Ans: A mixture is a combination of two or more substances that are not chemically combined and can be separated by physical methods.

Ans: A solution is a homogeneous mixture of two or more substances where the solute is uniformly dissolved in the solvent.

Ans: Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a particular temperature.

Ans: Homogeneous mixture: uniform composition throughout, no visible boundaries between components, e.g., salt solution, air. Heterogeneous mixture: non-uniform composition, components can be distinguished visually, e.g., soil, sand and water.

Ans: A colloid has particles of intermediate size (1–100 nm) that are dispersed but do not settle. It shows the Tyndall effect (scattering of light). A true solution has particles smaller than 1 nm, is transparent, and does not show the Tyndall effect. Examples of colloids: milk, fog, smoke.

Ans: (1) Filtration – separates an insoluble solid from a liquid using filter paper; e.g., separating chalk powder from water. (2) Distillation – separates miscible liquids with different boiling points by heating and condensing the vapour; e.g., separating alcohol from water.

Ans: Chromatography: separates components of a mixture based on their different rates of movement through a medium (e.g., paper). Used to separate dyes, pigments. The component that is more soluble in the solvent travels farther. Crystallisation: a pure solid is obtained from a solution by slowly cooling a hot saturated solution. As temperature drops, the solute comes out of solution as pure crystals. Used to purify substances like salt, sugar, and alum.

Ans: Solutions: homogeneous, particle size < 1 nm, particles cannot be seen, do not scatter light, do not settle, cannot be separated by filtration. Example: salt in water. Suspensions: heterogeneous, particle size > 100 nm, particles visible to naked eye, settle on standing, can be filtered. Example: chalk in water. Colloids: heterogeneous in nature but appear homogeneous, particle size 1–100 nm, scatter light (Tyndall effect), do not settle, cannot be filtered through ordinary filter paper. Example: milk, fog, blood. The Tyndall effect (visible beam of light through colloid) is used to distinguish colloids from true solutions.

Ans: Chromatography is a technique used to separate mixtures based on differences in the rates at which components move through a stationary phase under the influence of a moving phase (solvent/mobile phase). Principle: Different components have different solubilities in the mobile phase and different affinities for the stationary phase — so they travel at different speeds and get separated. Types: Paper chromatography (filter paper as stationary phase, solvent as mobile phase). Applications: (1) Separation of dyes/pigments in ink, flowers, leaves (e.g., separating chlorophyll pigments). (2) Detecting drugs in blood/urine (forensics, anti-doping). (3) Separating amino acids in a mixture. (4) Identifying food colouring agents.

Ans: Physical Change: No new substance is formed. Change is usually reversible. Chemical composition remains the same. Properties may change temporarily. Examples: Melting of ice, dissolving sugar in water, tearing paper, magnetising iron. Chemical Change: New substance(s) with different properties are formed. Usually irreversible. Chemical composition changes. Examples: Burning of wood (→CO₂, H₂O, ash), rusting of iron (→Fe₂O₃), souring of milk (→lactic acid), digestion of food. Key difference: In physical changes, only state or appearance changes; in chemical changes, bonds are broken/formed and new substances result.

Ans: A suspension is a heterogeneous mixture in which insoluble solid particles are spread throughout a liquid. The particles are visible, settle down when left undisturbed and can be separated by filtration. Example: muddy water or chalk powder in water.

Ans: Simple distillation is used when the boiling points differ by more than 25 K. If the difference is less than 25 K, fractional distillation is used.

Ans: An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal, e.g. brass (copper and zinc). It is a mixture because its composition can vary and the components keep their properties.

Ans: Mass of solution = 36 + 100 = 136 g. Mass percentage = (36 ÷ 136) × 100 = 26.47%. So the solution is about 26.5% sodium chloride by mass.

Ans: Colloidal particles are very small (1–1000 nm) and are in constant random Brownian motion due to collisions with molecules of the dispersion medium. They also carry like charges that repel each other, so they stay spread evenly and do not settle.

Ans: Sublimation is the change of a solid directly into vapour on heating, without passing through the liquid state; the vapour changes back to solid on cooling. Substances that sublime: ammonium chloride, camphor, naphthalene, iodine (also anthracene). It is used to separate a sublimable solid from a non-sublimable impurity.

Ans: Fractional distillation separates miscible liquids whose boiling points differ by less than 25 K. The mixture is heated and the vapours rise through a fractionating column packed with glass beads, which gives a large surface area for repeated condensation and vaporisation. The more volatile liquid (lower boiling point) reaches the top first and is collected, while the less volatile one stays lower. Applications: (1) separation of gases of air, (2) separation of different fractions of petroleum, (3) alcohol–water mixture.

Ans: Ammonium chloride sublimes on heating, while common salt does not. Take the mixture in a china dish and cover it with an inverted funnel plugged with cotton. Heat gently. Ammonium chloride changes directly to vapour and condenses as pure solid on the cooler inner walls of the funnel. Common salt is left behind in the china dish.

Ans: Step 1 (magnetic separation): bring a magnet near the mixture to pull out the iron filings. Step 2 (filtration): add water, stir to dissolve the salt and filter. Sand is left on the filter paper as residue. Step 3 (evaporation/crystallisation): heat the filtrate (salt solution) in a china dish to evaporate the water, leaving common salt behind. For purer crystals, concentrate the solution and cool it slowly so that crystals form.

Ans: (1) Mass by mass percentage = (mass of solute ÷ mass of solution) × 100. (2) Mass by volume percentage = (mass of solute ÷ volume of solution) × 100. (3) Volume by volume percentage = (volume of solute ÷ volume of solution) × 100. Numerical: a solution has 20 g of sugar in 80 g of water. Mass of solution = 100 g, so mass percentage = (20 ÷ 100) × 100 = 20%. If 5 g of a solid is dissolved to make 100 mL of solution, mass by volume percentage = 5%.