Tests for Ions and Water

Explained

Tests for ions and for water

Every test in this topic follows the same shape: add a named reagent, observe a named result. The marks come from naming both precisely, and from one procedural detail that is easy to skip, which is adding the correct acid first.

Positive ions, using sodium hydroxide solution

Add sodium hydroxide solution and look at the precipitate that forms.

• Copper(II) ions give a blue precipitate.
• Iron(II) ions give a green precipitate.
• Iron(III) ions give a brown precipitate.

Write the oxidation number. Iron(II) and iron(III) give different colours, so writing only "iron" does not identify anything.

Ammonium ions are different. Warm the solution gently with sodium hydroxide and test the gas given off with damp red litmus paper. Ammonia turns it blue. The warming is part of the method, not an optional extra, because the gas is not released otherwise.

Negative ions, and the acid that comes first

For sulfate ions, add dilute hydrochloric acid, then barium chloride solution. A white precipitate confirms sulfate.

For halide ions, add dilute nitric acid, then silver nitrate solution. Chloride gives a white precipitate, bromide cream and iodide yellow.

The acid is not there by accident. It removes carbonate ions, which would otherwise produce a white precipitate of their own and give a false positive. Note also that the two tests use different acids, and using the wrong one introduces the very ion you are testing for.

Carbonates

Add dilute acid to the solid. Carbonates fizz, releasing carbon dioxide, which turns limewater milky. Two observations, and both are usually needed: the effervescence and the limewater result.

Testing for water

Anhydrous copper(II) sulfate turns from white to blue in the presence of water. Blue cobalt chloride paper turns pink.

Both show that water is present, but neither shows that a liquid is pure water. For purity you need the physical test: pure water boils at exactly 100 degrees Celsius and freezes at exactly 0 degrees Celsius at normal pressure. A question asking you to show a sample is pure water needs both the chemical test and the boiling point.

Writing an answer that scores

Name the reagent in full, say what you do to it if anything, and give the exact observation including the colour. "Add sodium hydroxide solution, a blue precipitate forms, so copper(II) ions are present" contains all three parts and is hard to mark down.