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Tests for anions and gases, and the order of testing

Edexcel IAL Chemistry · WCH13 Practical Skills in Chemistry I

Anion tests must be done in a sensible order, with the right acid, and the results described precisely. Gas tests need the exact wording of the observation.

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Anion tests and why the order matters

Carbonate first: add dilute acid; effervescence, and the gas turns limewater cloudy (CO2).
Sulfate: add dilute hydrochloric acid, then barium chloride solution; a white precipitate of BaSO4. The acid removes carbonate (and sulfite), which would also give a white precipitate with Ba2+.
Halides: add dilute nitric acid, then silver nitrate solution. Not HCl, which would add chloride ions; not H2SO4, which could give a silver sulfate precipitate. Cl- white, Br- cream, I- yellow. Then ammonia: AgCl dissolves in dilute NH3, AgBr only in concentrated NH3, AgI in neither.
H+ ions: add sodium carbonate or magnesium; effervescence. An indicator may fail if the solution is coloured (June 2024: blue copper(II) sulfate solution masked the colour).

Gas tests

Gas	Test and result
H2	lighted splint: squeaky pop
O2	glowing splint relights
CO2	limewater turns cloudy (milky)
NH3	damp red litmus turns blue; white smoke with HCl
HCl	steamy (misty) fumes in moist air; white smoke with ammonia
Cl2	damp blue litmus turns red then is bleached white
NO2	brown gas
H2O	anhydrous copper(II) sulfate white to blue; cobalt chloride paper blue to pink

A glowing splint is not lit; a lit splint is not glowing.

Heating to constant mass

To make sure a thermal decomposition is complete: heat the solid in a crucible, cool it (in a desiccator if it absorbs water), weigh, then heat again, cool and reweigh, repeating until two masses agree. Simply heating to a higher temperature is not the same thing (October 2021 report). Group 2 carbonates give CO2; Group 2 nitrates give brown NO2 and O2.

Worked example

A white solid gives a lilac flame. With dilute HCl it fizzes, and the gas turns limewater cloudy. Identify the solid.

Lilac flame: K+.
Effervescence with acid and CO2: carbonate.
Neutral compound: K2CO3.

Answer: Potassium carbonate, K2CO3

Key idea: Test carbonate first. Sulfate: HCl then BaCl2, white ppt. Halide: HNO3 then AgNO3, then ammonia. Learn the exact gas test wording. Heat to constant mass by repeated heating and weighing.

Check yourself

Why is dilute nitric acid, not hydrochloric acid, added before silver nitrate in the halide test?
- Nitric acid dissolves silver halides
- Hydrochloric acid reacts with silver nitrate to give a gas
- Hydrochloric acid would add chloride ions, giving a white precipitate whatever the sample
- Nitric acid is a catalyst
Show answer
Hydrochloric acid would add chloride ions, giving a white precipitate whatever the sample. The acid removes carbonate, but it must not add a halide.
A precipitate with silver nitrate dissolves in concentrated ammonia but not in dilute ammonia. Which halide was present?
- Bromide
- Chloride
- Iodide
- Fluoride
Show answer
Bromide. AgCl dissolves in dilute NH3, AgBr only in concentrated NH3, AgI in neither.
Why is dilute hydrochloric acid added before barium chloride solution in the sulfate test?
- To make the precipitate dissolve
- To provide chloride ions for the precipitate
- To remove carbonate ions, which would also give a white precipitate
- To speed up the precipitation
Show answer
To remove carbonate ions, which would also give a white precipitate. BaCO3 is also white, so carbonate must be removed first.
What is the test for oxygen?
- It gives a squeaky pop with a lighted splint
- It relights a glowing splint
- It turns limewater cloudy
- It bleaches damp litmus
Show answer
It relights a glowing splint. A glowing (not burning) splint bursts back into flame.
How do you show that a solid has been heated to constant mass?
- Heat it once at a very high temperature
- Weigh it before heating only
- Heat it until it changes colour
- Heat, cool and weigh repeatedly until two consecutive masses are the same
Show answer
Heat, cool and weigh repeatedly until two consecutive masses are the same. Matching masses show no more gas or water is being lost.
A blue solution contains copper(II) sulfate and possibly excess sulfuric acid. Which is the best test for H+ ions?
- Add barium chloride: white precipitate
- Add sodium carbonate: effervescence of carbon dioxide
- Add methyl orange: red colour
- Add silver nitrate: white precipitate
Show answer
Add sodium carbonate: effervescence of carbon dioxide. Barium chloride tests for sulfate (present anyway); indicator colours are masked by the blue solution.
Describe the test for chlorine gas and its result.
Show answer
Damp blue litmus paper turns red and is then bleached (white).. Chlorine is acidic and a bleach.

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Last updated 8 November 2026. Spotted a mistake? Tell us.