Overview
This lecture covers the key theory and important questions from Chapter 1: Chemical Reactions and Equations for Class 10, including concepts, definitions, formulas, types of reactions, and methods for balancing equations.
Types of Changes
- A change occurs when the final state of a substance is different from the initial state.
- Physical change involves changes in physical properties (e.g., melting ice).
- Chemical change involves changes in chemical properties and composition (e.g., burning paper).
Identifying Physical vs Chemical Changes
- Melting ice: physical change.
- Burning candle: chemical (combustion) and physical (wax melting).
- Rusting of iron: chemical change.
- Tearing paper: physical change.
- Cooking food, curd from milk: chemical changes.
Chemical Reactions & Equations
- A chemical reaction is a process in which substances transform into new substances.
- A chemical equation represents a reaction using symbols and formulas.
- Physical states are noted as (s) solid, (l) liquid, (g) gas, (aq) aqueous.
Characteristics of Chemical Reactions
- Indications include change in colour, temperature, evolution of gas, formation of precipitate, and change in state.
- Exothermic reactions release heat; endothermic reactions absorb heat.
Balancing Chemical Equations
- The number of atoms of each element must be equal on both sides.
- Law of Conservation of Mass: mass is neither created nor destroyed.
- Methods include making tables and adjusting coefficients.
Types of Chemical Reactions
- Combination: two or more reactants form one product (e.g., Mg + O₂ → MgO).
- Decomposition: one reactant breaks into two or more products (thermal, electrolytic, photolytic).
- Displacement: a more reactive element replaces a less reactive one in a compound.
- Double displacement: compounds exchange ions, often forming a precipitate.
Important Activities & Examples
- Magnesium burns with a white flame to form MgO.
- Calcium oxide reacted with water produces slaked lime and heat.
- Lime water turns milky in presence of CO₂ (lime water test).
- Lead nitrate heated forms lead oxide (yellow), NO₂ (brown), and O₂.
- Photolytic decomposition: AgCl turns grey in sunlight due to decomposition.
Reactivity Series & Displacement
- Higher metals in the reactivity series displace lower metals in compounds.
- Example trick: "Katrina asked for a car, Alto Zen Ferrari..." to remember the series order.
Precipitate & Catalyst
- Precipitate: insoluble solid formed in a reaction.
- Catalyst: substance that speeds up a reaction without being consumed.
Oxidation and Reduction
- Oxidation: addition of oxygen or removal of hydrogen.
- Reduction: addition of hydrogen or removal of oxygen.
- Redox reaction: both oxidation and reduction occur simultaneously.
- Oxidizing agent: causes oxidation (itself reduced); Reducing agent: causes reduction (itself oxidized).
Corrosion & Rancidity
- Corrosion: degradation of metals due to air/moisture (e.g., rusting of iron).
- Rancidity: spoilage of oily/fatty foods due to oxidation, causing bad smell/taste.
- Prevent corrosion by painting/oiling; prevent rancidity by refrigeration, nitrogen packing, or antioxidants.
Key Terms & Definitions
- Chemical Reaction — Transformation of substances forming new products.
- Chemical Equation — Symbolic representation of a chemical reaction.
- Precipitate — Insoluble solid formed in a reaction.
- Catalyst — Substance increasing reaction rate without being consumed.
- Oxidation — Addition of oxygen/removal of hydrogen.
- Reduction — Addition of hydrogen/removal of oxygen.
- Redox Reaction — Simultaneous oxidation and reduction.
- Corrosion — Destructive process where metals react with environment.
- Rancidity — Spoilage of oils/fats due to oxidation.
Action Items / Next Steps
- Practice balancing chemical equations.
- Memorize the reactivity series using the provided trick.
- Review key reaction examples and their color changes.
- Complete the homework questions and revise important activities from the chapter.