Class 9 Science Ch 2 Notes in English

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📘 Chapter 2: Is Matter Around Us Pure?

🔹 1. Pure Substances vs Mixtures

• A pure substance contains only one type of particle.

  • It has a fixed composition and specific properties.
  • Examples: Water, iron, oxygen.

• A mixture contains two or more different particles.

  • It can be separated by physical methods.
  • Examples: Salt water, air, milk.

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🔹 2. Types of Pure Substances

👉 Element

• It is made up of only one kind of atom.
• It cannot be broken down into simpler substances by chemical or physical methods.
• Examples: Hydrogen, carbon, gold.

👉 Compound

• It is made up of two or more elements chemically combined in a fixed ratio.
• The properties of a compound are different from the properties of the elements that form it.
• Examples: Water (H₂O), carbon dioxide (CO₂), sodium chloride (NaCl).

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🔹 3. Types of Mixtures

👉 Homogeneous Mixture

• The composition is uniform throughout.
• The different components are not visible separately.
• Examples: Sugar dissolved in water, air, vinegar.

👉 Heterogeneous Mixture

• The composition is not uniform.
• The components are easily visible and can be separated.
• Examples: Sand and salt, oil and water, soil.

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🔹 4. Solutions

• A solution is a homogeneous mixture of two or more substances.
• The substance that dissolves is called the solute.
• The substance in which solute dissolves is called the solvent.

Properties of Solutions:

1. They are homogeneous.
2. The particles are very small and cannot be seen.
3. The solute does not settle down.
4. Cannot be separated by filtration.

Examples:

• Salt in water, sugar in water.

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🔹 5. Concentration of a Solution

• It is the amount of solute present in a given quantity of solution.
• A solution can be dilute, concentrated, or saturated depending on the amount of solute.

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🔹 6. Suspensions

• A suspension is a heterogeneous mixture in which the particles are visible and settle down over time.

Properties:

1. It is heterogeneous.
2. Particles are visible.
3. Particles settle down on standing.
4. Can be separated by filtration.

Examples:

• Mud in water, chalk in water.

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🔹 7. Colloids

• A colloid is a mixture where particles are evenly spread but not dissolved.

Properties:

1. It appears homogeneous but is actually heterogeneous.
2. Particles are not visible to the naked eye.
3. Particles do not settle.
4. They show Tyndall effect (scatter light).
5. Cannot be separated by filtration.

Examples:

• Milk, fog, blood.

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🔹 8. Separation of Mixtures

Some common methods:

1. Evaporation – to separate a solid dissolved in a liquid.
2. Centrifugation – to separate cream from milk.
3. Filtration – to remove insoluble solid from liquid.
4. Decantation and sedimentation – to separate heavier particles from liquids.
5. Chromatography – to separate colors in a dye.
6. Distillation – to separate two miscible liquids with different boiling points.
7. Separating funnel – to separate two immiscible liquids like oil and water.

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🔹 9. Physical and Chemical Changes

• Physical Change: No new substance is formed, and the change is reversible.

  • Example: Melting of ice.

• Chemical Change: A new substance is formed, and the change is usually irreversible.

  • Example: Burning of paper.

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🔹 10. Types of Pure Substances and Mixtures Summary

• Pure substances include elements and compounds.
• Mixtures are of two types: homogeneous and heterogeneous.
• Mixtures can be separated using physical methods based on their properties.