12.5 Identification of ions and gases

Learning objectives

12.5.1Anion tests• Describe anion tests: carbonate + dilute acid then CO2 test; halides (Cl-/Br-/I-) acidified with dilute HNO3 then aqueous AgNO3; nitrate with Al foil + aqueous NaOH then NH3 test; sulfate acidified with dilute HNO3 then aqueous Ba(NO3)2; sulfite with acidified KMnO4 12.5.2Cation tests using aqueous sodium• Describe cation tests using aqueous sodium hydroxide and aqueous ammonia for Al3+, NH4+, Ca2+, Cr3+, Cu2+, Fe2+, Fe3+ and Zn2+12.5.3Gas tests: NH3 with damp red litmus• Describe gas tests: NH3 with damp red litmus; CO2 with limewater; Cl2 with damp litmus; H2 with lighted splint; O2 with glowing splint; SO2 with acidified aqueous potassium manganate(VII)12.5.4Use of a flame test to identify the• Describe use of a flame test to identify the cations: (a) lithium, Li+ (b) sodium, Na+ (c) potassium, K+ (d) calcium, Ca2+ (e) barium, Ba2+ (f) copper(II), Cu2+

An anion test is identified by the complete sequence of reagents and its positive observation. Use a fresh portion of the unknown for each test so one reagent does not contaminate the next.

Anion	Ordered test	Positive result
carbonate, $\mathrm{CO_3^{2-}}$	add dilute acid; test the gas with limewater	effervescence; the carbon dioxide turns limewater milky
chloride, $\mathrm{Cl^-}$	acidify with dilute nitric acid; add aqueous silver nitrate	white precipitate
bromide, $\mathrm{Br^-}$	acidify with dilute nitric acid; add aqueous silver nitrate	cream precipitate
iodide, $\mathrm{I^-}$	acidify with dilute nitric acid; add aqueous silver nitrate	yellow precipitate
nitrate, $\mathrm{NO_3^-}$	add aqueous sodium hydroxide, then aluminium foil; warm carefully; test the gas with damp red litmus	ammonia is produced and turns the paper blue
sulfate, $\mathrm{SO_4^{2-}}$	acidify with dilute nitric acid; add aqueous barium nitrate	white precipitate
sulfite, $\mathrm{SO_3^{2-}}$	add a small volume of acidified aqueous potassium manganate(VII)	purple solution turns colourless

The three halides use the same reagents, so the precipitate colour makes the identification: chloride white, bromide cream, iodide yellow.

Do not replace dilute nitric acid with hydrochloric acid before the silver nitrate test: hydrochloric acid introduces chloride ions. Likewise, sulfuric acid would introduce sulfate before a sulfate test.

Add each reagent dropwise to a fresh portion of the solution, record the precipitate colour, then add the reagent in excess and record whether the precipitate dissolves. For ammonium, warm with aqueous sodium hydroxide and test the gas.

Cation	Aqueous sodium hydroxide	Aqueous ammonia
$\mathrm{Al^{3+}}$	white precipitate; soluble in excess to a colourless solution	white precipitate; insoluble in excess
$\mathrm{NH_4^+}$	on warming, ammonia is produced	—
$\mathrm{Ca^{2+}}$	white precipitate; insoluble in excess	no precipitate or a very slight white precipitate
$\mathrm{Cr^{3+}}$	green precipitate; soluble in excess	green precipitate; insoluble in excess
$\mathrm{Cu^{2+}}$	light blue precipitate; insoluble in excess	light blue precipitate; soluble in excess to a dark blue solution
$\mathrm{Fe^{2+}}$	green precipitate; insoluble in excess; turns brown near the surface on standing	green precipitate; insoluble in excess; turns brown near the surface on standing
$\mathrm{Fe^{3+}}$	red-brown precipitate; insoluble in excess	red-brown precipitate; insoluble in excess
$\mathrm{Zn^{2+}}$	white precipitate; soluble in excess to a colourless solution	white precipitate; soluble in excess to a colourless solution

A white precipitate alone is not an identification. Use behaviour in excess to distinguish aluminium from zinc with ammonia, and use the sodium-hydroxide and ammonia results together to distinguish calcium.

For $\mathrm{NH_4^+}$ , the positive result is ammonia gas on warming with aqueous sodium hydroxide; confirm it because damp red litmus paper turns blue.

State both the initial observation and the result in excess. 'Soluble' refers to the precipitate dissolving after excess reagent is added, not to the original salt solution.

A gas identification must pair the specified test with its distinctive positive result.

Gas	Test	Positive result
ammonia, $\mathrm{NH_3}$	damp red litmus paper	turns blue
carbon dioxide, $\mathrm{CO_2}$	limewater	turns milky
chlorine, $\mathrm{Cl_2}$	damp litmus paper	bleached
hydrogen, $\mathrm{H_2}$	lighted splint	a ‘pop’ sound
oxygen, $\mathrm{O_2}$	glowing splint	relights
sulfur dioxide, $\mathrm{SO_2}$	acidified aqueous potassium manganate(VII)	purple turns colourless

Keep the splint states exact: hydrogen is tested with a lighted splint; oxygen with a glowing splint. Reversing them loses the identifying observation.

Litmus must be damp for ammonia and chlorine. Potassium manganate(VII) must be acidified for sulfur dioxide; an unqualified colour change is not the complete specified test.

Place a little salt on a clean wooden splint or clean nichrome/platinum wire and introduce it into a roaring Bunsen flame. Observe the characteristic flame colour.

Metal ion	Flame colour
lithium, $\mathrm{Li^+}$	red
sodium, $\mathrm{Na^+}$	yellow
potassium, $\mathrm{K^+}$	lilac
calcium, $\mathrm{Ca^{2+}}$	orange-red
barium, $\mathrm{Ba^{2+}}$	light green
copper(II), $\mathrm{Cu^{2+}}$	blue-green

Record the observed colour first, then match it to the table. Distinguish the paired descriptions precisely: calcium is orange-red, barium light green and copper(II) blue-green.

Use clean test equipment and a fresh sample. A contaminated wire or splint can add another flame colour, especially the strong yellow from sodium, and make the identification unreliable.

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—Cambridge IGCSE Chemistry (0620)Read

12.5 Identification of ions and gases

Learning objectives

Identify anions with ordered chemical tests

Identify aqueous cations with sodium hydroxide and ammonia

Identify six gases with their specific tests

Identify metal ions by flame colour

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