Tests for Water — Chemical Tests | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 8% of your exam marks.

stable

Low

Stable8%

Ion tests, flame tests and gas tests all appear regularly; expect to recall observations.

Anhydrous copper(II) sulfate is a fine white powder
A drop of any sample suspected to contain water is added to the white powder
If water is present, the powder turns blue as it absorbs water and becomes hydrated:

CuSO₄(s) + 5 H₂O(l) → CuSO₄·5H₂O(s)

Pure water boils at exactly 100 °C at standard atmospheric pressure
Heat the sample gently in a boiling tube with a thermometer in the liquid; record the temperature when the liquid is steadily boiling
If the boiling point is 100 °C, the sample is pure water
A dissolved solute pushes the boiling temperature higher than 100 °C and pulls the freezing temperature below 0 °C, so any reading away from those textbook values is evidence of an impurity
(Anhydrous copper(II) sulfate is the chemical test for the presence of water; a boiling point of 100 °C tests for purity. Both tests are normally needed to confirm "pure water".)