Exploring Mixtures and their Separation Class 9 Notes

These notes on Exploring Mixtures and their Separation for Class 9 walk through the chapter in order: how mixtures are classified, how to calculate concentration, and which separation method suits which mixture. You also get a quick revision list, solved questions and the mistakes to watch for.

Classifying mixtures

Matter around us is rarely pure. Most of it is a mixture, and we classify mixtures by how evenly their components are spread.

In well-stirred sugar water, every sip is equally sweet because the sugar is spread evenly. Vinegar is acetic acid in water; soda is carbon dioxide in water.

Colloids like milk look homogeneous to the naked eye but are technically heterogeneous. Their particles are bigger than those in a true solution but smaller than those in a suspension, and they do not settle when left undisturbed.

Concentration of a solution

A solution has a solute (the substance that dissolves) and a solvent (the substance that does the dissolving). In ORS, salt and sugar are solutes and water is the solvent; ORS works only when the ratio is right. In farming, too little pesticide may not protect crops, and too much can harm the environment. So concentration is stated exactly, in three ways.

• Mass by mass: % m/m = (mass of solute ÷ mass of solution) × 100
• Mass by volume: % m/v = (mass of solute ÷ volume of solution) × 100
• Volume by volume: % v/v = (volume of solute ÷ volume of solution) × 100

Worked examples from the chapter:

• 10 g salt in 90 g water: solution = 10 + 90 = 100 g. Concentration = (10 ÷ 100) × 100 = 10% m/m.
• 5 g glucose made up to 100 mL of solution: (5 ÷ 100) × 100 = 5% m/v. This is common for medical drips.
• 1 mL liquid pesticide in 100 mL of spray: (1 ÷ 100) × 100 = 1% v/v.
• Talcum powder with 4% m/m zinc oxide, 300 g pack: mass of solute = (4 × 300) ÷ 100 = 12 g.

Note the denominator is the solution, not the solvent.

Solubility and crystallisation

Solubility is the maximum amount of solute that can dissolve in a fixed amount of solvent at a given temperature. At that limit the solution is saturated. For most solids in liquids, solubility increases with temperature. For gases in liquids, it generally decreases as temperature rises.

Crystallisation gets a pure solid from its saturated solution. A hot saturated solution is cooled slowly, and the extra solute comes out as pure crystals. It is better than plain evaporation because it does not decompose the solid and it leaves soluble impurities behind in the liquid (filtrate). Natural crystals include rock salt, sugar (mishri) and snowflakes.

Copper sulfate crystals, step by step:

1. Take 1 g of impure copper sulfate in a beaker with 25 mL of water.
2. Add a drop of dilute sulfuric acid (it prevents unwanted reactions) and heat while stirring.
3. Keep adding copper sulfate until no more dissolves (saturated).
4. Filter the hot solution to remove insoluble impurities.
5. Cover the filtrate with a watch glass and let it cool slowly, undisturbed.
6. Filter out the shiny blue crystals, rinse with cold water and dry.

Distillation: separating miscible liquids

Distillation separates two miscible liquids (liquids that mix completely) or recovers a solvent from a solution. It works on the difference in boiling points.

1. Heating: the liquid with the lower boiling point turns to vapour first.
2. Vaporisation: vapours rise into the condenser; higher-boiling liquids stay behind.
3. Condensation: cold water in the condenser turns the vapour back into liquid.
4. Collection: the pure liquid, the distillate, is collected separately.

Simple distillation works when boiling points differ by at least about 25 °C, for example acetone (56 °C) and water (100 °C). If the difference is less than 25 °C, use fractional distillation, which refineries use to separate crude oil into petrol, diesel and kerosene. In Kannauj, Uttar Pradesh, the traditional Deg-Bhapka method of distillation captures the smell of earth after rain, called Mitti ka Ittar.

Paper chromatography

"Chroma" is Greek for colour; the method was first used to separate plant pigments and dyes. A spot of the mixture (such as black ink) is put on a strip of filter paper, and the paper is dipped in a solvent with the spot kept above the liquid level. The solvent climbs by capillary action and carries the components. Those more soluble in the solvent, or less attracted to the paper, travel higher, so the colours separate into spots. It can separate pigments from spinach leaves or petals.

Tip: draw the starting line in pencil, not pen, so it does not dissolve and spoil the result.

Heterogeneous mixtures, alloys and colloids

• Separating funnel for immiscible liquids like oil and water. Water is denser, forms the bottom layer and is drained through the stopcock; oil stays behind.
• Sublimation (solid turning directly to vapour on heating) separates camphor from sand. Camphor vapour turns back to solid on the cool walls of an inverted funnel; sand stays in the dish.
• Alloys such as brass, bronze and stainless steel are homogeneous mixtures of metals (or a metal and a non-metal) that cannot be separated by physical methods.

Suspensions like muddy water can be filtered. Colloids cannot, so we use centrifugation: spinning fast pushes denser particles to the bottom. This separates cream from milk and blood into plasma and cells.

Tyndall effect and coagulation

The Tyndall effect is the scattering of a light beam by particles in a colloid or suspension, making the beam's path visible, as in sunlight through a dusty room or a dense forest. True solutions do not show it; their particles are smaller than 1 nm.

Coagulation makes fine particles clump into bigger masses that settle. Alum (fitkari) added to muddy water makes mud particles clump and sink (sedimentation), so clear water can be poured off. Paneer is made the same way: acid (lemon or vinegar) in hot milk makes the proteins clump, separating paneer from whey.

In a colloid, the particles are the dispersed phase and the substance around them is the dispersion medium. An emulsion is a colloid in which both are liquids, such as milk or face cream.

Remember this

• Concentration always divides by the solution, then × 100.
• Boiling point gap of 25 °C or more: simple distillation; less: fractional.
• Colloid particles: 1 to 1000 nm; they pass through filter paper.
• Solutions show no Tyndall effect.

Important questions with answers

1. Why is salt water homogeneous? Salt spreads uniformly through the water, so the composition is the same everywhere and no grains are visible.

2. 20 g of sugar is dissolved in 180 g of water. Find % m/m. Solution = 20 + 180 = 200 g. (20 ÷ 200) × 100 = 10% m/m.

3. 2 mL of a liquid is mixed with water to make 250 mL of solution. Find % v/v. (2 ÷ 250) × 100 = 0.8% v/v.

4. Why is crystallisation better than evaporation for getting a pure solid? It avoids decomposing the solid, and soluble impurities stay in the filtrate.

5. When is fractional distillation used instead of simple distillation? When the boiling points of the miscible liquids differ by less than 25 °C.

6. What is the principle of paper chromatography? Components travel at different rates as the moving solvent carries them along the paper.

7. How would you separate mustard oil and water? Use a separating funnel; drain the denser water from the bottom first.

8. Why can milk not be separated with filter paper? Milk is a colloid; its particles are small enough to pass through the pores.

9. Why is a laser beam visible in milk but not in salt water? Milk's colloidal particles scatter light (Tyndall effect); salt water is a true solution with particles under 1 nm, too small to scatter light.

Common mistakes to avoid

• Dividing by the solvent's mass instead of the solution's.
• Calling milk a true solution.
• Drawing the chromatography line in pen.
• Saying gases dissolve better in hot water.

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