Class 9 Science Notes Chapter 2 (Is Matter Around Us Pure) – Science Book

Alright class, let's focus on Chapter 2, 'Is Matter Around Us Pure?'. This is a fundamental chapter for understanding the classification of matter based on its chemical composition, which is crucial not just for your Class 9 understanding but also forms the basis for many concepts tested in government exams.

Chapter 2: Is Matter Around Us Pure? - Detailed Notes for Exam Preparation

1. Introduction: Classification of Matter

• Matter can be classified based on its chemical composition into:
  • Pure Substances: Consist of only one type of particle.
  • Mixtures (Impure Substances): Consist of two or more types of pure substances mixed together physically.

2. Pure Substances

• Definition: A substance consisting of single type of particles (atoms or molecules) with a definite set of properties and a fixed composition. Cannot be separated into simpler substances by physical methods.
• Characteristics:
  • Homogeneous throughout.
  • Fixed melting point and boiling point (at constant pressure).
  • Fixed composition.
• Types of Pure Substances:
  • (a) Elements:
    • Definition: The basic form of matter that cannot be broken down into simpler substances by chemical reactions. Composed of only one type of atom.
    • Examples: Hydrogen (H), Oxygen (O), Iron (Fe), Gold (Au), Carbon (C).
    • Classification:
      • Metals: Generally lustrous, malleable, ductile, good conductors of heat and electricity, solid at room temp (except Mercury - Hg). Examples: Iron, Copper, Gold, Silver, Aluminium.
      • Non-metals: Generally non-lustrous, brittle, poor conductors (except Graphite - Carbon allotrope), exist as solids, liquids (Bromine - Br), or gases at room temp. Examples: Oxygen, Hydrogen, Carbon, Sulphur, Phosphorus.
      • Metalloids: Exhibit properties intermediate between metals and non-metals. Examples: Boron (B), Silicon (Si), Germanium (Ge), Arsenic (As).
  • (b) Compounds:
    • Definition: A substance composed of two or more different elements chemically combined in a fixed proportion by mass.
    • Characteristics:
      • Components can only be separated by chemical or electrochemical reactions, not physical methods.
      • The properties of a compound are entirely different from those of its constituent elements. (e.g., Water (H₂O) is a liquid, extinguishes fire, while H₂ and O₂ are gases, H₂ is combustible, O₂ supports combustion).
      • Energy (heat, light) is usually absorbed or released during the formation of a compound.
      • Have a fixed melting and boiling point.
      • Homogeneous in nature.
    • Examples: Water (H₂O), Carbon Dioxide (CO₂), Sodium Chloride (NaCl - Common Salt), Sugar (C₁₂H₂₂O₁₁).

3. Mixtures

• Definition: Substances consisting of two or more elements or compounds (pure substances) mixed together physically, not chemically combined.
• Characteristics:
  • Variable composition.
  • Show the properties of the constituent substances.
  • Constituents can be separated by physical methods.
  • No fixed melting or boiling point.
  • Formation usually involves no energy change.
  • Can be homogeneous or heterogeneous.
• Types of Mixtures:
  • (a) Homogeneous Mixtures (Solutions):
    • Definition: Mixtures which have a uniform composition and properties throughout. The particles are indistinguishable even under a microscope.
    • Examples: Salt dissolved in water, Sugar dissolved in water, Air (mixture of gases), Alloys (mixture of metals, e.g., Brass - Copper & Zinc).
  • (b) Heterogeneous Mixtures:
    • Definition: Mixtures which have a non-uniform composition and properties. The components remain distinct and visible boundaries of separation often exist.
    • Examples: Mixture of sand and salt, Mixture of oil and water, Muddy water, Smoke (solid particles in gas), Soil.

4. Solutions (Homogeneous Mixtures)

• Components:
  • Solute: The substance that is dissolved (present in lesser quantity).
  • Solvent: The substance in which the solute is dissolved (present in larger quantity). The medium of dissolution. (Water is a universal solvent).
• Properties:
  • Homogeneous.
  • Particle size is extremely small (< 1 nanometer or 10⁻⁹ m). Cannot be seen by naked eye or microscope.
  • Do not scatter a beam of light passing through them (Do not show Tyndall effect).
  • Stable: Solute particles do not settle down when left undisturbed.
  • Components cannot be separated by filtration.
• Types: Solid in solid (Alloys), Solid in liquid (Sugar solution), Liquid in liquid (Vinegar - Acetic acid in water), Gas in liquid (Aerated drinks - CO₂ in water), Gas in gas (Air).
• Concentration of a Solution: Represents the amount of solute present in a given amount (mass or volume) of solution or solvent.
  • Mass by Mass Percentage: (Mass of solute / Mass of solution) × 100
  • Mass by Volume Percentage: (Mass of solute / Volume of solution) × 100
  • Volume by Volume Percentage: (Volume of solute / Volume of solution) × 100
• Saturation:
  • Unsaturated Solution: More solute can be dissolved at a given temperature.
  • Saturated Solution: Cannot dissolve more solute at a given temperature. Contains the maximum amount of solute at that temperature.
  • Supersaturated Solution: Contains more solute than required for saturation at a given temperature (unstable).
• Solubility: The maximum amount of solute (in grams) that can be dissolved in 100 grams of a solvent at a specific temperature to form a saturated solution. Solubility generally increases with temperature for solids in liquids, and decreases with temperature for gases in liquids. Pressure significantly affects the solubility of gases in liquids (Henry's Law).

5. Suspensions (Heterogeneous Mixtures)

• Definition: A heterogeneous mixture in which solute-like particles settle out of a solvent-like phase sometime after their introduction. Particles are large enough to be seen with the naked eye.
• Properties:
  • Heterogeneous.
  • Particle size is large (> 100 nanometers). Visible to the naked eye.
  • Scatter a beam of light passing through them (Show Tyndall effect).
  • Unstable: Particles settle down when left undisturbed.
  • Components can be separated by filtration.
• Examples: Chalk powder in water, Muddy water, Milk of Magnesia.

6. Colloids / Colloidal Solutions (Appear Homogeneous, but are Heterogeneous)

• Definition: A mixture in which the particle size is intermediate between those in true solutions and suspensions (1 nm to 100 nm). The particles are uniformly spread throughout the medium.
• Components:
  • Dispersed Phase: The solute-like component or the particles.
  • Dispersion Medium: The solvent-like component in which the particles are dispersed.
• Properties:
  • Appear homogeneous but are actually heterogeneous.
  • Particle size: 1 nm - 100 nm. Cannot be seen by naked eye, but visible under ultramicroscope.
  • Scatter a beam of light passing through them (Tyndall Effect). This is a key identification test.
  • Quite stable: Particles do not settle down easily.
  • Components cannot be separated by filtration, but can be separated by Centrifugation.
  • Particles exhibit Brownian Movement (random zig-zag motion).
• Examples & Common Types:
  • Aerosol: Solid/Liquid in Gas (Smoke, Fog, Mist, Clouds)
  • Foam: Gas in Liquid (Shaving cream, Soap lather)
  • Emulsion: Liquid in Liquid (Milk, Face cream, Cod liver oil)
  • Sol: Solid in Liquid (Ink, Paint, Starch solution, Milk of Magnesia)
  • Gel: Liquid in Solid (Jelly, Cheese, Butter)

Table: Comparison of Solution, Colloid, and Suspension

7. Separation Techniques for Mixtures

The choice of technique depends on the nature of the components and the difference in their physical properties.

Note: Crystallization is often preferred over evaporation for obtaining pure solids as it avoids decomposition and removes soluble impurities.

8. Physical vs. Chemical Changes

• Physical Change:

  • A change in which only the physical properties (state, shape, size, colour) change.
  • No new substance is formed.
  • Generally reversible.
  • Composition remains the same.
  • Examples: Melting of ice, Boiling of water, Cutting of trees, Dissolving salt in water, Magnetizing iron.

• Chemical Change:

  • A change in which new substances with entirely new properties are formed.
  • Involves changes in chemical composition.
  • Generally irreversible.
  • Often accompanied by heat/light production or absorption, sound, gas evolution, colour change, precipitate formation.
  • Examples: Burning of wood/paper, Rusting of iron, Cooking of food, Curdling of milk, Digestion of food.

• Burning of a Candle: Involves both physical (melting of wax) and chemical (burning of wax vapour) changes.

9. Difference between Mixtures and Compounds (Key Points for Exams)

Multiple Choice Questions (MCQs) for Practice:

1. Which of the following is classified as a pure substance?
   (a) Milk
   (b) Air
   (c) Iron
   (d) Soil

2. The process used to separate cream from milk is:
   (a) Filtration
   (b) Evaporation
   (c) Centrifugation
   (d) Distillation

3. Brass is an example of a:
   (a) Compound
   (b) Element
   (c) Homogeneous mixture (Alloy/Solution)
   (d) Heterogeneous mixture

4. Which of the following will show the Tyndall effect?
   (a) Salt solution
   (b) Sugar solution
   (c) Starch solution (Colloid)
   (d) Copper sulphate solution

5. A key difference between a mixture and a compound is that:
   (a) Mixtures have a fixed composition, while compounds do not.
   (b) Compounds show the properties of their constituents, while mixtures do not.
   (c) Mixtures can be separated by physical means, while compounds cannot.
   (d) Compounds are always heterogeneous, while mixtures are always homogeneous.

6. Which technique is most suitable for separating a mixture of oil and water?
   (a) Sublimation
   (b) Separating funnel
   (c) Chromatography
   (d) Filtration

7. Burning of a candle involves:
   (a) Only a physical change
   (b) Only a chemical change
   (c) Both physical and chemical changes
   (d) Neither physical nor chemical change

8. Which of the following represents the solute in an aerated (carbonated) drink?
   (a) Water
   (b) Carbon dioxide
   (c) Sugar
   (d) Flavouring agent

9. The technique used to separate components of a mixture based on their different rates of adsorption on a stationary phase is called:
   (a) Distillation
   (b) Crystallization
   (c) Chromatography
   (d) Sublimation

10. Which of the following substances undergoes sublimation?
    (a) Sodium chloride
    (b) Sugar
    (c) Ammonium chloride
    (d) Copper sulphate

Answer Key:

1. (c)
2. (c)
3. (c)
4. (c)
5. (c)
6. (b)
7. (c)
8. (b)
9. (c)
10. (c)

Study these notes thoroughly, paying special attention to definitions, classifications, properties, examples, and the principles behind separation techniques. Understanding the differences between key concepts like mixtures vs. compounds, solutions vs. colloids vs. suspensions, and physical vs. chemical changes is vital for objective questions. Good luck with your preparation!