Chemical Tests for Water

These tests show whether water is present in a sample. They do not prove that a liquid is pure water.

Key idea: anhydrous substances

Anhydrous means “without water”. Certain anhydrous chemicals change colour when they gain water, so they act like flags for water.

Test 1: Anhydrous copper(II) sulfate

• Appearance: white powder (anhydrous). It turns blue when water is present.
• Method (liquid): place a small amount of the white powder in a dish. Add a drop of the sample. A blue colour shows water is present.
• Method (gas): bubble the gas through copper(II) sulfate. A blue colour shows the gas contains water vapour.

Word idea: white to blue means water is present.

Chemical change (for interest):

[ \text{CuSO}_4(s) + 5\text{H}_2\text{O}(l) \rightarrow \text{CuSO}_4\cdot 5\text{H}_2\text{O}(s) ]

Test 2: Anhydrous cobalt(II) chloride (cobalt chloride paper)

• Appearance: blue when dry (anhydrous). Turns pink when it absorbs water.
• Method (liquid): place a drop of the sample on blue cobalt chloride paper. Pink colour shows water is present.
• Method (gas): hold the blue paper in the gas stream. Pink colour shows water vapour is present.

Chemical change (for interest):

[ \text{CoCl}_2(s) + 6\text{H}_2\text{O}(l) \rightarrow \text{CoCl}_2\cdot 6\text{H}_2\text{O}(s) ]

Presence vs purity

• Copper(II) sulfate and cobalt chloride show that water is present in a sample.
• To test if a liquid is pure water, check its fixed points: it boils at 100°C and freezes at 0°C at standard pressure.

Common mistakes

• Thinking the tests identify pure water. They only show water is present.
• Mixing up colours: copper(II) sulfate goes white → blue; cobalt chloride paper goes blue → pink.
• Using chemicals that are already wet. They must be anhydrous to work.