Is Matter Around Us Pure
CBSE Class 9 · Science · Notes, formulas and practice questions
Revise how scientists classify matter: pure substances (elements and compounds) versus mixtures, and how solutions, suspensions and colloids differ. The chapter covers separation methods such as evaporation, centrifugation, sublimation, chromatography and distillation, along with concentration and the idea of physical and chemical changes.
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This chapter asks a single question: how pure is the matter around us, and how can we tell? Matter is first divided into pure substances and mixtures. Pure substances are further split into elements, like iron and oxygen, and compounds, such as water. Mixtures, by contrast, contain two or more substances simply mixed together, so they can be homogeneous, like salt solution, or heterogeneous, like sand in water. This classification is not an empty exercise: the method we use to separate a mixture depends entirely on which type of matter we are holding.
The chapter treats solutions in detail, and comparisons carry the marks. A solution is a homogeneous mixture with solute particles too small to be seen. A suspension is heterogeneous: its particles are large, settle on standing and can be filtered. A colloid is between the two, with particles that do not settle but scatter a beam of light. That scattering, the Tyndall effect, gives a quick test for distinguishing a colloid from a true solution. Concentration gives the amount of solute in a solution, usually as a percentage.
Once we can name a mixture, the next question is how to take it apart. The chapter presents physical methods chosen from the properties of the components. Evaporation removes water and leaves a dissolved solid such as salt. Centrifugation uses rapid spinning to separate components of different densities. Sublimation collects a solid that turns straight from solid to vapour, like ammonium chloride. Chromatography separates dissolved substances by their movement with a solvent. Distillation boils a liquid and condenses it back, freeing it from non-volatile impurities.
The chapter closes by separating a compound from a mixture and physical changes from chemical ones. In a compound, elements are combined in a fixed ratio and a new substance forms, so its properties are not those of the starting elements. In a mixture, the substances are simply blended and keep their own properties. A physical change alters only state or shape, while a chemical change creates a new substance. These distinctions matter because deciding whether matter is pure is ultimately a chemical judgement.
Key terms
- Pure substance
- A material that is made of only one kind of substance and has a fixed composition. Pure substances are elements, made of one type of atom, or compounds, made of two or more elements chemically combined in a fixed ratio. Distilled water and sodium chloride are pure substances; air and milk are not.
- Mixture
- A material that contains two or more substances physically combined, without any chemical reaction between them. Its composition is not fixed, and each component keeps its own properties. Mixtures can be homogeneous, with one uniform phase such as salt solution, or heterogeneous, with visible separate parts such as soil in water.
- Solution
- A homogeneous mixture in which a solute dissolves completely in a solvent. The solute is the substance that dissolves, while the solvent is the medium, usually present in larger amount. In a solution the solute particles are extremely small, so they cannot be filtered out and do not settle on standing.
- Concentration of a solution
- The amount of solute dissolved in a given quantity of solution. It is usually expressed as a percentage, either mass by mass or mass by volume. A concentrated solution has a relatively large amount of solute, while a dilute solution has little.
- Suspension
- A heterogeneous mixture in which insoluble solid particles are dispersed throughout a liquid. Its particles are large enough to be seen with the eye, scatter light, and settle when left undisturbed. A suspension can be separated by filtration; chalk powder in water is a familiar example.
- Colloid
- A mixture in which particles of one substance are dispersed evenly through another, with particle sizes between those of a solution and a suspension. The particles are too small to settle, but large enough to scatter light. Milk, blood and fog are common colloids.
- Tyndall effect
- The phenomenon in which a beam of light becomes visible as it passes through a colloid, because light is scattered by the dispersed particles. True solutions do not show this effect because their particles are too small. This effect can be used to distinguish a colloid from a true solution.
- Sublimation
- The direct change of a solid to vapour on heating and back to solid on cooling, without passing through the liquid state. Substances like ammonium chloride and camphor show this property. They can be separated from non-subliming impurities by heating the mixture and cooling the vapour on a cold surface.
- Distillation
- A separation method in which a liquid is boiled to vapour and then condensed back to liquid by cooling. It is used to separate a volatile liquid from a non-volatile solid dissolved in it, such as obtaining pure water from salty water. The non-volatile solute remains in the flask and the pure liquid collects in the receiver.
Formula sheet
| What | Formula | Notes |
|---|---|---|
| Mass by mass percentage of a solution | Mass % = (mass of solute / mass of solution) × 100 | Use the same unit for solute and solution. Mass of solution = mass of solute + mass of solvent. This formula is used when both solute and solution masses are known. |
| Mass by volume percentage of a solution | Mass by volume % = (mass of solute / volume of solution) × 100 | Here mass of solute is in grams and volume of solution is in millilitres. The result gives grams of solute per 100 mL of solution. |
Practice questions with answers
1. What is a pure substance? Why is milk not a pure substance?
A pure substance consists of only one kind of material and has a definite composition; it can be an element or a compound. Milk is not pure because it is a mixture of water, fats, proteins and lactose that are simply mixed together, and its composition varies.
2. 20 g of sugar is dissolved in 180 g of water. Calculate the mass by mass percentage of the solution.
Mass of solution = 20 g + 180 g = 200 g. Mass by mass percentage = (20 g / 200 g) × 100 = 10%. So the solution is 10% sugar by mass.
3. What is the concentration in mass by volume percentage of a solution containing 8 g of salt in 160 mL of solution?
Mass by volume percentage = (8 g / 160 mL) × 100 = 5%. This means every 100 mL of the solution contains 5 g of dissolved salt.
4. Why does a beam of light become visible when passed through milk but not through a salt solution?
Milk is a colloid, so its dispersed particles are large enough to scatter a light beam and make its path visible. This is the Tyndall effect. A salt solution is a true solution with particles far too small to scatter light, so the beam passes through unseen.
5. A mixture of iron filings and sulphur powder is prepared in the laboratory. Is it a compound? How would you separate the iron filings from the mixture?
It is not a compound because its composition can vary and the two substances can still be identified and separated by physical means. Moving a magnet through the mixture attracts the iron filings and leaves the sulphur powder behind. Magnetism is a physical property, so no new substance is formed.
6. How is cream separated from milk? State the principle used.
Milk is rotated rapidly in a centrifuge. The heavier skimmed milk is forced outward, while the lighter fat globules that form cream collect nearer the centre and can be skimmed off. Centrifugation separates components by using the difference in their densities.
7. How would you obtain pure water from a salt solution?
Boil the salt solution in a distillation flask; water changes to vapour while the non-volatile salt remains behind. The vapour is cooled in a condenser and collected as pure distilled water. The salt is left in the distillation flask.
8. A mixture contains ammonium chloride, sand and common salt. How would you separate each component?
Place the mixture in a china dish and cover it with an inverted funnel. On heating, ammonium chloride sublimes and deposits as crystals on the cool funnel, leaving sand and salt behind. Add water to the residue, filter to remove insoluble sand, and evaporate the filtrate to recover common salt.
9. How can two dissolved dyes be separated by chromatography? What property of the dyes makes this possible?
Put a small drop of the dye mixture on a strip of filter paper and dip the lower end of the strip into a suitable solvent without wetting the spot. The solvent rises by capillary action and carries the dyes at different speeds, separating them into distinct coloured spots. This happens because the dyes have different solubilities and different ability to be adsorbed by the paper.
10. Classify cutting of wood and burning of wood as physical or chemical change, giving a reason in each case.
Cutting wood is a physical change because the pieces are still wood and no new substance is formed. Burning wood is a chemical change because wood reacts with oxygen to form new substances such as ash, carbon dioxide and water vapour, and the original wood cannot be recovered.
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