Identifying Unknown Substances

The hardest questions on Paper 6 (and the 4-6 mark deduction items on Paper 4) give you a table of observations on substance X and ask what X is. No single fact answers it. The skill is running the tests in a sensible order and converting each observation into one logged deduction, because the mark scheme pays per inference, not per final answer.

Purity first: melting and boiling points

A pure substance has a sharp melting point and a sharp boiling point: fixed values you can look up. Pure water melts at 0°C and boils at 100°C. An impurity:

• lowers the melting point, and
• raises the boiling point, and
• spreads both over a range instead of a single temperature.

So a white solid melting sharply at 80°C is probably one pure compound; one melting between 68°C and 74°C is impure. This is why melting points matter to drug manufacturers (a sharp value is evidence of a pure product), and the application sentence is worth a mark when the context is pharmaceutical. Chromatography gives the complementary check: a pure substance shows a single spot, as covered in chromatography and Rf values.

The systematic order

Unknown substances surrender their identity fastest when tests run from cheap information to specific confirmation:

1. Look. Colour and state narrow things immediately: blue solid suggests copper(II); green, iron(II) or chromium; red-brown, iron(III); white, a compound of the colourless cations (Group I, calcium, zinc, aluminium, ammonium).
2. Add dilute acid. Fizzing that turns limewater milky means a carbonate. One test has just identified the anion.
3. Cation tests. Flame test on the solid; then aqueous sodium hydroxide, then aqueous ammonia, each added slowly then in excess, on a solution.
4. Anion tests. Acidified silver nitrate (halides), acidified barium nitrate (sulfate), NaOH with aluminium foil (nitrate).
5. Confirm. A second, independent test for the same ion turns a probable answer into a certain one.

The full observation tables sit on tests for ions, gases and water and in the qualitative analysis guide. This page is about sequencing them, and about writing the deduction trail the examiner can mark.

Writing the deduction trail

Mark schemes for deduction questions are lists of paired statements: observation → inference. Train the habit of writing both:

Observation	Deduction (write this down)
blue solution	contains Cu2+
effervescence with acid; gas turns limewater milky	carbonate present; gas is CO2
white precipitate with NaOH, dissolves in excess	Zn2+ or Al3+
white precipitate with ammonia, insoluble in excess	Al3+ (eliminates Zn2+)
cream precipitate with acidified silver nitrate	Br− present

Note the elimination logic in rows three and four: two tests, one conclusion. Questions are engineered around these forks: zinc/aluminium (resolved by excess ammonia), the three halides (resolved by precipitate colour), carbonate/sulfite (resolved by testing the gas).

Worked exam question

Substance T is a green solid. T dissolves in water to give a green solution. Aqueous sodium hydroxide gives a green precipitate, insoluble in excess. Dilute hydrochloric acid produces no reaction. Acidified aqueous barium nitrate gives a white precipitate. (a) Identify the cation in T, with a reason. [2] (b) Identify the anion in T, with a reason. [2] (c) Name substance T. [1]

Model answer, mark by mark:

• (a) M1: iron(II), Fe2+. M2: because the hydroxide precipitate is green and insoluble in excess NaOH. (The green colour of the solid and solution supports it; the precipitate is the evidence to quote.)
• (b) M3: sulfate, SO42−. M4: white precipitate with acidified barium nitrate; no fizzing with acid rules out carbonate.
• (c) M5: iron(II) sulfate.

The “no reaction with acid” line is not filler. It is the negative result that eliminates carbonate, and quoting it is what makes M4 secure.

The mistakes that cost marks

1. Final answer with no trail. If “iron(II) sulfate” is wrong, an answer with no logged deductions scores zero; one with correct per-step inferences keeps most of the marks.
2. Ignoring negative results. “No precipitate”, “no reaction” lines are printed because they eliminate candidates. Use them explicitly.
3. Impurities described as raising the melting point. Impurities lower melting point and raise boiling point. Half the candidates flip one of them.
4. Testing in excess forgotten. The NaOH and ammonia tests only distinguish ions when you record what happens in excess. “White precipitate” alone leaves three cations standing.
5. Two ions, one test. Concluding “zinc” from solubility in excess NaOH alone. Aluminium does that too. Name the second test that splits them.

How examiners want it phrased

Student wording	Mark-scheme wording
”It’s pure because it looks clean"	"It melts at a single sharp temperature, so it is pure"
"It might be zinc or aluminium"	"White precipitate soluble in excess NaOH: Zn2+ or Al3+; insoluble in excess ammonia, so Al3+"
"Nothing happened with acid"	"No effervescence with dilute acid, so carbonate ions are absent"
"The stuff in it changes the boiling"	"An impurity lowers the melting point and raises the boiling point, over a range of temperatures”

Deduction questions reward calm bookkeeping more than brilliance, which is why they respond so fast to coaching. The whole strategy above sits inside experimental techniques and chemical analysis. Bring one unsolved deduction question to a free trial lesson and we will walk the elimination grid together, line by line.

Test yourself

Run these three cold. The answers are hidden until you click.

Q1 (2 marks). State the effect of an impurity on the melting point and on the boiling point of a substance.

Show answer

• an impurity lowers the melting point (and spreads it over a range) [1] • and raises the boiling point (over a range) [1]

Q2 (3 marks). Solid X dissolves in water to give a blue solution. Adding acidified aqueous barium nitrate to the solution gives a white precipitate. Identify the cation, the anion, and substance X.

Show answer

• blue solution: copper(II), Cu2+ [1] • white precipitate with acidified barium nitrate: sulfate, SO42− [1] • X is copper(II) sulfate [1]

Q3 (2 marks). A solution gives a white precipitate with aqueous sodium hydroxide which dissolves in excess. Describe one further test, with its results, that shows whether the cation is Zn2+ or Al3+.

Show answer

• add aqueous ammonia slowly, then in excess, to a fresh sample [1] • the white precipitate dissolves in excess ammonia if Zn2+; it does not dissolve if Al3+ [1]