Chemistry 11–12 · Year 12

Identifying the eight cations of the 2017 syllabus by flame and precipitation tests

Module 8: Applying Chemical Ideas

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Each cation gives a fixed pattern of flame colour, precipitate colour and solubility in excess reagent, so a short sequence of tests identifies which of the eight is present.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

lead(II) nitrate toxic and a reproductive hazard
heating a lead salt in a flame releases toxic lead compounds into the air
barium nitrate toxic
silver nitrate corrosive
chromate is a classified carcinogen (drop scale, or omit and rely on sulfate and flame)
sodium hydroxide corrosive, ammonia vapour irritant
naked flame

Controls

drop scale in well plates
no flame test on the lead(II) or silver solutions, or on an unknown until the hydroxide test has ruled them out
the lead(II) nitrate solution is made up by the teacher or technician from the solid; the bench is washed down with dilute detergent solution after use
gloves for lead, barium, silver and chromate
heavy-metal waste container; nothing to the sink
eye protection; hair tied back for flame tests
chromate step is optional and teacher-dispensed

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions. Potassium chromate is used only where the school's CSIS register allows it, at drop scale; the scheme identifies all eight cations without it. CSIS Volume 2 Appendix F lists lead and lead salts (solid) among the chemicals to be used with caution, largely by science teachers, and says lead salts are toxic by inhalation and absorbed through the skin: learners handle only the 0.1 mol/L solution, at drop scale with gloves, and never heat or flame-test it.

What you need

0.1 mol/L solutions in dropper bottles: barium nitrate, calcium nitrate, magnesium nitrate, lead(II) nitrate, silver nitrate, copper(II) nitrate, iron(II) sulfate (fresh), iron(III) nitrate; three unknowns chosen from these
Reagents in dropper bottles: 1 mol/L sodium hydroxide, 2 mol/L ammonia, 0.1 mol/L sodium sulfate, 0.1 mol/L hydrochloric acid, 0.1 mol/L potassium chromate
Well plate, nichrome wire or soaked splints, Bunsen burner, cobalt-blue glass, white and black cards, centrifuge tubes

How to do it

Flame test only the barium, calcium and copper(II) solutions, in a well-ventilated room, and record the colour: barium apple-green, calcium brick-red, copper blue-green. Do not flame-test the lead(II), silver, magnesium or iron solutions: they give no distinctive colour, and heating lead salts puts lead into the air (CSIS Volume 2 Appendix F: lead salts are toxic by inhalation).
Add sodium hydroxide dropwise then in excess to two drops of each known; record precipitate colour and whether it redissolves.
Repeat with ammonia solution to excess.
Add hydrochloric acid to each: at 0.1 mol/L only silver gives a white precipitate, because equal drops of 0.1 mol/L lead(II) nitrate and 0.1 mol/L acid can form at most 0.025 mol/L of lead(II) chloride against a solubility of 0.039 mol/L (1.08 g per 100 mL of water at 20 degrees Celsius), so the lead tube stays clear; add sodium sulfate: barium, lead and calcium give white precipitates (calcium slowly); add chromate (teacher-dispensed): barium and lead give yellow and silver brick-red.
Build a flow chart from the results that separates all eight cations, then apply it to the three unknowns and report each with the evidence; flame-test an unknown only after the hydroxide test has ruled out lead(II) (a white precipitate that redissolves in excess) and silver (a brown precipitate).
Explain the chemistry of two results: the deep blue copper-ammonia complex and the rust-brown iron(III) hydroxide.

What you should see

Sodium hydroxide: magnesium gives a white precipitate, calcium a white precipitate or cloudiness, barium usually none (barium hydroxide is fairly soluble); lead(II) gives white that redissolves in excess; silver gives brown silver oxide; copper(II) pale blue; iron(II) dirty green, darkening to brown in air; iron(III) rust-brown. Ammonia: copper(II) gives pale blue then a deep royal-blue solution in excess; silver's brown precipitate redissolves in excess; magnesium and lead give white precipitates, iron(II) green and iron(III) brown; calcium and barium show no change. Hydrochloric acid: silver alone gives a white precipitate (it dissolves in ammonia); lead(II) gives none at 0.1 mol/L, because the mixture holds less lead(II) chloride than a saturated solution, so lead is named from its hydroxide, sulfate and chromate results instead. Sulfate: barium dense white, lead white, calcium a slow white cloud. Chromate: barium and lead bright yellow, silver brick-red. Flame: barium pale green, calcium orange-red, copper blue-green. These results let the flow chart name each of the three unknowns.

What changes

What you change: the cation solution tested and the reagent added
What you measure: flame colour, precipitate colour and its behaviour in excess reagent
What you keep the same: concentrations
drop volumes
order of tests
clean wire and wells

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

A white precipitate identifies the cation (several give white; the reagent sequence and solubility in excess are what identify it).
No flame colour means no metal is present (magnesium, iron, lead and silver give no distinctive flame colour).
Excess reagent always dissolves the precipitate (only amphoteric hydroxides and complex-forming ions redissolve).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-15 CH11/12-1 CH11/12-2 CH11/12-3 CH11/12-4
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taught; its solubility-rules experiment names six of this entry's eight cations (not magnesium or lead)CH-12-01
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-04 CH-12WS-02 CH-12WS-03 CH-12WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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