IGCSE Chemistry Revision Notes (Grade 8) – Paper 2
Q1 – Identifying Metals and Gases
Key Concept:
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Metals react with acids → salt + hydrogen gas.
Carbonates decompose on heating → oxide + carbon dioxide.
Gases tested by simple lab tests (CO₂ → limewater cloudy, H₂ → squeaky
pop).
Examples:
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Magnesium + hydrochloric acid → magnesium chloride + hydrogen.
Calcium carbonate heated → calcium oxide + carbon dioxide.
Hydrogen gas tested with a lit splint → squeaky pop sound.
Diagram Idea:
Test tube with acid + magnesium ribbon, bubbles of hydrogen tested with burning
splint.
Q2 – pH Values and Acid Strength
Key Concept:
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pH decreases as acid concentration increases.
Strong acids fully ionise, weak acids partially ionise.
Sulfuric acid gives 2 H⁺ ions per molecule → lower pH than HCl of same
concentration.
Examples:
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0.1 M HCl has pH ≈ 1, while 0.1 M H₂SO₄ has pH < 1.
Vinegar (weak acid) has higher pH than hydrochloric acid (strong acid) of
same concentration.
Lemon juice (weak acid) vs battery acid (strong acid).
Diagram Idea:
Graph: x-axis = acid concentration, y-axis = pH, showing pH falls as concentration
rises.
Q3 – Thermal Decomposition of Zinc Carbonate
Key Concept:
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Metal carbonates → metal oxide + CO₂ on heating.
Zinc carbonate → zinc oxide (yellow when hot, white when cold) + CO₂.
Chemical change = new substance, Physical change = reversible colour
change.
Examples:
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Copper carbonate → copper oxide (black) + CO₂.
Calcium carbonate → calcium oxide (white) + CO₂.
Magnesium carbonate → magnesium oxide + CO₂.
Diagram Idea:
Flowchart: ZnCO₃ → ZnO + CO₂, with limewater test showing cloudiness.
Q4 – Exothermic vs Endothermic
Key Concept:
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Exothermic: release heat, products lower in energy than reactants.
Endothermic: absorb heat, products higher in energy.
Examples:
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Combustion of fuels (exothermic).
Neutralisation reactions (exothermic).
Photosynthesis (endothermic, requires sunlight).
Thermal decomposition of calcium carbonate (endothermic).
Diagram Idea:
Energy profile:
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Exothermic curve slopes down.
Endothermic curve slopes up.
Q5 – Haber Process (Ammonia Manufacture)
Key Concept:
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Reaction: N₂ + 3H₂ ⇌ 2NH₃ (exothermic).
High pressure → shifts equilibrium to ammonia side.
Lower temperature favours yield but slows rate, so compromise (450 °C).
Iron catalyst speeds up reaction.
Examples:
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Contact Process (SO₂ → SO₃) also uses compromise temperature.
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Carbon monoxide and hydrogen (methanol production) uses similar
equilibrium principles.
Fertiliser plants use high pressure reactors.
Diagram Idea:
Diagram of reaction with arrows: ↑ pressure → ↑ yield, ↓ temp → ↑ yield but slow,
catalyst → faster.
Q6 – Reactions of Acids
Key Concept:
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Acid + metal → salt + hydrogen.
Acid + base/alkali → salt + water.
Acid + carbonate → salt + water + CO₂.
Examples:
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HCl + sodium hydroxide → sodium chloride + water.
H₂SO₄ + magnesium → magnesium sulfate + hydrogen.
HNO₃ + calcium carbonate → calcium nitrate + water + CO₂.
Diagram Idea:
Table: Reaction type vs products (e.g., “acid + metal → salt + H₂”).
Q7 – Making Carbonyl Chloride (COCl₂)
Key Concept:
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CO made by incomplete combustion of methane, or methane + steam.
High temperature → faster reaction (activation energy).
High pressure → costly and dangerous.
Examples:
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Incomplete combustion of wood → carbon monoxide.
Steam reforming of methane → CO + H₂ (used in hydrogen production).
Industrial processes often use catalysts and high temperatures.
Diagram Idea:
Equation: CH₄ + H₂O ⇌ CO + 3H₂ (with arrows showing high T, equilibrium).
Q8 – Photochromic Glass
Key Concept:
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Contains silver chloride and copper chloride.
In sunlight (UV): AgCl decomposes → silver (black), glass darkens.
In low light: reaction reverses, glass clears.
Example of a photochemical reversible reaction.
Examples:
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Silver salts in photography darken when exposed to light.
Plants need light for photosynthesis (photochemical reaction).
Glow-in-the-dark paints work with light-activated reactions.
Diagram Idea:
Picture of sunglasses:
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Bright sunlight → lenses dark.
Indoors → lenses clear.