Chemistry 11–12 · Year 12

Identifying anions in solution: the eight syllabus anions by acid, silver, barium and iron(III) 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

Anions are identified by which cation precipitates them, the colour of the precipitate and whether it dissolves in acid or ammonia, with gas tests for carbonate and hydroxide.

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

silver nitrate corrosive and staining
barium nitrate toxic
1 mol/L nitric acid corrosive
ammonia vapour
sodium hydroxide 0.1 mol/L irritant

Controls

drop scale
gloves for silver and barium
heavy-metal waste container
eye protection
wafting only for the smell test

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.

What you need

0.1 mol/L sodium salts in dropper bottles: chloride, bromide, iodide, hydroxide, ethanoate, carbonate, sulfate, phosphate (trisodium phosphate); three unknowns
Reagents: 0.1 mol/L silver nitrate, 0.1 mol/L barium nitrate, 1 mol/L nitric acid, 2 mol/L ammonia, 0.1 mol/L iron(III) chloride, limewater, red litmus and pH paper, dilute sulfuric acid 1 mol/L
Well plate, test tubes, delivery tube, warm water bath

How to do it

Test each known with pH paper and record the reading.
Add nitric acid to two drops of each: carbonate fizzes (the gas clouds limewater). Then add silver nitrate to these acidified drops: chloride white, bromide cream, iodide yellow, the rest none; test each silver precipitate with ammonia.
Add silver nitrate to fresh, un-acidified drops: phosphate gives a yellow precipitate that dissolves when nitric acid is added, hydroxide a brown one, and carbonate a pale yellow one that also dissolves in nitric acid but with fizzing; the acid test has already found the carbonate, so it is not mistaken for phosphate.
Add barium nitrate to fresh drops: sulfate, carbonate and phosphate give white precipitates; add nitric acid: only barium sulfate remains.
Ethanoate: add a drop of iron(III) chloride to the neutral solution and record the red-brown colour; confirm by warming two drops with dilute sulfuric acid and wafting for the vinegar smell of ethanoic acid.
Hydroxide: record the pH and the red litmus change.
Build a flow chart and identify the three unknowns with evidence.

What you should see

Silver nitrate in nitric acid: chloride gives a white precipitate soluble in dilute ammonia, bromide cream and only slightly soluble, iodide yellow and insoluble; the other anions give nothing in acid. In neutral solution phosphate gives yellow silver phosphate that dissolves when nitric acid is added, carbonate gives pale yellow silver carbonate that dissolves in nitric acid with fizzing (so the acid test separates the two), and hydroxide gives brown silver oxide. Barium nitrate: sulfate gives a white precipitate insoluble in acid; carbonate and phosphate give white precipitates that dissolve in acid (carbonate with fizzing that clouds limewater). Ethanoate turns red-brown with iron(III) and smells of vinegar when warmed with dilute sulfuric acid. Hydroxide reads pH 13 and turns red litmus blue; on pH paper trisodium phosphate reads about 12 to 13, carbonate about 11 to 12 and ethanoate about 9 (computed for 0.1 mol/L solutions from standard acid dissociation constants: 12.6, 11.7 and 8.9), and the other anions near 7.

What changes

What you change: the anion tested and the reagent added
What you measure: precipitate colour, solubility in acid or ammonia, gas evolution, pH
What you keep the same: concentrations
acidification before the silver test
drop volumes
fresh limewater

Common misconceptions

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

Silver nitrate identifies chloride uniquely (bromide, iodide, phosphate, carbonate and hydroxide also precipitate; the acid and ammonia steps separate them).
Fizzing with acid proves carbonate is the only anion present (a mixture can hold carbonate and others; the scheme continues after the gas test).
Ethanoate cannot be tested because it forms no precipitate (the iron(III) colour and the vinegar smell on acidifying are its tests).

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 taughtCH-12-01 CH-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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