2hChemical tests

Five gas tests, and each needs both the reagent and the observation. Naming the test without the result answers half the question.

The splint tests

Hydrogen needs a lighted splint; oxygen needs a glowing one. Swapping them is the single most common error in this sub-topic.

The logic helps: hydrogen burns, so it needs a flame to ignite it. Oxygen supports burning, so it revives a splint that is barely alight.

The litmus tests

Both need the paper damp. A dry gas cannot affect dry litmus — the gas has to dissolve in the moisture to act as an acid or alkali.

Ammonia turns red litmus blue, because it is the only common alkaline gas.

Chlorine does two things in sequence: it dissolves to form an acidic solution, turning blue litmus red, and then bleaches the colour away entirely. Describing only the bleaching misses the first half.

Note which colour paper each test starts with. Ammonia uses red paper, chlorine uses blue.

Limewater

Carbon dioxide reacts with the calcium hydroxide in limewater to form calcium carbonate, which is insoluble and appears as a white suspension.

That is what the milkiness is. Naming the insoluble product turns an observation into an explanation.

Flame tests

Lithium is red and calcium is orange-red. If a question offers both, the shade is the whole answer.

Cleaning the wire

1. Dip a nichrome or platinum wire in concentrated hydrochloric acid.
2. Heat it in the flame.
3. Repeat until the wire produces no colour.

That last step is the one candidates skip, and it is usually worth a mark.

It matters because sodium contamination is everywhere and its yellow is intense enough to mask other colours. A yellow flame is the result most likely to be a false positive — if you see one, check the wire was clean before concluding sodium is present.

Use the edge of a blue Bunsen flame. A yellow flame would hide the result.

Testing for the ammonium ion

1. Add sodium hydroxide solution.
2. Warm the mixture.
3. Test any gas with damp red litmus — it turns blue.

The warming is essential. Without it little ammonia is released and the test can read as negative when the ion is present.

Ammonium is also the only ion here that produces a gas rather than a precipitate, which by itself distinguishes it.

Metal hydroxide precipitates

Adding sodium hydroxide to a solution of a metal ion forms an insoluble hydroxide whose colour identifies the metal:

Iron(II) is green, iron(III) is brown. Reversing them is a frequent and costly error — and it is the same element in both cases, distinguished only by the charge on the ion.

Combining tests

When a question gives you two results, both must fit. A compound giving a blue-green flame and a blue precipitate contains copper(II) — two independent tests agreeing makes the identification secure.

Halide ions

1. Add dilute nitric acid.
2. Then add silver nitrate solution.

The colours deepen going down Group 7, and white, cream and yellow are close enough that comparing against a known sample is sensible when you are unsure.

Sulfate ions

1. Add dilute hydrochloric acid.
2. Then add barium chloride solution.
3. A white precipitate of barium sulfate confirms it.

Why acidify first

This is the step students skip and examiners test.

Carbonate ions also give a white precipitate with silver nitrate and with barium chloride. Adding acid first destroys any carbonate present, so a white precipitate afterwards can only mean chloride or sulfate.

Without that step the result is ambiguous, and an ambiguous test is not a test. A question asking why the acid is added wants the interfering ion named.

Carbonate ions

1. Add dilute acid to the solid.
2. Effervescence is seen.
3. Bubble the gas through limewater — it turns milky.

Fizzing alone is not proof. Many things fizz; only carbon dioxide turns limewater milky.

Testing for water

Anhydrous copper(II) sulfate turns from white to blue when water is added.

This shows water is present. It does not show the sample is pure — salt solution would give exactly the same result.

Testing for pure water

Purity needs a physical test:

Pure water boils at exactly 100 °C at normal atmospheric pressure, and melts at exactly 0 °C.

An impure sample boils over a range, and at a higher temperature.

The word exactly carries the answer. "It boils at about 100 °C" demonstrates nothing, because an impure sample does too.

Presence and purity are separate questions needing separate tests — a chemical test can answer the first, only a physical measurement answers the second.