mixture separates into two visible layers. Behaviour towards light: A true solution does not scatter light, so the path of a torch beam is invisible through it. Both colloids and suspensions scatter light and show the Tyndall effect, which is why the beam of a car headlamp is visible through fog and dusty air. Separation of particles: The particles of a true solution pass through both filter paper and parchment and cannot be filtered out; they are recovered by evaporation. Colloidal particles pass through filter paper but can be separated by centrifugation. Suspended particles are large enough to be trapped by ordinary filter paper, so simple filtration is sufficient.
During a school science exhibition, Meera places a small dot of black ink from a marker pen about 3 cm above the lower edge of a strip of filter paper. She dips the lower edge of the strip in water in a beaker, taking care that the ink dot stays above the water level. As the water rises up the paper, the single black dot spreads into three separate bands — blue, pink and yellow — at different heights. Answer the following: (a) Name the separation technique Meera has used. (b) State the principle on which the separation depends. (c) Why must the ink dot be kept above the level of the water in the beaker? (d) What does the appearance of three bands tell her about the black ink?
Answer
(a) The technique is chromatography — more specifically, ascending paper chromatography. (b) It works because the different coloured components of the ink have different solubilities in the rising solvent and are adsorbed to different extents by the filter paper. A component that dissolves more readily and sticks less to the paper is carried further up, so each component travels a different distance and the mixture spreads into separate bands. (c) If the dot were below the water level, the dyes would simply dissolve away into the water in the beaker instead of being carried up the paper, and no separation would be seen. Keeping the dot above the water level forces the solvent to rise through the dot by capillary action and carry the components upward. (d) The three bands show that the black ink is not a pure substance but a mixture of at least three different coloured dyes. Since they were separated by a purely physical method, the dyes were only mixed together, not chemically combined.
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