S4 Chemistry | Ion Tests (Flame Tests and Precipitation) Prep Guide (for tutors) | A four-step route for an unknown salt in 60 minutes

2026-11-06

Tutor lesson-prep guides · Secondary Chemistry

Students memorise that copper gives a blue precipitate, then meet “white precipitate soluble in excess sodium hydroxide” and cannot separate zinc from aluminium, because they memorised colours instead of a route. Identification questions test a decision path: colour first, then excess, then gas.

Objectives — Students can: (1) name four flame colours (sodium golden yellow, potassium lilac, calcium brick red, copper blue-green); (2) use sodium hydroxide and then excess sodium hydroxide to separate six hydroxide precipitates, especially the two soluble whites (zinc, aluminium) from the insoluble white (calcium); (3) write a four-step route for an unknown salt and describe the carbonate and ammonium gas tests correctly.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Ionic equations	What precipitate does copper(II) give with hydroxide ions?	Blue copper(II) hydroxide
Solubility	Which hydroxides are insoluble in water?	Most heavy metal hydroxides
Gas tests	How is carbon dioxide tested?	Bubble through limewater; it turns milky

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Entry: two test tubes	Show one blue and one white precipitate and ask which ions could be present	Name at least two possibilities
5-17 min	Four flame colours	Demonstrate four flame rods (sodium golden yellow, potassium lilac, calcium brick red, copper blue-green) and clean the wire in concentrated acid	Name the four colours and order them
17-32 min	Six precipitates and the excess test	Add sodium hydroxide one by one: copper blue, iron(II) green, iron(III) reddish-brown, zinc, aluminium and calcium white; then add excess, zinc and aluminium dissolve, calcium does not	Fill a six-row table and ring the two that dissolve
32-44 min	Carbonate and ammonium	Model acid giving carbon dioxide (limewater turns milky) and alkali with warming giving ammonia (damp red litmus turns blue)	Write both observation sentences
44-55 min	Four-step route for an unknown salt	Model elimination: colour, precipitate, excess, gas	Write the route for one unknown salt independently
55-60 min	Exit check	Three questions collected and marked at once	Complete independently

03 Teaching script (read it out)

Scene 1: Four flame colours belong to four rods, not four words

Sodium is golden yellow, potassium lilac (seen through cobalt glass, otherwise sodium yellow swamps it), calcium brick red, copper blue-green.
Give two operational reasons while teaching: the platinum or nichrome wire must be cleaned in concentrated acid and heated to no colour between samples, otherwise the previous sample contaminates the next; and potassium lilac is easily masked by a trace of sodium, so cobalt glass filters the yellow.
The common slips are saying “purple” for potassium and “green” for copper, each missing a word. Insist on the full colour name.

Get the student to: write the four colours in two columns, metal and colour, and annotate the potassium row with “needs cobalt glass”.

Scene 2: Learn the precipitate table in two layers: colour, then excess

With sodium hydroxide: copper(II) blue, iron(II) green, iron(III) reddish-brown, and those three are settled by colour alone. The three whites (zinc, aluminium, calcium) are the hard part.
Layer two: add excess sodium hydroxide. The white precipitates of zinc and aluminium dissolve as soluble complex ions, while calcium hydroxide does not. So white plus soluble gives two candidates, and white plus insoluble means calcium.
To separate zinc from aluminium you need a further test; at S4 it is enough to say honestly that both fit the observation so far.

Get the student to explain: why “white precipitate, so calcium” is wrong – two other whites dissolve in excess alkali.

Scene 3: An unknown salt is eliminated step by step, not guessed

Four steps: read the colour first (blue points to copper and rules out the rest); add sodium hydroxide and read the precipitate colour; add excess and see whether it dissolves; add acid for a gas and warm with alkali for ammonia.
Each step needs an observation and a conclusion, for example “excess sodium hydroxide dissolved the white precipitate, so zinc or aluminium may be present”.
Gas tests must reach the observation sentence: carbon dioxide turns limewater milky, ammonia turns damp red litmus blue. “A gas was produced” earns nothing.

Get the student to: take an unknown salt containing two ions and write the four steps with the observation and conclusion for each.

04 Question types and worked steps

Type	Example	Steps	Answer
Flame test	A salt gives a brick red flame. Which metal ion is present?	Match the four-colour table, brick red is not plain red	Calcium
Precipitate colour	Sodium hydroxide gives a reddish-brown precipitate. Which ion?	Reddish-brown maps to one ion only	Iron(III)
Excess test	A white precipitate dissolves in excess sodium hydroxide. What is possible?	White plus soluble means zinc or aluminium	Zinc or aluminium
Gas test	Acid gives a gas that turns limewater milky. Which ion?	Milky limewater means carbon dioxide	Carbonate

Sanity check (teach this): Sanity check: flame colours must be named in full (golden yellow, lilac, brick red, blue-green), never just yellow or purple; each precipitate colour maps to one ion, so “pale precipitate” is not an answer; a white precipitate needs the excess test before any conclusion; a gas test must state the observation, not the gas name alone; ion symbols carry charge (iron(III) is Fe3+); the conclusion says which ion is present, not which compound it is, unless the data justify that.

05 Independent practice (3 questions)

Question	Answer	What to watch
Four colour match	Match four metal ions to four flame colours	Check the colour names are complete
Six-row table	Precipitate colour and excess behaviour for six ions	Check the three whites are separated
Four-step route	Write the full route for one unknown salt	Check every step has observation plus conclusion

06 Common mistakes and what to say

Mistake	Student writes	Say this
Every white is calcium	Concludes calcium whenever a white precipitate appears	There are three whites: zinc, aluminium and calcium. Add excess sodium hydroxide and see whether it dissolves; if it does, it is zinc or aluminium.
Flame colour abbreviated	Says potassium is purple	It is lilac, and you look through cobalt glass. One missing word is a lost mark.
Gas named, observation missing	Writes “acid produced a gas, so it is a carbonate”	Add the observation: limewater turned milky. Without the observation there is no mark.
Excess test skipped	Concludes on the first white precipitate	You skipped step three. You must add excess sodium hydroxide before ruling anything out.

07 Homework (paper, section and question numbers)

Secondary Chemistry chapter index — All chapters in this subject
Prep guide master index — Similar chapters in other subjects
Redo this chapter — Design an unknown salt with two ions, then write the four steps and the expected observations

08 Exit check (last 5 minutes)

Question	Answer	Decision
What flame colour does potassium give	Lilac, seen through cobalt glass	“Purple” means return to act one
Which two ions give a white precipitate soluble in excess alkali	Zinc and aluminium	“Only calcium” means return to act two
How is a carbonate confirmed	Acid, and limewater turns milky	“A gas appears” means return to act three

09 Tutor FAQ

How are zinc and aluminium separated?

Ammonia added to excess behaves differently and can help at S4; if the question only asks for a white precipitate soluble in excess, either answer is accepted.

Must the flame test use platinum wire?

Platinum is best, nichrome works, but the wire must be cleaned in concentrated acid and heated until no colour remains, or the sample is contaminated.

Are ionic equations required?

Only when the question asks, for example copper(II) plus hydroxide gives copper(II) hydroxide; otherwise the observation is enough.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.