Chemical Tests for Ions and Gases

A lighted splint that pops, a glowing splint that relights, a flame that turns lilac -- these are not party tricks, they are precise, examinable evidence for exactly which substance you are looking at. This guide walks through every standard chemical test at IGCSE level: flame tests, the sodium hydroxide tests for cations, the silver nitrate and barium chloride tests for anions, the five gas tests, and the test for water, finishing with how to combine several results to identify a completely unknown compound.

Why Do We Need Chemical Tests?

Every test in this chapter follows the same basic idea: add a reagent (or apply a flame, a glowing splint, or a piece of damp indicator paper) to an unknown sample, and look for a positive result -- a colour, a precipitate, or a change that only happens if a particular ion or gas is present. A positive result must be specific: if several different ions could cause the same colour change, that test alone cannot identify the substance, and you may need to combine more than one test, exactly as in the worked example at the end of this guide.

Flame Tests: Five Cations, Five Colours

A flame test shows the presence of certain metal ions by the characteristic colour they give a Bunsen flame. A clean platinum or nichrome wire is dipped first into concentrated hydrochloric acid, then into the solid or solution being tested, and held in a roaring (non-luminous) flame.

One catch worth knowing: lithium is not the only ion that gives a red flame -- strontium gives a very similar red, so the only reliable way to be sure which one you have is to compare the colour side by side with a known compound.

Testing Cations with Sodium Hydroxide

Most metal hydroxides are insoluble, so adding sodium hydroxide solution to a solution containing a metal ion produces a precipitate of that metal's hydroxide -- and the precipitate's colour identifies the ion. Copper(II) ions give a blue precipitate, Cu²⁺(aq) + 2OH^-(aq) → Cu(OH)₂(s). Iron(II) ions give a green precipitate that slowly darkens to orange-brown on standing in air, as it oxidises to iron(III) hydroxide. Iron(III) ions give an orange-brown precipitate directly, Fe³⁺(aq) + 3OH^-(aq) → Fe(OH)₃(s). Ammonium ions behave differently -- there is no precipitate at all; instead, sodium hydroxide reacts with the ammonium salt to release ammonia gas, NH₄⁺(aq) + OH^-(aq) → NH₃(g) + H₂O(l), which turns damp red litmus paper blue.

Testing Anions: Halides, Sulfates and Carbonates

Negative ions have their own standard tests, and getting the order of reagents right matters -- the wrong acid can add the very ion you're testing for.

Two golden rules protect against false positives. For sulfate, always acidify with hydrochloric or nitric acid -- never with sulfuric acid, since it already contains the sulfate ion you're testing for. For halides, always acidify with dilute nitric acid -- never with hydrochloric, hydrobromic or hydroiodic acid, since each would add a halide ion of its own.

The Five Gas Tests

Five gases have simple, specific tests you need to know by heart.

A couple of precise wordings are worth locking in: examiners specifically want "squeaky pop" for hydrogen, not "bang" or "explosion", and "relights a glowing splint" for oxygen -- mixing up a lighted splint with a glowing one is one of the most common slip-ups in this whole topic. For chlorine, if blue litmus is used, it briefly turns red first (chlorine dissolving to form an acidic solution) before bleaching white.

Testing for Water

Water turns white anhydrous copper(II) sulfate blue, as it re-forms the hydrated crystal: CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s). This test only proves that water is present, though -- it says nothing about purity. To check whether a sample of water is pure, measure its melting or boiling point instead: pure water freezes at exactly 0°C and boils at exactly 100°C; an impure sample will typically freeze lower and boil higher than these exact values.

Putting It Together: Identifying an Unknown

Exam questions often chain several clues together. For example: A is an orange solid that dissolves to give an orange solution. Adding sodium hydroxide gives an orange-brown precipitate, B; adding dilute nitric acid then silver nitrate gives a white precipitate, C. Working through each clue in turn -- the orange-brown NaOH precipitate identifies Fe³⁺, so B is iron(III) hydroxide; the white silver nitrate precipitate identifies Cl^-, so C is silver chloride -- combining the two ions identifies A as iron(III) chloride, FeCl₃. This step-by-step approach, working out the cation and the anion separately before combining them, is the method behind every "identify the unknown" question at this level.

Key Takeaways

• Flame test colours: Li⁺ red, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red, Cu²⁺ blue-green.

• Sodium hydroxide precipitates: Cu²⁺ blue, Fe²⁺ green (darkens to orange-brown), Fe³⁺ orange-brown; NH₄⁺ releases ammonia gas instead.

• Silver nitrate (after nitric acid) gives white/cream/yellow precipitates for Cl^-/Br^-/I^-; barium chloride (after HCl or HNO₃) gives a white precipitate for SO₄^2-.

• Gas tests: H₂ squeaky pop, O₂ relights a glowing splint, CO₂ turns limewater milky, NH₃ turns damp litmus blue, Cl₂ bleaches damp litmus white.

• Anhydrous copper(II) sulfate turning blue shows water is present, but only melting/boiling point confirms it is pure.

Want more practice? Download the Science A Plus Edu Chemical Tests Revision Notebook, with 25 pages of notes, diagrams and revision activities, and read our companion guide to Acids, Bases and Salt Preparations, since many of the compounds you identify here are exactly the salts you learn to prepare there.