Class 9 Science Notes Chapter 2 (Chapter 2) – Lab Manual (English) Book

Alright class, let's focus on Chapter 2 from your Science Lab Manual. This chapter deals with fundamental concepts about the nature of matter – specifically solutions, suspensions, colloids, mixtures, compounds, and physical versus chemical changes. Understanding these experiments is crucial, not just for your practical exams, but also for building a strong foundation for competitive exams. Pay close attention to the observations and the inferences drawn from them.

Chapter 2: Solutions, Suspensions, Colloids, Mixtures, Compounds, Physical & Chemical Changes

Core Concepts:

• Pure Substance: Consists of only one type of particle (atoms or molecules). Can be elements or compounds. Has fixed composition and properties (e.g., fixed melting/boiling point).
• Mixture: Consists of two or more pure substances physically mixed in any proportion. Components retain their individual properties. Can be separated by physical methods. Mixtures can be homogeneous or heterogeneous.
• Homogeneous Mixture (Solution): Uniform composition throughout. Particles are not visible. Example: Salt dissolved in water.
• Heterogeneous Mixture: Non-uniform composition. Components are often visible. Examples: Sand in water (Suspension), Milk (Colloid).
• Solution: A homogeneous mixture of two or more substances.
  • Solute: Substance dissolved (present in lesser quantity).
  • Solvent: Substance in which solute dissolves (present in larger quantity).
  • Properties: Transparent, particles (< 1 nm) invisible, do not settle, pass through filter paper, do not show Tyndall effect.
• Suspension: A heterogeneous mixture where solute particles do not dissolve but remain suspended.
  • Properties: Opaque or translucent, particles (> 100 nm) visible, settle down (unstable), particles retained by filter paper, may show Tyndall effect initially but not after settling.
• Colloid (Colloidal Solution): A heterogeneous mixture where particle size is intermediate between solutions and suspensions. Appears homogeneous but is actually heterogeneous.
  • Properties: Translucent or opaque, particles (1 nm - 100 nm) not visible to naked eye but scatter light, stable (do not settle), pass through filter paper, show Tyndall effect.
  • Tyndall Effect: Scattering of a beam of light by colloidal particles, making the path of light visible.
• Compound: A pure substance formed when two or more elements combine chemically in a fixed ratio by mass. Properties are entirely different from constituent elements. Can only be broken down by chemical reactions.
• Physical Change: A change affecting only the physical properties (like state, shape, size, colour) without forming a new substance. Usually reversible. Example: Melting of ice, dissolving sugar in water, heating copper sulphate crystals (loss of water of crystallization).
• Chemical Change: A change resulting in the formation of one or more new substances with entirely new properties. Usually irreversible or reversible with difficulty. Involves energy changes. Example: Burning of magnesium, rusting of iron, reaction of acid with metal.

Experiment 1: Preparation and Study of True Solutions, Suspensions, and Colloids

• Aim: To prepare and differentiate between true solutions, suspensions, and colloidal solutions based on transparency, filtration, stability, and Tyndall effect.
• Materials: Common salt, sugar, alum, soil, chalk powder, fine sand, starch, egg albumin/milk, water, beakers, glass rod, filter paper, funnel, test tubes, torch/laser pointer.
• Procedure Summary:
  1. True Solution: Dissolve salt/sugar/alum in water. Stir well.
  2. Suspension: Mix soil/chalk powder/sand in water. Stir well.
  3. Colloid: Mix starch with cold water to make a paste, then add boiling water OR mix egg albumin/milk with water. Stir well.
• Observations & Comparison:

Experiment 2: Preparation of Mixture and Compound (Iron Filings & Sulphur Powder)

• Aim: To prepare a mixture and a compound using iron filings and sulphur powder and distinguish between them.
• Materials: Iron filings, sulphur powder, watch glass, china dish, burner, tripod stand, wire gauze, magnet, carbon disulphide (CS₂), test tubes.
• Procedure Summary:
  1. Mixture: Mix iron filings and sulphur powder in a watch glass (Fe + S).
  2. Compound: Mix iron filings and sulphur powder in a china dish, heat strongly until a black mass (Iron Sulphide, FeS) is formed. Let it cool.
• Observations & Comparison:

• Inference: A mixture retains the properties of its constituents and can be separated physically. A compound has properties different from its constituents and is formed by a chemical reaction.
• Precautions: Carbon disulphide is highly flammable and its vapours are toxic – use it carefully, away from flames, preferably in a fume hood. Heat the mixture of iron and sulphur gently at first, then strongly.

Experiment 3: Performing Chemical Reactions and Classifying Changes

• Aim: To carry out specific reactions and classify them as physical or chemical changes.

• Reactions & Observations:

  1. Iron Nail + Copper Sulphate Solution (CuSO₄):
     • Observation: The blue colour of the CuSO₄ solution fades and turns light green. A reddish-brown coating (copper) deposits on the iron nail.
     • Classification: Chemical Change (Displacement reaction: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)). New substances (Iron Sulphate and Copper) are formed.
  2. Burning Magnesium Ribbon (Mg):
     • Observation: Burns with a dazzling white flame, producing a white powdery ash (Magnesium Oxide, MgO).
     • Classification: Chemical Change (Combination reaction: 2Mg(s) + O₂(g) → 2MgO(s)). A new substance (MgO) is formed.
  3. Zinc Granules (Zn) + Dilute Sulphuric Acid (H₂SO₄):
     • Observation: Bubbles of a colourless, odourless gas (Hydrogen, H₂) evolve rapidly. The test tube feels warm.
     • Classification: Chemical Change (Displacement reaction: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)). New substances (Zinc Sulphate and Hydrogen gas) are formed, heat is evolved (exothermic).
  4. Heating Copper Sulphate Crystals (CuSO₄·5H₂O):
     • Observation: Blue crystals turn into a white anhydrous powder upon heating. Water droplets appear on the cooler parts of the test tube. Adding water to the white powder restores the blue colour.
     • Classification: Primarily a Physical Change (Loss of water of crystallization). Reversible. (Note: Very strong heating causes decomposition into CuO and SO₃, which is a chemical change, but the lab manual focuses on the reversible dehydration).
  5. Sodium Sulphate Solution (Na₂SO₄) + Barium Chloride Solution (BaCl₂):
     • Observation: A white, insoluble substance (precipitate) of Barium Sulphate (BaSO₄) is formed immediately upon mixing the two clear solutions.
     • Classification: Chemical Change (Double displacement/Precipitation reaction: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) ↓ + 2NaCl(aq)). A new, insoluble substance (BaSO₄) is formed.

• Key Indicators of Chemical Change: Formation of a new substance, change in colour, evolution of gas, formation of precipitate, change in temperature (heat evolved or absorbed), production of light or sound.

Multiple Choice Questions (MCQs):

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

2. When preparing a suspension of chalk powder in water, what is observed after some time?
   a) Chalk dissolves completely.
   b) The mixture remains homogeneous.
   c) Chalk particles settle down at the bottom.
   d) The mixture becomes transparent.

3. A student prepares a mixture of iron filings and sulphur powder. When a magnet is brought near it:
   a) Only sulphur powder is attracted.
   b) Only iron filings are attracted.
   c) Both iron and sulphur are attracted.
   d) Neither iron nor sulphur is attracted.

4. When iron filings and sulphur powder are heated strongly, the resulting substance (Iron Sulphide) is:
   a) Attracted by a magnet.
   b) Soluble in carbon disulphide.
   c) A black homogeneous mass.
   d) A physical mixture of iron and sulphur.

5. The reaction between zinc granules and dilute sulphuric acid is characterized by:
   a) Formation of a yellow precipitate.
   b) Evolution of a gas with a pungent smell.
   c) Evolution of a colourless and odourless gas.
   d) A decrease in temperature.

6. Heating blue copper sulphate crystals results in a white powder due to:
   a) Decomposition into copper oxide.
   b) Melting of the crystals.
   c) Loss of water of crystallization.
   d) Formation of a new compound with air.

7. Which of the following represents a chemical change?
   a) Dissolving sugar in water.
   b) Melting of wax.
   c) Burning of a candle.
   d) Boiling of water.

8. In the reaction between sodium sulphate solution and barium chloride solution, the white precipitate formed is:
   a) Sodium chloride
   b) Barium sulphate
   c) Sodium barium sulphate
   d) Barium sodium chloride

9. Which criterion is NOT suitable for distinguishing between a true solution and a colloid?
   a) Tyndall effect
   b) Stability
   c) Filtration through standard filter paper
   d) Transparency

10. When an iron nail is dipped in copper sulphate solution, the colour change observed in the solution is from:
    a) Blue to colourless
    b) Blue to light green
    c) Green to blue
    d) Colourless to blue

Answer Key for MCQs:

1. c) Milk
2. c) Chalk particles settle down at the bottom.
3. b) Only iron filings are attracted.
4. c) A black homogeneous mass.
5. c) Evolution of a colourless and odourless gas.
6. c) Loss of water of crystallization.
7. c) Burning of a candle. (Involves combustion, forming new substances like CO₂ and water vapour)
8. b) Barium sulphate
9. c) Filtration through standard filter paper (Both generally pass through)
10. b) Blue to light green

Study these notes thoroughly, focusing on the differences between the types of mixtures and changes. Understanding the observations and the reasons behind them is key for your exams. Good luck!