Is Matter Around Us Pure — Science Class 9 Notes (CBSE & HBSE)
Free NCERT Science notes for Is Matter Around Us Pure (Class 9) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Is Matter Around Us Pure (CBSE & HBSE)
CBSE stresses the classification of matter, distinguishing solutions/suspensions/colloids using the Tyndall effect and applying separation techniques to mixtures, while HBSE emphasises definitions, concentration calculation and the element-compound-mixture distinction.
Pure Substances, Mixtures, Solutions, Suspensions and Colloids
Pure Substance vs Mixture
A pure substance consists of a single type of particle and has a fixed composition (e.g., gold, water, oxygen). A mixture contains two or more pure substances mixed in any ratio with no fixed composition (e.g., air, sea water).
Types of Mixtures
| Type | Definition | Example |
|---|---|---|
| Homogeneous | Uniform composition throughout, no visible boundaries | Salt in water, air |
| Heterogeneous | Non-uniform composition, visible boundaries | Sand + water, oil + water |
Solutions
A solution is a homogeneous mixture of two or more substances.
- Solute — the component present in smaller amount (dissolves).
- Solvent — the component present in larger amount (dissolves the solute).
- Example: in sugar solution, sugar = solute, water = solvent. Air, alloys and soda water are also solutions.
Properties of a solution:
- It is homogeneous.
- Particle size is less than 1 nm — not visible.
- Particles do not scatter light (no Tyndall effect).
- Particles do not settle and cannot be separated by filtration.
Concentration of a Solution
Concentration = amount of solute present in a given amount of solution (or solvent).
Suspensions
A suspension is a heterogeneous mixture in which solute particles are large (>100 nm), visible to the eye, settle down on standing and can be filtered. They scatter light (show Tyndall effect). Example: chalk powder in water, muddy water.
Colloids
A colloid is a heterogeneous mixture that appears homogeneous. Particle size is between solution and suspension (1 nm–100 nm). Particles do not settle, cannot be filtered by ordinary filter paper, but scatter a beam of light (Tyndall effect). Example: milk, fog, smoke, ink.
Tyndall Effect
The scattering of a beam of light by colloidal particles, making the path of light visible, is the Tyndall effect. Seen when sunlight enters a dusty room or passes through fog/forest canopy.
| Property | Solution | Colloid | Suspension |
|---|---|---|---|
| Nature | Homogeneous | Heterogeneous | Heterogeneous |
| Particle size | < 1 nm | 1–100 nm | > 100 nm |
| Tyndall effect | No | Yes | Yes |
| Settling | No | No | Yes |
| Filtration | Cannot | Cannot (ordinary) | Can be filtered |
CBSE trap: A colloid looks homogeneous to the naked eye but is actually heterogeneous, and it shows the Tyndall effect — a favourite exam point.
Separation Techniques for Mixtures
Why Separate Mixtures?
Mixtures are separated to remove undesirable substances or to obtain useful pure components. The method chosen depends on the difference in physical properties of the components.
Common Separation Methods
| Method | Used to Separate | Principle |
|---|---|---|
| Evaporation | Soluble solid from a liquid (salt from water) | Liquid evaporates, solid remains |
| Centrifugation | Suspended particles too small to filter (cream from milk) | Heavier particles settle on fast spinning |
| Separating funnel | Two immiscible liquids (oil + water) | Difference in densities |
| Sublimation | Sublimable solid from non-sublimable (ammonium chloride + salt) | One component sublimes |
| Chromatography | Coloured components of a dye/ink | Different solubility/adsorption on paper |
| Distillation | Two miscible liquids with different boiling points (acetone + water) | Difference in boiling points |
| Fractional distillation | Liquids with close boiling points (components of air, petroleum) | Repeated distillation in a fractionating column |
| Crystallisation | Pure solid crystals from impure sample (sugar, copper sulphate) | Pure substance crystallises out |
Crystallisation vs Simple Evaporation
Crystallisation is better than evaporation because:
- Some solids decompose or get charred on direct heating (e.g., sugar).
- Soluble impurities may remain after evaporation; crystallisation gives pure crystals.
Obtaining Different Gases from Air
Air is first compressed and cooled to form liquid air, which is then warmed in a fractional distillation column; gases separate out at their different boiling points (nitrogen boils off first, then oxygen).
HBSE/CBSE point: Filtration separates an insoluble solid from a liquid (heterogeneous), but it CANNOT separate a soluble solid (solution) — use evaporation or crystallisation instead.
Physical and Chemical Changes; Elements and Compounds
Physical vs Chemical Changes
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance formed? | No | Yes |
| Reversibility | Usually reversible | Usually irreversible |
| Composition change | No change in composition | Composition changes |
| Energy change | Small/none | Often large (heat/light) |
| Examples | Melting ice, dissolving sugar, boiling water | Burning of paper, rusting of iron, cooking food |
The interconversion of states (e.g., melting, boiling) and dissolving are physical changes because no new substance is formed and the change can be reversed.
Classification of Pure Substances
Pure substances are classified into elements and compounds.
Elements
An element is the basic form of matter that cannot be broken down into simpler substances by chemical reactions. (First defined by Antoine Lavoisier.)
Elements are of three types:
- Metals: lustrous, malleable, ductile, sonorous, good conductors (e.g., iron, copper, gold). Mercury is the only liquid metal.
- Non-metals: dull, non-malleable, poor conductors (e.g., sulphur, oxygen, carbon).
- Metalloids: show properties of both (e.g., boron, silicon, germanium).
Compounds
A compound is a substance made of two or more elements chemically combined in a fixed ratio by mass.
Properties of compounds:
- Components cannot be separated by physical means.
- A compound has entirely new properties, different from its constituent elements.
- The composition is fixed (e.g., water is always H₂O).
Mixture vs Compound (key distinction)
| Property | Mixture | Compound |
|---|---|---|
| Composition | Variable (any ratio) | Fixed ratio |
| Properties | Shows properties of components | New properties |
| Separation | By physical methods | Only by chemical methods |
| Energy change on formation | No | Heat/light usually given out/absorbed |
CBSE classic: Iron + sulphur as a mixture still shows magnetism of iron and can be separated, but on heating they form iron sulphide (FeS), a compound with new properties that is not magnetic and cannot be separated physically.
Frequently asked questions
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Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Is Matter Around Us Pure.