Exploring Mixtures and their Separation (RBSE Class 9 · Science)

Almost nothing around you is pure — the air you breathe, the sea, your morning milk are all mixtures. Chemistry begins by asking two questions: what is mixed in here? and how do I pull it apart? This chapter answers both.

RBSE note (2026-27). Class 9 uses the new NCF (Curiosity) Science textbook. Exploring Mixtures and their Separation is the new book's treatment of what older books called "Is Matter Around Us Pure?". BSER (Ajmer) sets the exam.

1. Pure substances and mixtures

• A pure substance has a fixed composition and a single kind of particle — an element (e.g. copper, oxygen) or a compound (e.g. water, salt).
• A mixture contains two or more substances not chemically combined, in any ratio, each keeping its own properties.

Classifying mixtures

• Homogeneous — uniform throughout; you cannot see the parts (salt solution, air, brass).
• Heterogeneous — non-uniform; parts are visible or separable (sand + iron filings, oil + water).

2. Solutions

A solution is a homogeneous mixture of a solute (dissolved, smaller amount) in a solvent (dissolving medium, larger amount). In salt water, salt = solute, water = solvent.

Concentration = amount of solute in a given amount of solution. A common measure: $\text{mass \% of solute}=\frac{\text{mass of solute}}{\text{mass of solution}}\times 100$

• A saturated solution can dissolve no more solute at that temperature; an unsaturated one can dissolve more.
• Solubility usually increases with temperature for solids.

Suspensions and colloids

3. The Tyndall effect

When a beam of light passes through a colloid, the particles scatter the light and the beam becomes visible — the Tyndall effect. It is why a torch beam shows up in fog and sunlight streams through a forest canopy or a dusty room. A true solution shows no Tyndall effect.

4. Separating the components of a mixture

Choose the method by the property that differs between the components:

• Evaporation — recover a dissolved solid (salt from salt water) by boiling off the solvent.
• Filtration — separate an insoluble solid from a liquid (sand from water).
• Sedimentation and decantation — let heavy particles settle, then pour off the liquid.
• Centrifugation — spin fast to settle fine particles (cream from milk, blood components).
• Using a separating funnel — separate two immiscible liquids (oil and water) by density.
• Sublimation — separate a solid that sublimes (ammonium chloride, camphor) from one that does not (salt).
• Distillation — separate a solvent from a solution (or two miscible liquids) by boiling and condensing. Fractional distillation separates liquids with close boiling points (e.g. components of air, petroleum).
• Chromatography — separate dissolved substances that travel at different rates on paper (dyes in ink).

Rule of thumb: insoluble solid → filter; dissolved solid → evaporate; miscible liquids → distil; immiscible liquids → separating funnel; coloured mixtures → chromatography.

5. Physical vs chemical change (why mixtures matter)

Separating a mixture is a physical change — no new substance forms and the change is usually reversible (evaporated salt is still salt). Breaking a compound needs a chemical change. This is the key difference between a mixture (physically separable) and a compound (chemically bonded).

6. Quick recap

• Pure substance (element/compound) vs mixture (two+ substances, not combined).
• Mixtures are homogeneous (uniform) or heterogeneous (non-uniform).
• Solution = solute in solvent; concentration = solute per solution; saturated = no more dissolves.
• True solution < colloid < suspension by particle size; colloids show the Tyndall effect, true solutions don't.
• Pick a separation method by the differing property: filter (insoluble solid), evaporate (dissolved solid), distil (miscible liquids), separating funnel (immiscible liquids), chromatography (dyes), sublimation, centrifugation.
• Separating a mixture is a physical change.