Revision Notes — Is Matter Around Us Pure?
1. Introduction — What is Matter?
Matter is anything that has mass and occupies space. Everything around us — solids, liquids and gases — is made of matter. To study matter scientifically, we classify it according to composition: whether it is made of a single type of particle (pure substance) or a combination of different substances (mixture).
2. Pure Substances
Pure substance is a form of matter that has a fixed or constant composition and distinct properties. It may be either an element (e.g., oxygen, hydrogen) or a compound (e.g., water, sodium chloride).
- Element: Cannot be broken down into simpler substances by chemical means. Made of atoms of one kind. Example: iron (Fe), oxygen (O2).
- Compound: Substances formed by chemical combination of two or more elements in fixed proportion. Properties differ from constituent elements. Example: water (H2O), carbon dioxide (CO2).
Key point: The composition of a compound is always fixed (by mass ratio) while mixtures can have variable composition.
3. Mixtures
A mixture contains two or more substances that are physically combined and can be separated by physical methods. Mixtures do not have fixed composition and retain the properties of their components.
Types of Mixtures
- Homogeneous mixture: Composition is uniform throughout. Also called a solution. Example: sugar solution, air.
- Heterogeneous mixture: Composition is not uniform; different parts can be visually distinguished. Example: sand in water, salad.
Special types of mixtures
| Type | Definition | Example |
|---|---|---|
| Suspension | Heterogeneous mixture with large particles that settle on standing | Sand in water, muddy water |
| Colloid | Mixture with particle size between true solution and suspension; particles do not settle and show Tyndall effect | Milk, fog, butter, starch in water |
| Solution (True) | Homogeneous mixture with particles at molecular or ionic level; no Tyndall effect | Sugar in water, salt in water |
Tyndall effect: Scattering of light by colloidal particles. Useful to distinguish between true solutions and colloids.
4. Separation Techniques — When & Why
Mixtures can be separated into their components because the components retain their properties. Different techniques use physical properties such as particle size, solubility, boiling point, magnetism, volatility, and density.
Common separation methods (what, when, how)
- Filtration: Separates insoluble solid from liquid using a porous medium (filter paper). Used for sand and water, or precipitates from solutions.
- Evaporation: Removes liquid from a solution by heating to obtain soluble solid (e.g., obtaining salt from salt solution). Useful when the solvent has a much lower boiling point or when we only need the solute.
- Distillation: Separates components of a solution based on different boiling points. Simple distillation: when components have widely different boiling points (e.g., separating water from a non-volatile solute). Fractional distillation: separates miscible liquids with closer boiling points (e.g., separation of ethanol and water).
- Chromatography: Separates components of a mixture based on their affinity towards stationary and mobile phases; excellent for separating colored substances and inks.
- Sublimation: Used when one or more components sublime (change from solid to gas without becoming liquid) — e.g., separation of ammonium chloride from a mixture of ammonium chloride and sand.
- Centrifugation: Applies centrifugal force to separate suspensions with fine particles that settle slowly under gravity (e.g., blood components in lab).
- Crystallization: Purifies solid substances by forming crystals from a saturated solution when cooled or evaporated slowly — good for obtaining pure samples.
- Separation using magnet: Removes magnetic solids (like iron filings) from mixtures using a magnet.
Exam tip: For each separation method know the principle, apparatus, an example, and why it works (i.e., which physical property is exploited).
5. Worked Examples (Exam-style)
Example 1: How will you separate a mixture of sand, common salt and ammonium chloride?
Solution outline:
- First use sublimation: heat the mixture — ammonium chloride will sublime and can be collected by cooling (sublimation).
- Now you have sand + salt. Add water to dissolve salt; filter to remove sand (filtration).
- Evaporate the filtrate to obtain salt crystals (evaporation or crystallization).
Example 2: Separate a mixture of iron filings and sulphur powder.
Solution: Use a magnet to attract iron filings away from sulphur (magnetic separation). If a pure sulphur sample is needed, iron can be collected and iron removed.
6. Important Definitions (Quick Recall)
- Solute: Substance that dissolves to form a solution.
- Solvent: Substance in which solute dissolves (usually present in larger amount).
- Solution: Homogeneous mixture of two or more substances.
- Saturated solution: Contains maximum amount of solute that can dissolve at a given temperature.
- Pure substance: Material with definite and constant composition.
7. NCERT Exercise — Approach & Tips
When solving NCERT questions, follow these steps: read carefully, underline keywords (e.g., "separate," "sublime," "volatile"), draw small diagrams for apparatus-based questions, and mention the principle behind the technique. Many board questions are directly from NCERT or are simple variations.
Practice the following repeatedly: problems asking for stepwise separation procedures (mixtures given), distinguishing between mixtures and pure substances, and identifying colloids via Tyndall effect.
8. Quick Revision Checklist (For last-minute study)
- Definition: matter, pure substance, mixture, solution, colloid, suspension.
- Examples of elements and compounds.
- List & principle for at least 6 separation techniques and one example for each.
- Recall Tyndall effect and how to distinguish between solution and colloid.
- Know how to separate sand + salt; sand + salt + ammonium chloride; iron + sulphur.
9. Practice Questions (Short)
- Differentiate between a compound and a mixture with one example each.
- How would you separate a mixture of camphor and common salt?
- Give one example each of a colloid and a suspension.
- What is the Tyndall effect? How does it help to identify colloids?
(Answers: 1) Compound has fixed composition; mixture variable — e.g., water vs. salt + sand. 2) Sublimation (camphor sublimes). 3) Colloid: milk; Suspension: muddy water. 4) Scattering of light by colloidal particles.)
10. Diagrams & Apparatus — What to draw in exams
Simple labeled diagrams score marks. Be able to sketch apparatus for: filtration, simple distillation, fractional distillation (fractionating column), chromatography setup (paper chromatography), and sublimation setup (conical flask with receiver). Label key parts and indicate direction of flow or heating.
11. Common Mistakes to Avoid
- Confusing crystallization with evaporation — crystallization yields purer crystals while evaporation just removes solvent and can give impure residue.
- Forgetting to state the principle (e.g., boiling point difference) behind distillation.
- Mixing up colloids and suspensions — remember: suspensions settle, colloids don’t.
12. Final Exam Strategy
Allocate 10–15 minutes to revise this chapter before the exam. Focus on separation technique steps, definitions and 2–3 worked examples. Practice drawing apparatus cleanly and labeling parts. The chapter is scoring — clear, concise answers get full marks.