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General Science: Chemical Reactions and Equations
📅 Topic: General Science (Chemistry)
Welcome back. You are tackling the absolute foundation of Chemistry today. In Civil Services and competitive exams, questions from this chapter are rarely about balancing complex equations. Instead, they focus on observations (color changes, gas evolution) and everyday applications (corrosion, digestion).
1. How to Spot a Chemical Reaction
A chemical reaction has occurred if you observe any of these four changes:
- Change in State: (e.g., Hydrogen gas + Oxygen gas → Liquid Water).
- Change in Colour: (e.g., Iron nail turning brown in copper sulphate).
- Evolution of Gas: (e.g., Zinc + Acid → Hydrogen bubbles).
- Change in Temperature: (e.g., Quick lime + Water → Heat).
Activity: Burning Magnesium ribbon.
Observation: It burns with a dazzling white flame and forms a white powder (Magnesium Oxide).
Exam Trap: Why clean the ribbon first? To remove the layer of Magnesium Oxide that forms naturally, so it can burn properly.
2. Balancing Equations (The Logic)
Why balance? To satisfy the Law of Conservation of Mass. Mass cannot be created or destroyed; therefore, the number of atoms on the Left Hand Side (Reactants) must equal the Right Hand Side (Products).
- Skeletal Equation: Mg + O2 → MgO (Unbalanced).
- Balanced Equation: 2Mg + O2 → 2MgO.
3. Types of Chemical Reactions (The Core Topic)
This is where 80% of the questions come from.
A. Combination Reaction (Marriage)
Two reactants combine to form a single product (A + B → AB).
- Key Example (Whitewashing): Quick Lime (CaO) + Water → Slaked Lime (Ca(OH)2) + Heat.
- Real World: This Slaked Lime is applied to walls. It reacts with CO2 in the air to form Calcium Carbonate (CaCO3), giving a shiny finish after 2-3 days.
Exothermic Reactions: Reactions that release heat.
Examples: Burning natural gas, Respiration (Glucose breaks down to release energy), Decomposition of vegetable matter into compost.
B. Decomposition Reaction (Divorce)
A single reactant breaks down into simpler products. This requires energy (Heat, Light, or Electricity). These are usually Endothermic.
- Thermal Decomposition (Heat):
- Ferrous Sulphate (FeSO4): Green crystals heat up → turn White (lose water) → then turn Brown (Ferric Oxide). Observation: Smell of burning sulphur.
- Lead Nitrate: Heating colorless crystals → Brown fumes of Nitrogen Dioxide (NO2). (Memorize this color).
- Electrolytic Decomposition (Electricity):
- Electrolysis of Water: H2O → H2 + O2.
- Mentor's Touch: The volume of Hydrogen collected is double the volume of Oxygen.
- Photolytic Decomposition (Light):
- Silver Chloride (AgCl): White powder turns Grey in sunlight.
- Application: Used in Black and White Photography.
C. Displacement Reaction (The Bully)
A more reactive element kicks out a less reactive element.
The Classic Experiment: Iron nail dipped in Copper Sulphate (CuSO4) solution.
1. The Blue colour of the solution fades (turns light green).
2. The Iron nail becomes brownish (coated with Copper).
Equation: Fe + CuSO4 (Blue) → FeSO4 (Green) + Cu.
D. Double Displacement Reaction
Exchange of partners (ions) between reactants.
Precipitation Reaction: Any reaction that produces an insoluble solid.
Example: Sodium Sulphate + Barium Chloride → Barium Sulphate (White Precipitate) + Sodium Chloride.
4. Oxidation and Reduction (Redox)
- Oxidation: Gaining Oxygen OR Losing Hydrogen.
- Reduction: Losing Oxygen OR Gaining Hydrogen.
- Redox: When both happen simultaneously.
Example: CuO + H2 → Cu + H2O
Copper Oxide loses Oxygen → Reduced.
Hydrogen gains Oxygen → Oxidized.
5. Everyday Chemistry (Application)
A. Corrosion
When metal is attacked by moisture/acid.
- Rusting of Iron: Reddish-brown coating.
- Silver: Turns Black (Silver Sulphide).
- Copper: Turns Green (Copper Carbonate).
B. Rancidity
The oxidation of fats and oils in food, causing bad smell and taste.
- Prevention: Adding Anti-oxidants, air-tight containers.
- Chips Packets: Manufacturers flush bags with Nitrogen gas to prevent oxidation.
6. Mentor’s Final Drill (Exam-Ready Questions)
A: To prevent oxidation (rancidity) of the oil in the chips.
Q: What are the brown fumes evolved when heating Lead Nitrate?
A: Nitrogen Dioxide (NO2).
Q: Is Respiration exothermic or endothermic?
A: Exothermic (it releases energy).
Q: In the electrolysis of water, which gas is collected at the cathode in double the amount?
A: Hydrogen.
Q: What is the chemical formula of Marble and Chalk?
A: Calcium Carbonate (CaCO3).
Action Plan: Pay close attention to the colors mentioned (Blue CuSO4, Green FeSO4, Brown fumes). In objective exams, these visual cues are often the basis of the question. Keep moving forward!