Exploring Mixtures and their Separation — CBSE Class 9 Science Important Questions

Answer: (b) Salt dissolved in water.

Salt dissolves completely to give a uniform composition throughout, so it is homogeneous. The other three are heterogeneous because their components stay in separate, visible phases.

Answer: (b) Tyndall effect.

Colloidal particles ($1$1–$100$100 nm) are large enough to scatter light, so the beam's path becomes visible. True solutions do not show this effect.

Answer: (b) Coagulation. Alum makes the tiny suspended (colloidal) clay particles stick together into larger lumps that settle down. It is used in water treatment.

Answer: (b) Evaporation followed by crystallisation.

Sea water is evaporated to obtain crude salt, which is then dissolved and crystallised to remove soluble impurities. Crystallisation gives a purer solid than simple evaporation because impurities remain in the mother liquor.

Answer: (c) A is true but R is false. A colloid is indeed heterogeneous, but its particles do not settle on standing; they remain dispersed and can be separated only by centrifugation. Only suspension particles settle down.

A solution is a homogeneous mixture of two or more substances. Its components are the solute (present in smaller amount, gets dissolved) and the solvent (present in larger amount, does the dissolving).

Example: sugar dissolved in water — sugar is the solute and water is the solvent.

The two are separated by sublimation.

On heating, ammonium chloride changes directly from solid to vapour (it sublimes) and re-solidifies on a cool surface, while common salt does not sublime and is left behind. The difference in the ability to sublime allows the separation.

Mass of solute (salt) $=40$=40 g and mass of solvent (water) $=320$=320 g.

Mass of solution $=40+320=360$=40+320=360 g.

$\text{Mass by mass \%}=\frac{\text{mass of solute}}{\text{mass of solution}}\times100=\frac{40}{360}\times100$Mass by mass %=(mass of solute)/(mass of solution)×100=40/360×100

$=11.1\%\ (\text{approximately}).$=11.1% (approximately).

So the solution is about $11.1\%$11.1% salt by mass.

(a) By centrifugation: milk is spun very fast; the lighter cream collects at the centre (top) and the denser part is pushed outwards, so they separate.

(b) $\text{mass by volume \%}=\dfrac{20}{250}\times100=8\%$mass by volume %=20/250×100=8%.

(c) $\text{volume by volume \%}=\dfrac{30}{150}\times100=20\%$volume by volume %=30/150×100=20%.

The components differ in magnetism, ability to sublime and solubility, so they are separated in stages:

Step 1 — Magnet: Move a magnet through the mixture; the iron filings are attracted and removed.

Step 2 — Sublimation: Heat the remaining mixture; ammonium chloride sublimes and is collected as a solid on a cool surface.

Step 3 — Dissolving and filtration: Add water to the leftover (salt + sand). Salt dissolves; sand remains insoluble and is separated by filtration (sand is the residue).

Step 4 — Evaporation: Heat the filtrate to evaporate the water, leaving behind common salt.

All four components are thus recovered.

Distillation is used to separate a mixture of a non-volatile solid dissolved in a liquid, or two miscible liquids with a large difference in boiling points. The mixture is heated in a distillation flask; the liquid with the lower boiling point vaporises, passes through a condenser where it cools back to liquid, and is collected as the distillate. The non-volatile component is left behind.

Fractional distillation is used when two miscible liquids have boiling points differing by less than $25$25 K (e.g. separating the gases of air, or components of petroleum). It uses an extra fractionating column (packed with glass beads) fitted between the flask and the condenser. The beads provide surface for repeated evaporation and condensation, so the vapour becomes progressively richer in the lower-boiling liquid before it reaches the condenser.

Difference: simple distillation has no fractionating column and works only when boiling points are far apart; fractional distillation uses a column and separates liquids with close boiling points.

Mixtures can be classified as shown:

Comparison:

• True solution: particle size $<1$<1 nm; transparent; no Tyndall effect; does not settle; homogeneous. Example: sugar in water.
• Colloid: particle size $1$1–$100$100 nm; appears uniform but is heterogeneous; shows Tyndall effect; does not settle (needs centrifugation). Example: milk.
• Suspension: particle size $>100$>100 nm; particles visible; shows Tyndall effect; settles on standing and can be filtered; heterogeneous. Example: chalk in water.

Types of colloids (by dispersed phase and medium):

• Foam (gas in liquid): shaving cream.
• Emulsion (liquid in liquid): milk.
• Sol (solid in liquid): ink, paint.
• Aerosol (liquid in gas): fog or clouds.
• Gel (liquid in solid): jelly, cheese.

(i) The technique is chromatography (paper chromatography).

(ii) It depends on the difference in the rates at which the components move up the paper — i.e. their differing solubility in the solvent and their differing tendency to be adsorbed on the paper. Components that are more soluble and less strongly held travel faster and farther.

(iii) If the ink spot were dipped in the water, the dyes would simply dissolve into the water in the container and wash away instead of rising up the paper and separating into distinct bands. Keeping the spot above the water level lets the solvent rise through the spot and carry the dyes up.

(iv) It is used to separate coloured pigments from a plant extract (or to detect drugs/additives in blood and to separate the coloured constituents of a natural dye).