Is Matter Around Us Pure? Class 9 Study Guide
Most things around us are mixtures. We can separate them using differences in their properties.
A pure substance has a fixed composition, such as an element (oxygen, iron) or a compound (water, salt). A mixture has two or more substances that are not chemically joined, and their amounts can vary. A solution is a uniform mixture of a solute dissolved in a solvent, such as salt in water.
A suspension, such as muddy water, has particles large enough to settle and to be seen. A colloid, such as milk or fog, has particles too small to settle but large enough to scatter light, which is the Tyndall effect.
Solution, colloid or suspension?
| Property | Solution | Colloid | Suspension |
|---|---|---|---|
| Particles settle on standing | No | No | Yes |
| Scatters a beam of light | No | Yes | Yes |
| Can be seen by eye | No | No | Yes |
| Example | Sugar in water | Milk | Chalk in water |
Separating mixtures
Choose the method from the property that differs. Filtration separates a solid from a liquid. Evaporation recovers a dissolved solid. Distillation separates liquids with different boiling points. Chromatography separates coloured dyes in ink. A separating funnel separates liquids that do not mix, like oil and water.
Size, boiling point, solubility.
That uses the difference.
Use correct apparatus.
Is it pure?
Physical and chemical changes
In a physical change, no new substance forms, as in melting wax. In a chemical change, a new substance forms, as when iron rusts or milk turns sour. An element cannot be broken down into simpler substances by chemical means, while a compound can.
Worked examples and common mistakes
Example: Separate sand from salt solution: filter, then evaporate the filtrate to get salt. Example: Is air a mixture? Yes, because it contains gases in varying proportions. Common mistakes: calling a mixture of elements a compound, and calling any clear liquid pure. Clear salt water is a solution, not a pure substance.
Check your understanding: model answers
1. A colloid. 2. Use a magnet, because only the iron is attracted. 3. Chemical, since a new substance forms.
✨ Key Takeaways
- Pure substances have a fixed composition.
- Solutions are uniform and do not scatter light.
- Colloids show the Tyndall effect and do not settle.
- Choose a separation method from a property difference.
💬 Questions for Discussion & Reflection
- Is milk a solution, colloid or suspension?
- How can you separate iron filings from sand?
- Is rusting a physical or a chemical change?
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