IGCSE Chemistry 0620 Paper 6: Every Qualitative Analysis Test (And Why You Should Not Memorise Them)

Here is something most IGCSE Chemistry students find out far too late: you are given the qualitative analysis table in the exam.

Cambridge prints the "Notes for use in qualitative analysis" in the question paper itself, for both Paper 5 and Paper 6. Every precipitate colour, every excess behaviour, every gas test. It is right there, on the page, while you write.

Which means the hours you spent making flashcards for "iron(III) gives a red-brown precipitate" bought you almost nothing. The marks in qualitative analysis are not for knowing the table. They are for reading it under pressure without falling into the four traps built into it — and for writing observations in the exact language the mark scheme accepts.

This guide covers the whole table, the traps, and what to revise instead.

The tests you are given

Everything below is taken from the reference tables Cambridge prints in the 0620 paper. Read it to understand the structure — the patterns and the collisions — not to memorise it.

Cation tests: aqueous sodium hydroxide and aqueous ammonia

Eight cations are on the syllabus. Each is tested with two reagents, and it is the combination that identifies the ion — not either result alone.

Anion tests

Gas tests

Flame tests — only six

The four traps

The table is designed so that a careless reader gets the wrong answer. Every collision in it is deliberate. Here are the four that catch people.

Trap 1 — Zinc and aluminium are identical until you add excess

With sodium hydroxide, Zn²⁺ and Al³⁺ behave exactly the same: white precipitate, soluble in excess, colourless solution. There is no way to tell them apart from that test.

The separation is in ammonia:

• Zinc — white ppt that dissolves in excess ammonia
• Aluminium — white ppt that stays in excess ammonia

This is the most commonly examined discrimination in the whole topic. If a question gives you a white precipitate soluble in excess NaOH and then asks what to do next, the answer is always: repeat with aqueous ammonia and add it in excess.

Trap 2 — Three white precipitates, not one

With sodium hydroxide, a white precipitate could be calcium, zinc or aluminium. Students see "white" and commit to an answer. The excess behaviour splits them:

Trap 3 — Copper's dark blue is the giveaway, and it is easy to miss

Copper(II) gives a light blue precipitate with both reagents. With ammonia in excess, it dissolves to a dark blue solution — a distinctive, unmistakable result and a favourite of examiners. The mark is for noting that the solution went dark blue, not merely that the precipitate dissolved.

Trap 4 — Chromium(III) and iron(II) are both green

Both give green precipitates. Chromium(III) is soluble in excess NaOH; iron(II) is not. Iron(II) also has a tell of its own: it slowly turns brown near the surface on standing as it oxidises. If a question mentions a green precipitate that browned over time, that is iron(II), stated explicitly.

Writing observations that score

Qualitative analysis questions usually give you a table with an "observations" column. This is where technically-correct students lose marks, because they write conclusions where the paper asked for observations.

Three rules that follow from this:

1. Describe what a camera would record
   Colour, state, whether it dissolved, whether gas was produced. If you could not photograph it, it is a conclusion, not an observation.
2. Never leave a cell blank
   "No visible change" and "no precipitate" are valid observations and are frequently worth a mark. A blank cell is always zero.
3. Always state the excess behaviour
   "White precipitate" on its own is an incomplete observation for a test where excess matters. Add what happened when you kept adding.

What to revise instead of the table

Since the table is given, here is where your revision time should actually go:

1. The four collisions, as rules
   Zn vs Al, the three white precipitates, copper's dark blue, Cr vs Fe(II). Not as facts — as decision procedures. "White ppt soluble in excess → test with ammonia."
2. The acidification step
   Halides and sulfate both require acidifying with dilute nitric acid first. This is the single most-forgotten step in the topic, and it is worth a mark on its own. Nitric acid, specifically — not hydrochloric, which would introduce chloride ions and ruin a halide test.
3. Observation language
   Practise writing the six or seven standard observation phrases until they are automatic. This is a writing drill, not a chemistry one.
4. Planning a full identification
   Given an unknown salt, what order do you test in? Flame test first if available, then cation with NaOH, then ammonia if needed, then anions. Questions asking you to design the procedure are worth several marks.
5. Past paper questions, marked strictly
   Qualitative analysis appears in nearly every Paper 6. Work through five years of them and you will have seen every variation Cambridge uses.

Common questions

The one-line version

Stop memorising the table — you are given it. Learn the four collisions as decision rules, never forget to acidify, always state the excess behaviour, and write what a camera would see. That is the whole topic.

For the rest of the paper, see the complete Paper 6 guide, and the same treatment for Physics 0625. For the theory side, our most-tested 0620 topics and how to get an A* in Chemistry cover Papers 2 and 4.