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.

Chemical test: anhydrous copper(II) sulfate

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)

The colour change from white to blue is the positive result for water
This test confirms water is present but does not say whether it is pure

Physical test: boiling point

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".)

Exam tip

Testing for the presence of water

What comes up: Describe the chemical test for water and state the positive result.

Write (two marks): (1) Add anhydrous copper(II) sulfate (white powder) to the sample. (2) The powder turns from white to blue.

Watch out: The colour-change mark depends on you naming anhydrous (or white) copper sulfate in the first step — if you skip naming the reagent, the observation mark is not awarded. An alternative accepted by the mark scheme is anhydrous cobalt chloride (blue) turning pink on contact with water. Both are acceptable answers, but do not mix them up (cobalt chloride starts blue, not white). This test confirms water is present; to confirm purity you must also measure the boiling point (100 °C) or freezing point (0 °C).