Chemical Tests for Water

Introduction

Many colourless liquids look just like water: ethanol, some salt solutions and various oils. How can a chemist tell whether a liquid contains water without tasting it (which is never safe in a laboratory)? Two simple chemical tests do the job, and both rely on the same idea: certain salts change colour when they gain water. These tests are used in schools, and the same chemistry is used in moisture indicators found in packaging and laboratory equipment.

Core explanation

Anhydrous copper(II) sulfate test. Anhydrous copper(II) sulfate, CuSO₄, is a white powder. When water is added, it forms hydrated copper(II) sulfate, which is blue :

CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s) white → blue

The five water molecules become part of the crystal as water of crystallisation . A drop of a liquid containing water turns the white powder blue. A liquid with no water, such as pure hexane, leaves it white.

Cobalt chloride paper test. Filter paper soaked in cobalt(II) chloride solution and then dried is blue when dry. When it touches water, it turns pink , because the anhydrous blue form becomes the hydrated pink form. The paper must be dried (usually kept in a sealed container with a drying agent) before use, or it will already be pink from moisture in the air.

Both reactions are reversible. Heating hydrated copper(II) sulfate drives off the water and turns it back to white; heating pink cobalt chloride paper turns it blue again. The forward reaction (adding water) is exothermic — you can feel the powder warm up. The reverse reaction (heating to remove water) is endothermic.

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

What the tests show — and what they do not. Both tests show that water is present . They do not show that a liquid is pure water. Seawater, lemonade and a salt solution all contain water, so all turn the tests positive. To check purity, you need a physical test such as measuring the boiling point (pure water boils at 100 °C at standard pressure) or melting point (0 °C).

Safety at a conceptual level. Copper(II) sulfate is harmful if swallowed and irritates the eyes; cobalt chloride is toxic and a suspected carcinogen, so the paper is handled with tweezers or gloves and hands are washed afterwards. Eye protection is worn whenever solids are heated.

Test Colour without water Colour with water --- --- --- Anhydrous copper(II) sulfate white blue Cobalt chloride paper blue pink

Step-by-step reasoning

To decide what a result tells you:

1. Add the unknown liquid to the test material. 2. If there is no colour change, the liquid contains no water. 3. If the colour changes, water is present. 4. To find out whether the liquid is pure water, carry out a separate physical test, such as checking its boiling point.

Visual explanation

Picture a watch glass holding a small heap of white powder. A single drop of liquid lands on it and a bright blue patch spreads outwards from the drop. Beside it, a strip of blue paper touched by a wet glass rod turns pink where it was touched, leaving a blue border.

Real-world analogy

The cobalt chloride test works like a mood ring that responds to moisture instead of temperature. Its colour tells you whether conditions are wet or dry, and it changes back when the conditions change.

Real-world example

Small sachets of silica gel packed with electronics and shoes sometimes contain coloured indicator beads. Older versions used cobalt chloride, which is blue when dry and pink when saturated; because of cobalt's toxicity, modern ones often use a safer orange-to-green dye. The colour tells you when the drying agent needs replacing.

Why?

Why is the forward reaction exothermic? When water molecules bond into the copper(II) sulfate crystal, attractions form between the water molecules and the copper ions, and forming bonds releases energy. Removing the water requires the same energy to be supplied, so heating is needed.

Common misconception

"If anhydrous copper(II) sulfate turns blue, the liquid is pure water." The test only shows that water is present. Any aqueous solution would give the same result; purity must be checked by boiling or melting point.

Worked example

Question: A student adds drops of three liquids — ethanol (pure), seawater and distilled water — to separate samples of anhydrous copper(II) sulfate. Predict each result.

Reasoning: Pure ethanol contains no water, so no change. Seawater and distilled water both contain water, so both turn the powder blue.

Answer: Ethanol — stays white; seawater — blue; distilled water — blue. Only a boiling-point test could distinguish seawater from distilled water.

Quick check

1. What colour change is seen when dry cobalt chloride paper is touched with water? Answer: Blue to pink.

Exam focus

Learn both colour changes in the correct direction: copper(II) sulfate goes white to blue; cobalt chloride goes blue to pink. A common exam question asks how to show a liquid is pure water — the answer is a boiling point of exactly 100 °C (or melting point of 0 °C), not the chemical test.

Advanced insight

The colour of copper(II) and cobalt(II) compounds depends on the groups arranged around the metal ion. In hydrated copper(II) sulfate, water molecules surround each copper ion and change how it absorbs visible light, producing the blue colour. In anhydrous copper(II) sulfate, the copper ions are surrounded by sulfate oxygen atoms instead, and the solid absorbs little visible light, so it appears almost white.

Summary

Water is detected chemically with anhydrous copper(II) sulfate, which turns from white to blue, or dry cobalt chloride paper, which turns from blue to pink. Both changes happen because the salts gain water of crystallisation, and both are reversible by heating. The tests prove water is present but not that a liquid is pure; purity is shown by a boiling point of 100 °C or a melting point of 0 °C.

Practice questions

1. Write a word equation for the reaction of anhydrous copper(II) sulfate with water. Answer: Anhydrous copper(II) sulfate + water → hydrated copper(II) sulfate. 2. Why must cobalt chloride paper be kept dry before use? Answer: Moisture in the air would turn it pink in advance, so it could no longer show a colour change with the test liquid. 3. How could you turn blue hydrated copper(II) sulfate back into the white anhydrous form? Answer: Heat it to drive off the water of crystallisation. 4. A liquid turns anhydrous copper(II) sulfate blue and boils at 102 °C. What can you conclude? Answer: The liquid contains water but is not pure water; it contains dissolved substances that raise the boiling point.