Class 9 Science Chapter 2: Important Questions with Answers

The component of a solution that dissolves the other (usually in larger amount).

Oxygen 21%, nitrogen 78%.

(mass of solute / volume of solution) × 100.

Sunlight passing through the mist in a dense forest canopy.

Gel (liquid in solid).

Centrifugation.

Separating drugs from blood.

−183 °C.

Lavoisier, a French chemist.

Mercury and bromine.

Homogeneous: uniform composition, no visible parts — salt in water. Heterogeneous: non-uniform, visibly distinct parts — oil + water.

Homogeneous; particles smaller than 1 nm; do not scatter light; cannot be filtered and do not settle (stable).

Suspension particles are visible, settle and can be filtered; colloid particles are not visible, do not settle and cannot be filtered (need centrifugation).

Mass % = 36/(36 + 100) × 100 = 26.47% (≈ 26.5%).

Blood and urine tests; butter from cream; squeezing water from clothes in washing machines.

Some solids decompose or, like sugar, char on heating to dryness; dissolved impurities contaminate the solid on evaporation.

Lustre; conduct heat and electricity; ductile; malleable (also sonorous).

Mixture: variable composition, shows constituents' properties. Compound: fixed composition, totally new properties.

| Property | Solution | Colloid | Suspension |
|Mixture|homogeneous|heterogeneous (looks homogeneous)|heterogeneous|
|Particles|smaller than 1 nm|not visible|visible|
|Tyndall|no|yes|yes|
|Stable?|yes|yes|no (settles)|
|Filterable?|no|no (centrifuge)|yes|

Distillation: miscible liquids with enough boiling-point difference — acetone + water; acetone vaporises and condenses in the condenser. If the difference is less than 25 K, use a fractionating column (glass beads) — fractional distillation. Air: compress + cool → liquid air → warm slowly in a fractional distillation column; N₂ (−196 °C), then Ar (−186 °C), then O₂ (−183 °C) separate.

5 g Fe + 3 g S: group I only mixes; group II heats red hot. Group I's material is attracted by a magnet and gives H₂ (odourless) with acid — a mixture. Group II's is not magnetic, gives H₂S (rotten-egg smell), uniform throughout — a compound. A mixture shows its constituents' properties; a compound has new ones.

Method to separate solutes that dissolve in the same solvent (kroma = colour). Pencil line 3 cm above the strip's edge, ink drop on it, strip in water with the drop above water level; water rises and the more soluble colour rises faster — colours separate. Uses: colours in a dye, natural pigments, drugs from blood.

A pure single form of matter made of one kind of particles; it cannot be separated into other kinds of matter by any physical process (e.g. sodium chloride, sugar).

Homogeneous: uniform composition throughout, no visible boundaries (salt/sugar in water). Heterogeneous: non-uniform composition, physically distinct parts visible (sodium chloride + iron filings, oil + water).

Homogeneous mixtures have uniform composition — e.g. sugar in water, air, brass. Heterogeneous mixtures have non-uniform composition with visible parts — e.g. salt + sulphur, chalk in water, oil + water.

Solution: homogeneous, particles < 1 nm, no Tyndall effect, stable, not filterable. Sol (colloid): heterogeneous, particles not visible but scatter light, stable, not filterable. Suspension: heterogeneous, particles visible, scatter light, settle (unstable), filterable.

Mass of solution = 36 + 100 = 136 g. Concentration (mass %) = 36/136 × 100 = 26.47%.

By distillation — petrol (lower boiling point) vaporises first, condenses and is collected; kerosene remains in the flask.

(i) Centrifugation (churning) (ii) Evaporation (and crystallisation for pure salt) (iii) Sublimation.

Mixtures in which a solid is dissolved in a liquid along with impurities — to obtain the pure solid as crystals (e.g. salt from sea water, alum, copper sulphate).

Physical: cutting of trees, melting of butter, boiling water to steam, dissolving salt, making fruit salad. Chemical: rusting of almirah, electrolysis of water into hydrogen and oxygen, burning of paper and wood.

Pure substances: distilled water, sugar, salt, gold, copper wire, ice. Mixtures: air, milk, soil, tea, sea water, brass, wood.

(a) Evaporation / crystallisation (b) Sublimation (c) Filtration or centrifugation (d) Chromatography (e) Centrifugation (f) Separating funnel (g) Filtration (h) Magnetic separation (using a magnet) (i) Winnowing (j) Centrifugation (or sedimentation and decantation, then filtration).

Heat water (solvent). Add sugar (solute) — it dissolves because it is soluble, forming a solution. Add tea leaves; their colour and flavour dissolve. Add milk and boil. Pour through a strainer: the insoluble tea leaves stay as residue, and the tea collected is the filtrate.

(a) 62 g in 100 g → 31 g in 50 g. (b) Crystals of KCl appear, because its solubility falls from 54 g (353 K) to about 35 g at room temperature; the excess comes out. (c) At 293 K: KNO₃ 32 g, NaCl 36 g, KCl 35 g, NH₄Cl 37 g — ammonium chloride is highest. (d) Solubility of a salt generally increases with temperature (for NaCl it hardly changes).

(a) Holds the maximum solute it can at that temperature — 36 g NaCl in 100 g water at 293 K. (b) Single kind of particles, fixed properties — sugar, sodium chloride, iron. (c) Heterogeneous mixture whose particles are invisible but scatter light and do not settle — milk, fog. (d) Heterogeneous mixture whose visible particles settle — chalk in water, muddy water.

Homogeneous: soda water, air, vinegar, filtered tea. Heterogeneous: wood, soil.

Heat it: pure water boils sharply at 373 K (100 °C) at 1 atm and leaves no residue on evaporation; it also freezes at 273 K (0 °C).

Pure substances: ice, iron, calcium oxide, mercury (and hydrogen chloride as a compound; hydrochloric acid as sold is its solution in water). Not pure: milk, brick, wood, air.

Solutions: sea water, air, soda water.

Milk and starch solution (both colloids).

Elements: sodium, silver, tin, silicon. Compounds: calcium carbonate, methane, carbon dioxide. Mixtures: soil, sugar solution, coal, air, soap (as used, a mixture), blood.

Chemical: growth of a plant, rusting of iron, cooking of food, digestion of food, burning of a candle. Physical: mixing iron filings and sand, freezing of water.