Exploring Mixtures and their Separation

This article follows the chapter's own sequence: classify mixtures as homogeneous or heterogeneous, then narrow in on homogeneous mixtures — solutions — their concentration (three percentage methods) and solubility (the solubility curve). Next come the separation techniques for homogeneous mixtures: crystallization, distillation, and paper chromatography. Then heterogeneous mixtures: the separating funnel, sublimation, and alloys as a special homogeneous mixture that no physical method can undo. Finally, the trickiest heterogeneous cases — suspensions, cleared by centrifugation or coagulation — and colloids, revealed by the Tyndall effect.

Classifying mixtures: homogeneous and heterogeneous

Stir sugar into water and the first sip tastes exactly like the last, because the composition is identical everywhere — a mixture like this, with uniform composition throughout, is a homogeneous mixture, or a solution (vinegar and soda are further examples). A solution never separates into visibly different layers on standing. Stir sand into water instead: the grains stay visible, unevenly spread, and settle with time. A mixture with non-uniform composition and visibly distinct parts is a heterogeneous mixture. The working test: does every portion look and behave identically, or can you point to different-looking regions? (The chapter later sharpens this test using particle size and the Tyndall effect.)

Concentration of a solution: three percentage methods

A solution has a solute (the substance that dissolves) and a solvent (the substance that dissolves it) — in sugar-water, sugar is the solute, water the solvent. Knowing "it's a solution" is not enough for real use: a saline drip, a pesticide spray, or an Oral Rehydration Solution (ORS) each demand a specific proportion of solute to solvent, not just any proportion. The mass of solute dissolved in a fixed amount of solvent or solution is the solution's concentration, expressed here in three percentage forms. Mass by mass percentage (% m/m) states how many grams of solute are present in every 100 grams of the total solution, and is the standard choice for labelling packaged foods and mixtures of two solids:

where: mass of solution equals mass of solute plus mass of solvent. NCERT's Example 5.1: 10 g salt in 90 g water gives mass of solution $10 + 90 = 100\ \text{g}$, so $\%\,m/m = (10/100)\times 100 = 10\%\,m/m$. Mass by volume percentage (% m/v) states how many grams of solute sit in every 100 mL of solution — used wherever measuring a liquid's volume beats weighing it, as with medicines:

where: hospital saline is 0.9% m/v (0.9 g salt per 100 mL). NCERT's Example 5.2: 5 g glucose made up to 100 mL gives $\%\,m/v = (5/100)\times 100 = 5\%\,m/v$. Volume by volume percentage (% v/v) states how many millilitres of solute sit in every 100 mL of solution — used for two miscible liquids, as in perfumes and vinegar:

where: NCERT's Example 5.3: 1 mL pesticide in 100 mL spray gives $\%\,v/v = (1/100)\times 100 = 1\%\,v/v$. The proportion matters in the field — too little fails to protect the crop, too much damages crop, soil and environment.

Solubility and the solubility curve

Add solute to a fixed amount of solvent and past a certain point no more will dissolve — the solution has become saturated. The maximum mass of solute that dissolves in a fixed quantity of solvent (100 g or 100 mL) at a given temperature is that solute's solubility there. Temperature matters: a solid solute's solubility in a liquid solvent generally rises with temperature, while a gas's solubility in a liquid generally falls with temperature (why a warm fizzy drink loses its dissolved carbon dioxide faster). A solubility curve plots solubility (g per 100 g solvent) against temperature. Table 5.4 of the chapter gives real data for four salts: potassium nitrate rises steeply (21 g at 10 degrees Celsius to 167 g at 80 degrees Celsius) while sodium chloride barely moves (36 g to 37 g) — proof that "solubility rises with temperature" is a general trend, not a fixed rate common to every solute.

Crystallization: separating a dissolved solid by cooling

Take a hot, saturated solution of a solid solute and cool it slowly. Solubility usually falls as temperature drops, so the solvent can no longer hold all the solute, and the excess separates out as pure, geometrically shaped crystals — crystallization. It separates two co-dissolved solids (one present in much smaller quantity) and purifies a solid, since impurities generally stay behind in the leftover liquid or are filtered out of the hot solution first. Copper sulfate is grown this way: hot saturated solution, filter hot, then cool slowly and undisturbed — slow cooling gives large crystals, rapid ice-cooling gives many small, poor ones. The same idea, scaled up, crystallizes salt from evaporating seawater.

Distillation: separating miscible liquids by boiling point

Two liquids that mix completely — miscible liquids — cannot be pulled apart by a separating funnel, since there is no boundary between them. Instead, heat the mixture: the lower-boiling liquid vaporises first, and a condenser cools that vapour back to a pure liquid, collected apart from the flask. This is distillation, working cleanly when the boiling points differ by at least about 25 degrees Celsius — acetone (about 56) and water (100) differ by 44, comfortably enough. Distillation also recovers a pure liquid from a dissolved solid's solution, such as water from salt water.

Below a 25 degree Celsius gap, simple distillation cannot separate liquids cleanly, since the second starts vaporising before the first finishes. This calls for fractional distillation — how a refinery splits crude oil into petroleum gas, petrol, kerosene, diesel, lubricating oil, paraffin wax and bitumen, closely spaced fractions separated in one tall column instead of repeated simple distillations.

Paper chromatography: separating dissolved colours

Touch a wet cloth to sketch-pen ink and the colour spreads, sometimes into several colours. Paper chromatography systematises this: a spot of the mixture sits near the bottom of a paper strip, and only the paper's lower edge dips into a solvent, commonly water. As the solvent rises by capillary action, it carries dissolved components upward at different rates, spreading one spot into separated bands — components that cling more to the paper move slower, those that dissolve more readily move faster. The same idea separates the pigments hidden inside a spinach leaf's green or a flower petal's colour, though some pigments need alcohol instead of water.

Separating heterogeneous mixtures: immiscible liquids and sublimation

Pour water into oil and the two form distinct layers — immiscible liquids. A separating funnel exploits this: the denser liquid settles at the bottom; its stopcock drains that layer first, is closed once it is nearly drained, a small mixed portion is discarded, and the upper liquid is collected separately. Mustard oil floats above water this way. Some solids, heated, pass straight from solid to vapour without ever forming a liquid — sublimation; cooling reverses this directly, called deposition. Camphor and naphthalene both sublime, so heating crushed camphor and sand drives off camphor vapour, which deposits solid on a cool inverted funnel, leaving sand behind. Dry ice sublimes the same way at room temperature. Metals do not dissolve into each other at room temperature, but melted together they solidify into a new homogeneous material, an alloy — brass (about 80% copper, 20% zinc), bronze (about 80% copper, 20% tin), stainless steel (iron with chromium, nickel, a little carbon). No physical method here can separate an alloy back into pure metals; the atoms are mixed at a scale no physical separation reaches.

Suspensions: centrifugation and coagulation

In muddy water the solid particles do not dissolve; they stay suspended, visible and large enough to settle with time — a suspension. Filtration removes the largest particles but often leaves the water cloudy, since some particles are too fine for the filter.

Centrifugation clears fine suspensions by spinning at high speed; the outward effect drives denser particles down to the tube's bottom, leaving lighter liquid on top — how blood separates into plasma, then platelets and white cells, then the densest red cells, and how a hand-powered paperfuge helps diagnose malaria without electricity.

Coagulation clears a suspension chemically: a coagulant, such as powdered alum (fitkari) in muddy water, clumps fine particles into flakes that settle under gravity and can be decanted or filtered off. Making paneer from milk uses the same idea — lemon juice or vinegar coagulates milk proteins.

Colloids and the Tyndall effect

Some mixtures fit neither box. Blood separates into components by centrifugation, like a suspension, yet its particles are too small to see, like a solution's. Blood — like milk, tomato sauce and ice cream — is a colloid, a third category between solutions and suspensions. The dividing line is particle size: solution particles below about 1 nanometre; colloid particles about 1 to 1000 nanometres; suspension particles above 1000 nanometres, large enough to see and settle. Unlike a suspension, colloid particles stay evenly, permanently dispersed — they never settle, even though larger than a solution's particles.

Pass a fine light beam through a salt solution, milk, and chalk powder in water. In the solution the beam is invisible from the side; in the milk and the chalk suspension the path glows, because dispersed particles scatter light sideways. This is the Tyndall effect, named after John Tyndall — the same effect behind a visible torch beam through dusty air or sunlight through gaps in leaves. A true solution never shows it, since its particles are too small to scatter light. A colloid's two parts have names: the dispersed phase is the fine, solute-like particles; the dispersion medium is the substance they are spread through. When both are liquids the colloid is an emulsion — milk disperses oil droplets in water (oil-in-water), butter disperses water in oil (water-in-oil); an emulsifying agent (milk's protein) keeps the droplets from separating back out.

Board level - Preparing a solution of a stated mass by mass percentage

A student must prepare 250 g of a 12% m/m salt solution. Find the mass of salt and of water required.

Step 1. Use the rearranged formula: mass of solute $= (\%\,m/m \times \text{mass of solution})/100$.

Step 2. Substitute: mass of salt $= (12 \times 250)/100 = 30\ \text{g}$.

Step 3. Mass of water $= 250 - 30 = 220\ \text{g}$.

30 g of salt dissolved in 220 g of water makes 250 g of a 12% m/m solution.

Board level - Reading and extending Table 5.4 for potassium nitrate

Using Table 5.4 (potassium nitrate: 21 g at 10 degrees Celsius, 62 g at 40 degrees Celsius, per 100 g water), find the mass needed to saturate 50 g of water at 40 degrees Celsius, and the mass that crystallizes out on cooling to 10 degrees Celsius.

Step 1. At 40 degrees Celsius, solubility is 62 g per 100 g water; scaled to 50 g water: $62 \times (50/100) = 31\ \text{g}$ needed.

Step 2. At 10 degrees Celsius, solubility is 21 g per 100 g water; scaled to 50 g water: $21 \times (50/100) = 10.5\ \text{g}$.

Step 3. Crystallized mass $= 31 - 10.5 = 20.5\ \text{g}$.

31 g of potassium nitrate saturates 50 g of water at 40 degrees Celsius; 20.5 g crystallizes out on cooling to 10 degrees Celsius.

Higher-order thinking (HOTS / olympiad-style) - Ranking liquid pairs by ease of separation using Table 5.5

Table 5.5 gives boiling points: water 100, acetone 56, alcohol 78, chloroform 61, benzene 80 (degrees Celsius). For the pairs water-acetone, acetone-alcohol, alcohol-chloroform and alcohol-benzene, decide which need only simple distillation, and rank all four easiest to hardest to separate.

Step 1. Boiling point differences: water-acetone $100-56=44$; acetone-alcohol $78-56=22$; alcohol-chloroform $78-61=17$; alcohol-benzene $80-78=2$ (all degrees Celsius).

Step 2. Apply the 25 degree Celsius rule: only water-acetone (44) clears it; the other three need fractional distillation.

Step 3. Rank by difference, largest first, since a larger gap always separates more cleanly: water-acetone, then acetone-alcohol, then alcohol-chloroform, then alcohol-benzene.

Water-acetone needs only simple distillation; the rest need fractional distillation, ranked easiest to hardest as water-acetone, acetone-alcohol, alcohol-chloroform, alcohol-benzene. Timing note: about 4 minutes — a minute for the differences, a minute to apply the rule, the rest to order them. The trap is ranking by boiling point instead of by the difference: a high-boiling liquid can still be nearly impossible to separate from a partner that boils almost as high, exactly the alcohol-benzene case.

• Treating evaporation, crystallization and distillation as interchangeable. Evaporation drives off and destroys the solvent; crystallization recovers a pure solid by cooling a saturated solution; distillation recovers a pure liquid solvent too, by boiling and condensing.
• Assuming a mixture that looks uniform must be a true solution. Milk and blood look homogeneous but are colloids; only the Tyndall effect and particle size confirm a true solution.
• Forgetting the 25 degree Celsius rule has two sides. At or above roughly 25 degrees Celsius, simple distillation works; below that, only fractional distillation does.
• Confusing sublimation with ordinary evaporation. Sublimation is solid turning straight to vapour without ever becoming liquid, and only some substances (camphor, naphthalene, dry ice) do it.
• Mixing up crystallization with concentrating by evaporation. Crystallization works by cooling a hot saturated solution, not by boiling water away.
• Saying alloys can be separated back into pure metals. An alloy forms only by melting metals together at high temperature, and no physical method here can undo that mixing.

• Classify a given mixture as homogeneous or heterogeneous and justify it.
• Define solute, solvent, solution and concentration, and calculate % m/m, % m/v and % v/v from given masses or volumes.
• Rearrange any percentage-concentration formula to find an unknown mass or volume.
• Read a solubility curve or table, and calculate the mass of crystals deposited when a saturated solution is cooled.
• Name the physical difference each of crystallization, distillation and paper chromatography exploits.
• State and apply the 25 degree Celsius rule for choosing simple versus fractional distillation.
• Describe how a separating funnel and sublimation each work, and why alloys cannot be separated physically.
• Distinguish solutions, colloids and suspensions by particle size and the Tyndall effect, and explain centrifugation and coagulation.

Ask the tutor: if you are unsure which separation technique fits a mixture your teacher gave you, ask the tutor to walk through the identify-the-physical-difference checklist from this article step by step.