Describe how to test for the purity of water using melting point and boiling point

Chemistry of the Environment – Water

💧 Water is the life‑sustaining liquid that covers 71 % of Earth’s surface. In the IGCSE syllabus, you’ll learn how to check if a water sample is pure or if it contains impurities that change its physical properties. The two most common tests are the melting point (freezing point) and the boiling point.

Why Melting and Boiling Points Matter

Think of pure water as a perfectly choreographed dance troupe. Every dancer (molecule) moves in sync, so the group starts to freeze at exactly \$0^\circ\text{C}\$ and boils at exactly \$100^\circ\text{C}\$ (at 1 atm). If the troupe is mixed with other dancers (impurities), the dance gets out of sync – the group will freeze later and boil earlier. This shift is what we measure to judge purity.

Step‑by‑Step: Melting Point Test

  1. Fill a clean, dry capillary tube with the water sample.

  2. Place the tube in a melting point apparatus (or a simple ice bath with a thermometer).

  3. Record the temperature at which the first ice crystals disappear – that’s the melting point.

  4. Compare with the standard value: \$0^\circ\text{C}\$. A deviation of more than ±0.5 °C suggests impurities.

Step‑by‑Step: Boiling Point Test

  1. Heat a small amount of the water in a clean beaker.

  2. Use a thermometer to monitor the temperature as the water starts to boil.

  3. Note the temperature at which a steady, vigorous boil occurs – that’s the boiling point.

  4. Compare with the standard value: \$100^\circ\text{C}\$ at 1 atm. A lower boiling point indicates the presence of a non‑volatile impurity (like salt). A higher boiling point suggests a volatile impurity (like alcohol).

Key Observations & What They Mean

PropertyPure WaterImpure Water
Melting Point\$0^\circ\text{C}\$> \$0^\circ\text{C}\$ (depressed)
Boiling Point\$100^\circ\text{C}\$< \$100^\circ\text{C}\$ (volatile impurity) or > \$100^\circ\text{C}\$ (non‑volatile impurity)

Exam Tips Box

🔍 Tip 1: Always state the standard values (\$0^\circ\text{C}\$ and \$100^\circ\text{C}\$) before giving your answer.

🔍 Tip 2: Show the direction of the shift (higher or lower) and explain what type of impurity it indicates.

🔍 Tip 3: Use the analogy of a dance troupe or a detective to make your explanation memorable.

Quick Practice Question

A water sample freezes at \$2^\circ\text{C}\$ and boils at \$98^\circ\text{C}\$. What can you infer about the impurities present?


(Answer: The sample likely contains a non‑volatile impurity that raises the freezing point and a volatile impurity that lowers the boiling point.)