IGCSE Chemistry · Topic 10.1.2 & 10.1.3 · Core
Testing Water Purity
Pure water has fixed boiling & melting points. Impurities destroy that precision.
🏎️ The Precision Engine Analogy
Imagine pure water is a perfectly-tuned Formula 1 engine. It changes gear at exactly 0°C (freezing) and exactly 100°C (boiling). These aren't suggestions — they are the engine's precise specifications.
Now throw dirt into that engine (dissolved salts, minerals). The engine misfires. It struggles to freeze — needing colder temperatures. It struggles to boil — needing hotter temperatures. The specifications are ruined.
Pure Water Specifications
Boiling point100.0°C (exact)
Melting point0.0°C (exact)
Impure Water (Sea Water)
Boiling point102.5°C (raised)
Melting point−1.9°C (lowered)
⚠️ The Core Rule — Memorise This
🧊
Melting Point
Impurities → LOWER
Below 0°C = impure
♨️
Boiling Point
Impurities → RAISE
Above 100°C = impure
⚠️ Examiner's Trap — Don't Lose These Marks
✗"Distilled water has no impurities." — Too absolute. Say: "Distilled water contains fewer chemical impurities than tap water."
✗"Impure water boils above 100°C at a fixed point." — It boils over a range of temperatures, not at a fixed point.
✗"Impurities raise the melting point." — Impurities LOWER the melting point (depress it), they RAISE the boiling point.
IGCSE Chemistry 0620 · Topic 10.1.2 & 10.1.3 · Water Purity · Limitless Chemistry Professor
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