Tests for Ions, Gases and Water

Qualitative analysis is the most memorisation-dense corner of 0620, and the most bankable: the tests never change, the observations are fixed phrases, and a typical Paper 4 or Paper 6 carries 6-10 marks of them. The full reagent-by-reagent walkthrough lives in our qualitative analysis guide; this page is the syllabus table plus the marking habits that turn knowledge into marks.

Testing for cations

Flame tests: clean wire, dip in the solid, hold in a roaring Bunsen flame:

Ion	Flame colour
lithium, Li+	red
sodium, Na+	yellow
potassium, K+	lilac
copper(II), Cu2+	blue-green
calcium, Ca2+	orange-red
barium, Ba2+	apple-green

Aqueous sodium hydroxide, added slowly then in excess:

Ion	Precipitate with NaOH	In excess NaOH
copper(II)	light blue	insoluble
iron(II)	green	insoluble
iron(III)	red-brown	insoluble
calcium	white	insoluble
zinc	white	dissolves, colourless solution
aluminium	white	dissolves, colourless solution
chromium(III)	green	dissolves
ammonium, NH4+	no precipitate; warm → ammonia gas given off	no precipitate

Aqueous ammonia breaks the ties: zinc hydroxide dissolves in excess ammonia, aluminium hydroxide does not. So a white precipitate soluble in excess of both reagents is zinc; soluble in excess NaOH only is aluminium; insoluble in both is calcium (confirm with the flame test).

Testing for anions

Anion	Test	Result
carbonate, CO32−	add dilute acid	effervescence; gas turns limewater milky
chloride, Cl−	acidify with dilute nitric acid, add silver nitrate	white precipitate
bromide, Br−	same	cream precipitate
iodide, I−	same	yellow precipitate
sulfate, SO42−	acidify with dilute nitric acid, add barium nitrate	white precipitate
sulfite, SO32−	add dilute acid, warm	sulfur dioxide given off; turns acidified potassium manganate(VII) from purple to colourless
nitrate, NO3−	add NaOH and aluminium foil, warm	ammonia given off; damp red litmus turns blue

The acidification step is not decoration: nitric acid removes carbonate ions that would otherwise give a false white precipitate. “Acidify first” is a marking point.

Testing for gases

Gas	Test	Result
hydrogen	lighted splint	pops (burns with a squeaky pop)
oxygen	glowing splint	relights
carbon dioxide	bubble through limewater	turns milky
ammonia	damp red litmus paper	turns blue
chlorine	damp litmus paper	bleached (turns white)

Two-part answers, always: what you do and what you see. “Limewater” alone is half a test.

Testing for water

• Anhydrous copper(II) sulfate: white → blue.
• Anhydrous cobalt(II) chloride: blue → pink.

Both show water is present. To show a liquid is pure water, measure its boiling point: exactly 100°C at standard pressure. Purity from sharp melting and boiling points is developed further under identifying unknown substances.

Worked exam question

Solid S dissolves in water. Adding aqueous sodium hydroxide to the solution gives a white precipitate which dissolves in excess. Adding aqueous ammonia gives a white precipitate which does not dissolve in excess. Dilute nitric acid followed by aqueous silver nitrate gives a yellow precipitate. Identify solid S. [3]

Model answer, mark by mark:

• M1: white precipitate soluble in excess NaOH → zinc or aluminium.
• M2: precipitate insoluble in excess ammonia → aluminium (zinc would dissolve), so the cation is Al3+.
• M3: yellow precipitate with acidified silver nitrate → iodide, I−. Solid S is aluminium iodide.

The structure of the answer matters: state the deduction from each observation, then the compound. A bare “aluminium iodide” with no reasoning risks losing the intermediate marks if the final answer is wrong.

The mistakes that cost marks

1. One-half gas tests. “Use a splint” without lighted/glowing, or “limewater” without “turns milky”. Action plus observation, every time.
2. Zinc and aluminium left unresolved. Excess NaOH cannot separate them; excess ammonia can. Questions are built around exactly this fork.
3. Acidifying with the wrong acid. Sulfuric acid before barium nitrate adds sulfate ions; hydrochloric acid before silver nitrate adds chloride ions. Use dilute nitric acid for both precipitate tests.
4. Colour drift. Iron(II) green, iron(III) red-brown: reversed in thousands of scripts every series. Two from “2” has an e like green; build whatever mnemonic sticks, then test yourself cold.
5. “Cloudy” for limewater without “white/milky”. The accepted observations are “milky” or “white precipitate forms”.

How examiners want it phrased

Student wording	Mark-scheme wording
”Test it with a splint"	"Insert a lighted splint; hydrogen burns with a squeaky pop"
"NaOH makes it go blue and solid"	"A light blue precipitate forms, insoluble in excess: copper(II) ions present"
"Silver nitrate finds chloride"	"Acidify with dilute nitric acid, add aqueous silver nitrate; a white precipitate shows chloride ions"
"The white powder goes blue, so water"	"Anhydrous copper(II) sulfate turns from white to blue, showing the presence of water”

These tables are the raw material; deploying them on multi-step unknowns is the skill, and the parent experimental techniques topic ties the two together. Most students can secure all of qualitative analysis in two or three focused sessions of drilling. Start with a free trial lesson and we will baseline exactly which tests you already own.

Test yourself

Three tests, from memory. Click each answer to check both halves of your phrasing.

Q1 (2 marks). Describe the test for oxygen and the test for hydrogen. Give the result of each.

Show answer

• oxygen: a glowing splint relights [1] • hydrogen: a lighted splint gives a squeaky pop [1]

Q2 (3 marks). Describe the test for chloride ions in solution. Explain why the solution is acidified first.

Show answer

• acidify with dilute nitric acid, then add aqueous silver nitrate [1] • a white precipitate shows chloride ions [1] • the nitric acid removes carbonate ions, which would otherwise give a false white precipitate [1]

Q3 (2 marks). State the flame colour given by potassium ions and by copper(II) ions.

Show answer

• potassium, K+: lilac [1] • copper(II), Cu2+: blue-green [1]