Chapter: Is Matter Around Us Pure
Class IX – Science (Chemistry)
1. What is a Pure Substance?
Key Definitions
- Pure Substance: A substance that contains only one kind of particles and has a fixed composition.
- Pure substances may be elements or compounds.
Explanation (Point-wise)
- Pure substances have uniform composition throughout.
- They have fixed melting and boiling points.
- They cannot be separated into other substances by physical methods.
Examples
- Elements: Iron, Gold, Oxygen
- Compounds: Water (H₂O), Sodium chloride (NaCl)
Important Points / Exam Tips
- Pure ≠ Single element only (compounds are also pure).
- NCERT highlights that solutions are not pure substances.
- 🔑 Keyword: Fixed composition
2. Mixtures
Key Definitions
- Mixture: A combination of two or more substances in any proportion, where components retain their individual properties.
Explanation (Point-wise)
- Components can be separated by physical methods.
- Composition is variable.
- No fixed melting or boiling point.
Examples
Important Points / Exam Tips
- Most substances around us are mixtures, not pure.
- 🔑 Keyword: Physical separation
3. Types of Mixtures
3.1 Homogeneous Mixture
Definition
- A mixture with uniform composition throughout.
Explanation
- Components are not visible separately.
- Single-phase system.
Examples
- Salt in water
- Sugar solution
Exam Tip
- Solutions are homogeneous mixtures.
3.2 Heterogeneous Mixture
Definition
- A mixture with non-uniform composition.
Explanation
- Components are visibly different.
- More than one phase present.
Examples
- Sand in water
- Oil and water
Exam Tip
- Presence of phases is the key clue.
4. Solution
Key Definitions
- Solution: A homogeneous mixture of solute and solvent.
- Solute: Substance that dissolves.
- Solvent: Substance in which solute dissolves.
Explanation (Point-wise)
- Particles are very small (< 1 nm).
- Cannot be seen with naked eye.
- Does not scatter light.
- Stable (no settling).
Examples
- Salt solution
- Sugar in water
Important Points / Exam Tips
- True solutions do not show Tyndall effect.
- 🔑 Keywords: Solute, Solvent, Homogeneous
5. Concentration of a Solution
Definition
- Amount of solute present in a given amount of solution.
Methods of Expressing Concentration
| Method |
Formula |
| Mass % |
(Mass of solute / Mass of solution) × 100 |
| Volume % |
(Volume of solute / Volume of solution) × 100 |
| Mass by Volume % |
(Mass of solute / Volume of solution) × 100 |
Important Points / Exam Tips
- Numerical questions often come from concentration formulas.
- Units must be written correctly.
6. Suspension
Definition
- A heterogeneous mixture in which insoluble particles are suspended in a liquid.
Explanation (Point-wise)
- Particles are large and visible.
- Scatter light.
- Unstable; particles settle on standing.
- Can be separated by filtration.
Examples
- Chalk in water
- Muddy water
Exam Tip
- Settling on standing is a key identifying feature.
7. Colloid
Definition
- A mixture in which particles are uniformly spread but not truly dissolved.
Explanation (Point-wise)
- Particle size is intermediate.
- Show Tyndall effect.
- Stable (do not settle).
- Cannot be filtered easily.
Examples
Diagram / Flowchart (Description)
- Light beam passing through colloid becomes visible due to scattering.
Important Points / Exam Tips
- Colloids appear homogeneous but are actually heterogeneous.
- 🔑 Keyword: Tyndall effect
8. Comparison: Solution, Colloid, Suspension
| Property |
Solution |
Colloid |
Suspension |
| Nature |
Homogeneous |
Heterogeneous |
Heterogeneous |
| Particle Size |
Very small |
Intermediate |
Large |
| Visibility |
Not visible |
Not visible |
Visible |
| Tyndall Effect |
No |
Yes |
Yes |
| Stability |
Stable |
Stable |
Unstable |
9. Separating the Components of a Mixture
9.1 Evaporation
- Used to separate soluble solid from liquid.
- Example: Salt from seawater.
9.2 Centrifugation
- Used when particles are very fine.
- Example: Cream from milk.
9.3 Filtration
- Used for insoluble solids.
- Example: Sand from water.
9.4 Separating Funnel
- Used for immiscible liquids.
- Example: Oil and water.
9.5 Sublimation
- Used when one component sublimates.
- Example: Ammonium chloride from salt.
9.6 Chromatography
- Used to separate coloured components.
- Example: Dyes in ink.
Exam Tip
- Always match method with property difference.
10. Physical and Chemical Changes
Physical Change
- No new substance formed.
- Change is usually reversible.
Chemical Change
- New substance formed.
- Usually irreversible.
Examples
- Melting of ice → Physical
- Rusting of iron → Chemical
11. Elements
Definition
- A pure substance made of only one kind of atom.
Types of Elements
- Metals: Iron, Copper
- Non-metals: Oxygen, Sulphur
- Metalloids: Silicon
Exam Tip
- Elements cannot be broken down by chemical methods.
12. Compounds
Definition
- A pure substance formed by chemical combination of two or more elements in a fixed ratio.
Explanation
- Properties are different from constituent elements.
- Can be broken down only by chemical methods.
Examples
- Water (H₂O)
- Sodium chloride (NaCl)
13. Differences: Mixture vs Compound
| Mixture |
Compound |
| Variable composition |
Fixed composition |
| Physical methods used |
Chemical methods used |
| Properties retained |
New properties formed |
Quick Revision Summary
- Pure substances have fixed composition (elements, compounds).
- Mixtures are of two types: homogeneous and heterogeneous.
- Solutions, colloids, and suspensions differ in particle size and properties.
- Tyndall effect is shown by colloids.
- Different separation techniques are based on physical properties.
- Elements and compounds are pure substances; mixtures are not.
