This lesson is all about how substances transform. It covers the difference between physical and chemical changes, the main types of chemical reactions, the reactivity of metals, and the properties of some important gases.

1. Short Notes: Core Concepts

A. Physical vs. Chemical Changes

• Physical Change: A change in the form or state of a substance, but no new substance is formed. It's usually reversible.

  • Examples: Melting ice (solid to liquid), boiling water (liquid to gas), crushing a stone.

• Chemical Change (Reaction): A process where the original substances (reactants) are transformed into new substances (products) with different properties. It's usually not easily reversible.

  • Signs of a chemical change: Gas bubbles, heat/light produced, colour change, formation of a solid (precipitate).

  • Examples: Burning wood, rusting iron, reacting an acid with a metal.

B. Types of Chemical Reactions

1. Chemical Combination: Two or more simple substances combine to form a more complex one.

   • General Form: A + B → AB

   • Example: C + O₂ → CO₂ (Carbon burns in oxygen to form carbon dioxide)

2. Chemical Decomposition: A complex substance breaks down into two or more simpler substances, often with heat.

   • General Form: AB → A + B

   • Example: CaCO₃ → CaO + CO₂ (Heating limestone)

3. Single Displacement: One element replaces another element in a compound. This is determined by reactivity.

   • General Form: A + BC → AC + B

   • Example: Zn + CuSO₄ → ZnSO₄ + Cu (Zinc displaces copper)

4. Double Displacement: The positive and negative ions of two ionic compounds switch places.

   • General Form: AB + CD → AD + CB

   • Example: NaCl + AgNO₃ → NaNO₃ + AgCl (A white precipitate of AgCl is formed)

C. Activity Series of Metals

• A list of metals arranged in order of their decreasing reactivity.

• Key Idea: A more reactive metal (higher on the list) can displace a less reactive metal (lower on the list) from its salt solution.

• Uses:

  • Predicting displacement reactions.

  • Understanding metal extraction (very reactive metals like Na need electrolysis; less reactive ones like Fe can be reduced with carbon; unreactive ones like Au are found native).

D. Extraction of Iron

• Furnace: Blast Furnace.

• Main Ore: Haematite (Fe₂O₃).

• Raw Materials: Iron ore, Coke (Carbon), and Limestone (CaCO₃).

• Process: Hot air is blasted in. Coke burns to produce heat and Carbon Monoxide (CO). The CO acts as the reducing agent, removing oxygen from the iron ore to produce molten iron. Fe₂O₃ + 3CO → 2Fe + 3CO₂. Limestone removes impurities like sand (SiO₂) to form slag.

E. Important Gases

2. Tips & Tricks for the Exam

• Identify the Reaction Type: Look at the reactants and products. Are they combining, breaking apart, or swapping partners? This tells you the reaction type.

• Balancing Equations: Make sure the number of atoms of each element is the same on both sides of the arrow. Start with the most complex molecule.

• Activity Series Logic: Memorize the relative positions of key metals (e.g., Mg > Zn > Fe > Cu). The one higher up is "stronger" and can "kick out" the one below it.

• Gas Tests are Essential: The tests for H₂, O₂, and CO₂ are classic exam questions. Memorize them perfectly.

3. Important Points & Common Exam Questions

• Classify a Reaction: "The reaction 2H₂O₂ → 2H₂O + O₂ is an example of which type of reaction?"

  • Answer: Decomposition (one substance breaks down into two).

• Predict a Displacement Reaction: "A piece of iron (Fe) is placed in a solution of copper sulphate (CuSO₄). State what would be observed and give a reason for your answer based on the activity series."

  • Answer: A brown coating of copper will form on the iron, and the blue solution will fade. This is because iron is more reactive than copper and displaces it from the solution.

• Iron Extraction: "What is the reducing agent used in the blast furnace for the extraction of iron?"

  • Answer: Carbon Monoxide (CO).

• Identify a Gas: "A gas is produced in a reaction which relights a glowing splint. Identify the gas."

  • Answer: Oxygen.