Class 9 Science – Chapter 2: Is Matter Around Us Pure? (Detailed Notes)

Class 9 Science – Chapter 2: Is Matter Around Us Pure? (Detailed Notes)

1. What is a Pure Substance?

• A pure substance is made up of only one kind of particles (atoms or molecules).
• It has a fixed composition and cannot be separated into other kinds of matter by physical methods.
• Examples: Water, oxygen, gold, copper, etc.

2. Types of Substances

(a) Pure Substances

• Elements and compounds are pure substances.

(b) Mixtures

• Mixtures are made up of two or more substances (elements or compounds) mixed together in any proportion.
• They can be separated by physical methods.
• Mixtures can be homogeneous or heterogeneous.

3. Types of Mixtures

(a) Homogeneous Mixture

• Composition is uniform throughout.
• Particles are not visible separately.
• Examples: Salt in water, sugar in water, air.

(b) Heterogeneous Mixture

• Composition is not uniform.
• Different components are visible.
• Examples: Oil and water, sand and iron filings.

4. Solution

• A homogeneous mixture of two or more substances is called a solution.
• Example: Salt solution, sugar solution.

Components of a Solution:

• Solvent: The component present in larger amount (e.g., water).
• Solute: The component present in smaller amount (e.g., salt).

Properties of Solution:

• Homogeneous in nature.
• Cannot be separated by filtration.
• Stable; solute does not settle down.

5. Types of Solutions

• Solid in liquid: Salt in water
• Gas in liquid: Carbon dioxide in soft drinks
• Gas in gas: Air

6. Concentration of Solution

• It refers to the amount of solute present in a given quantity of solvent or solution.

Formula:

• Concentration (%) = (Mass of solute / Mass of solution) × 100

7. Suspension

• A heterogeneous mixture in which particles are large enough to be seen with naked eye.
• Particles settle down when left undisturbed.
• Examples: Sand in water, chalk in water.

Properties of Suspension:

• Heterogeneous
• Particles are visible and settle down
• Can be separated by filtration

8. Colloidal Solutions (Colloids)

• Appear homogeneous but are actually heterogeneous.
• Particles are small and cannot be seen with naked eye, but large enough to scatter light.
• Examples: Milk, fog, blood, shaving cream.

Properties of Colloids:

• Heterogeneous in nature.
• Stable – particles do not settle down.
• Cannot be filtered by ordinary filter paper.
• Show Tyndall effect (scattering of light).

9. Separation of Components of Mixtures

Methods depend on physical properties like boiling point, solubility, size of particles, etc.

Important Methods:

• Filtration: Solid-liquid separation (e.g., sand from water).
• Evaporation: Solute recovery from a solution (e.g., salt from salt water).
• Centrifugation: Used for separating suspended particles (e.g., cream from milk).
• Decantation and Loading: Separation of solid from liquid by sedimentation.
• Distillation: For separating two miscible liquids with different boiling points.
• Chromatography: To separate different dyes in ink.
• Separating Funnel: For separating two immiscible liquids (e.g., oil and water).
• Sublimation: To separate sublimable substance from non-sublimable one (e.g., camphor from salt).

10. Types of Pure Substances

(a) Elements

• Substances that cannot be broken down into simpler substances.
• Made of only one kind of atom.
• Examples: Hydrogen, oxygen, gold, iron.

Types of Elements:

• Metals: Lustrous, malleable, ductile, good conductors (e.g., copper, gold).
• Non-metals: Non-lustrous, brittle, poor conductors (e.g., sulfur, carbon).
• Metalloids: Show properties of both metals and non-metals (e.g., silicon, boron).

(b) Compounds

• Formed when two or more elements combine chemically in a fixed ratio.
• Properties of compound are different from its constituent elements.
• Examples: Water (H₂O), Carbon dioxide (CO₂), Sodium chloride (NaCl).

11. Differences Summary Table

Mixture	Compound
Elements are mixed in any ratio	Elements combine in fixed ratio
No new substance is formed	New substance is formed
Can be separated by physical methods	Cannot be separated easily
Properties same as components	Properties different from components