Class 9 Science Notes Chapter 2 (Chapter 2) – Examplar Problem (English) Book

Alright class, let's dive deep into Chapter 2, 'Is Matter Around Us Pure?'. This is a fundamental chapter, and understanding these concepts clearly is crucial, not just for your Class 9 exams, but also for various government exams where basic science is often tested.

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

1. Classification of Matter

Matter can be broadly classified based on its chemical composition into:

• Pure Substances: Consist of only one kind of particle (atoms or molecules). They have a fixed composition and fixed properties (like melting point, boiling point).
  • Cannot be separated into simpler substances by physical methods.
• Mixtures (Impure Substances): Consist of two or more pure substances mixed together physically in any proportion.
  • Components retain their individual properties.
  • Can be separated into their components by physical methods.
  • Composition is variable.
  • Do not have fixed melting or boiling points.

2. Pure Substances

Pure substances are further divided into:

• Elements:

  • The basic form of matter that cannot be broken down into simpler substances by chemical reactions.
  • Made up of only one kind of atom.
  • Examples: Hydrogen (H), Oxygen (O), Iron (Fe), Gold (Au), Carbon (C).
  • Types of Elements:
    • Metals: Generally solid (except Mercury), lustrous (shiny), malleable (can be beaten into sheets), ductile (can be drawn into wires), good conductors of heat and electricity, typically have high melting/boiling points, sonorous (make ringing sound). Examples: Iron, Copper, Gold, Silver, Aluminium.
    • Non-metals: Exhibit variety in properties. Can be solid, liquid (Bromine), or gas (Oxygen, Hydrogen). Generally non-lustrous (except Iodine, Graphite), brittle (break easily), poor conductors of heat and electricity (except Graphite). Examples: Carbon, Sulphur, Phosphorus, Oxygen, Chlorine.
    • Metalloids: Show intermediate properties between metals and non-metals. Examples: Boron (B), Silicon (Si), Germanium (Ge), Arsenic (As).

• Compounds:

  • Substances formed when two or more elements combine chemically in a fixed ratio by mass.
  • The properties of a compound are entirely different from those of its constituent elements. (e.g., Water (H₂O) is a liquid, extinguishes fire, while Hydrogen (H₂) burns and Oxygen (O₂) supports combustion).
  • Have a fixed composition.
  • Have fixed melting and boiling points.
  • Can only be broken down into their constituent elements by chemical or electrochemical reactions, not by physical methods.
  • Examples: Water (H₂O), Carbon Dioxide (CO₂), Sodium Chloride (NaCl - common salt), Sugar (C₁₂H₂₂O₁₁).

Difference between Mixtures and Compounds

3. Mixtures

Mixtures are further classified based on the uniformity of their composition:

• Homogeneous Mixtures (Solutions):

  • Have a uniform composition throughout.
  • Particles are not visible even under a microscope.
  • Components cannot be separated by filtration.
  • Do not scatter light (do not show the Tyndall effect).
  • Stable (particles do not settle down).
  • Examples: Salt dissolved in water, sugar dissolved in water, air (mixture of gases), alloys (solid solutions like brass, steel).
  • Solutions: A homogeneous mixture of two or more substances.
    • Solute: The substance that is dissolved (present in lesser quantity).
    • Solvent: The substance in which the solute is dissolved (present in larger quantity).
    • Aqueous Solution: Solution where water is the solvent.
    • Non-Aqueous Solution: Solution where the solvent is other than water (e.g., alcohol, ether).
    • Concentration of a Solution: 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
    • Saturated Solution: A solution that cannot dissolve any more solute at a given temperature.
    • Unsaturated Solution: A solution that can dissolve more solute at a given temperature.
    • Solubility: The maximum amount of solute that can be dissolved in 100 grams of a solvent at a specific temperature to form a saturated solution.

• Heterogeneous Mixtures:

  • Have a non-uniform composition.
  • Have visible boundaries of separation between components.
  • Particles may be visible to the naked eye or under a microscope.
  • Examples: Mixture of sand and water, muddy water, mixture of oil and water, chalk powder in water.
  • Types include:
    • Suspensions:
      • Heterogeneous mixture where solid particles are dispersed in a liquid without dissolving.
      • Particle size is large (> 100 nm).
      • Particles are visible to the naked eye.
      • Unstable: Particles settle down when left undisturbed (sedimentation).
      • Can be separated by filtration.
      • Scatter light (show Tyndall effect when particles are suspended).
      • Examples: Chalk-water mixture, muddy water, milk of magnesia.
    • Colloids (or Colloidal Solutions):
      • Appear homogeneous but are actually heterogeneous.
      • Particle size is intermediate (between 1 nm and 100 nm), too small to be seen by naked eye but large enough to scatter light.
      • Stable: Particles do not settle down.
      • Cannot be separated by filtration, but can be separated by centrifugation.
      • Show Tyndall Effect: The scattering of a beam of light by colloidal particles, making the path of light visible.
      • Exhibit Brownian Movement: The random, zig-zag motion of colloidal particles due to collision with the molecules of the dispersion medium.
      • Components:
        • Dispersed Phase: The solute-like component or particles.
        • Dispersion Medium: The solvent-like component in which particles are dispersed.
      • Common Examples based on Phase/Medium:
        • Aerosol: (Solid in Gas - Smoke, dust), (Liquid in Gas - Fog, mist, clouds)
        • Foam: (Gas in Liquid - Whipped cream, soap lather)
        • Emulsion: (Liquid in Liquid - Milk, face cream)
        • Sol: (Solid in Liquid - Milk of magnesia, mud, starch solution)
        • Gel: (Liquid in Solid - Jelly, cheese, butter)
        • Solid Sol: (Solid in Solid - Coloured gemstones, milky glass)

Comparison: Solution vs Colloid vs Suspension

4. Separating the Components of a Mixture

Various physical methods are used based on the differences in physical properties of the components:

• Evaporation: To separate a volatile component (solvent) from a non-volatile component (solute). Principle: Solvent evaporates, leaving solute behind. Application: Salt from seawater, ink dye from water.
• Centrifugation: To separate suspended particles from a liquid when particles are very small and pass through filter paper. Principle: Denser particles settle at the bottom and lighter particles stay at the top when rotated rapidly. Application: Separating cream from milk, separating blood components, diagnostic labs, washing machine dryer.
• Decantation (using Separating Funnel): To separate a mixture of two immiscible liquids (liquids that do not mix). Principle: Difference in densities; heavier liquid forms lower layer. Application: Separating oil from water, kerosene from water.
• Sublimation: To separate a mixture containing a sublimable volatile component from a non-sublimable impurity. Principle: Some solids directly change into gas on heating (sublime). Application: Separating ammonium chloride, camphor, naphthalene, or iodine from common salt or sand.
• Chromatography: To separate solutes that dissolve in the same solvent. Principle: Different components have different solubilities in the same solvent and different adsorption rates on an adsorbent material, causing them to separate as the solvent moves. Application: Separating colours in a dye, pigments from natural colours, drugs from blood. (Paper chromatography is a common type).
• Distillation (Simple): To separate a mixture of two miscible liquids having a sufficient difference in their boiling points (generally > 25 K or 25°C). Also used to separate liquids from non-volatile solutes. Principle: Liquid with lower boiling point vaporizes first, condenses, and is collected. Application: Separating acetone (BP ~56°C) and water (BP 100°C), obtaining pure water (distilled water).
• Fractional Distillation: To separate a mixture of two or more miscible liquids with a small difference in boiling points (< 25 K). Principle: Uses a fractionating column which provides a large surface area for repeated vaporization and condensation, allowing better separation based on boiling points. Application: Separation of different gases from air, separation of different fractions from petroleum (petrol, diesel, kerosene), separation of ethanol and water.
• Crystallisation: To obtain a pure solid in the form of crystals from its solution. Principle: Pure solid separates out from a saturated solution upon cooling. Better than evaporation as it removes soluble impurities and avoids decomposition of solids that decompose on heating. Application: Purification of salt from seawater, obtaining pure crystals of copper sulphate or alum from impure samples.

5. Physical and Chemical Changes

• Physical Change:
  • A change where only the physical properties (like state, shape, size, colour, density) of a substance change.
  • No new substance is formed.
  • Generally reversible.
  • Composition remains the same.
  • Examples: Melting of ice, boiling of water, freezing of water, condensation of steam, dissolving salt/sugar in water, cutting of trees, tearing paper, stretching a rubber band.
• Chemical Change:
  • A change where one or more new substances with entirely different properties are formed.
  • Involves changes in chemical composition.
  • Generally irreversible.
  • Often accompanied by heat/light production or absorption, sound production, gas evolution, colour change, precipitate formation.
  • Examples: Burning of wood/paper/fuel, rusting of iron, digestion of food, cooking of food, ripening of fruits, formation of curd from milk, reaction of acid with base.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on Chapter 2 for your practice:

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

2. A mixture of sulphur and carbon disulphide is:
   (a) Homogeneous and shows Tyndall effect
   (b) Heterogeneous and shows Tyndall effect
   (c) Homogeneous and does not show Tyndall effect
   (d) Heterogeneous and does not show Tyndall effect

3. Which method is most suitable for separating cream from milk?
   (a) Filtration
   (b) Evaporation
   (c) Distillation
   (d) Centrifugation

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

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

6. The process used to separate a mixture of common salt and ammonium chloride is:
   (a) Crystallisation
   (b) Sublimation
   (c) Chromatography
   (d) Fractional distillation

7. Which of the following represents a chemical change?
   (a) Melting of wax
   (b) Bending of an iron rod
   (c) Rusting of iron
   (d) Dissolving sugar in water

8. A solution contains 40 g of common salt in 320 g of water. The mass by mass percentage of the solution is:
   (a) 12.5%
   (b) 11.1%
   (c) 10%
   (d) 8%

9. Separation of components of air is primarily done using:
   (a) Simple Distillation
   (b) Chromatography
   (c) Fractional Distillation
   (d) Sublimation

10. Fog is a colloidal solution of:
    (a) Gas in Liquid
    (b) Solid in Gas
    (c) Liquid in Gas
    (d) Gas in Gas

Answer Key for MCQs:

1. (c) Iron (Element)
2. (c) Homogeneous and does not show Tyndall effect (Sulphur dissolves in CS₂, forming a true solution)
3. (d) Centrifugation
4. (c) Homogeneous mixture (Solution/Alloy) (Brass is an alloy of Copper and Zinc)
5. (c) Starch solution (It's a colloid)
6. (b) Sublimation (Ammonium chloride sublimes, salt does not)
7. (c) Rusting of iron (A new substance, iron oxide, is formed)
8. (b) 11.1% [Mass of solute = 40g, Mass of solvent = 320g. Mass of solution = 40 + 320 = 360g. % = (40/360) * 100 = 11.1%]
9. (c) Fractional Distillation (After liquefying air)
10. (c) Liquid in Gas (Water droplets dispersed in air)

Make sure you revise these concepts thoroughly. Pay special attention to the differences between mixtures and compounds, the properties of solutions, colloids, and suspensions, and the principles behind each separation technique. Good luck with your preparation!