Chemistry · 12 Experimental techniques and chemical analysis

12.5 Identification of ions and gases

Before you start

Ions carry charge, and a salt in solution contains a cation and an anion (e.g. $FeSO_{4}$ gives $Fe^{2 +}$ and $SO_{4}^{2 -}$ ).
A precipitate is an insoluble solid formed when two solutions are mixed.
Acids react with carbonates to give carbon dioxide.

Syllabus objectives covered (4)

Describe tests to identify the anions: (a) carbonate, CO3 2–, by reaction with dilute acid and then testing for carbon dioxide gas (b) chloride, Cl –, bromide, Br –, and iodide, I –, by acidifying with dilute nitric acid then adding aqueous silver nitrate (c) nitrate, NO3 –, reduction with aluminium foil and aqueous sodium hydroxide and then testing for ammonia gas (d) sulfate, SO4 2–, by acidifying with dilute nitric acid and then adding aqueous barium nitrate (e) sulfite, SO3 2–, by reaction with acidified aqueous potassium manganate(VII)
Describe tests using aqueous sodium hydroxide and aqueous ammonia to identify the aqueous cations: (a) aluminium, Al 3+ (b) ammonium, NH4 + (c) calcium, Ca2+ (d) chromium(III), Cr3+ (e) copper(II), Cu2+ (f) iron(II), Fe2+ (g) iron(III), Fe3+ (h) zinc, Zn2+
Describe tests to identify the gases: (a) ammonia, NH3, using damp red litmus paper (b) carbon dioxide, CO2, using limewater (c) chlorine, Cl 2, using damp litmus paper (d) hydrogen, H2, using a lighted splint (e) oxygen, O2, using a glowing splint (f) sulfur dioxide, SO2, using acidified aqueous potassium manganate(VII)
Describe the 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+

Part 1 of 6

What this topic is actually asking of you

Qualitative analysis is detective work: you are given an unknown substance and you have to say what is in it. Every test follows the same shape — do something, observe something, conclude something.
Here is the thing nobody tells you clearly, and it changes how you should revise. In Paper 6 (Alternative to Practical) you are given a printed table of all these tests and results. In Paper 4 you are not.
So this material has two jobs. For Paper 4 you need it in your head. For Paper 6 you need to be fast and accurate at reading someone else's table under time pressure — a different skill, and one people lose marks on by half-reading a row.
Either way, the answer is never just a name. A full-mark answer says what you add, what you see, and what that means. Miss the observation and you have described a recipe, not a test.

Part 2 of 6

Cations with aqueous sodium hydroxide

The method for every cation on the list is the same: to the solution, add aqueous sodium hydroxide a few drops at a time, note what happens, then keep adding until it is in excess and note what happens again.
Those two stages are the whole test. Adding a little separates the ions by precipitate colour; adding excess separates the ones whose precipitates look identical.
Add a little $NaOH (aq)$ and you get a coloured clue straight away. Copper(II) gives a light blue precipitate; iron(II) green; iron(III) red-brown; chromium(III) green. Aluminium, calcium and zinc all give a white precipitate — indistinguishable at this stage.
Now add excess. The white precipitates split apart: aluminium dissolves to a colourless solution, zinc dissolves to a colourless solution, and calcium stays put. Chromium(III) also dissolves, giving a green solution.
Ammonium is the odd one out: no precipitate at all, because there is no insoluble hydroxide to form. Instead, warm the mixture and ammonia gas is given off — which you then identify with damp red litmus.
$NH_{4}^{+} + OH^{-} \overset{}{\to} NH_{3} + H_{2} O$
After sodium hydroxide, aluminium and zinc still look identical. That is exactly the gap the second reagent fills.

Part 3 of 6

Cations with aqueous ammonia — the tie-breaker

Repeat the whole procedure with aqueous ammonia instead of sodium hydroxide, again noting what happens with a little and then with excess.
Two results do the heavy lifting. Aluminium gives a white precipitate that is insoluble in excess ammonia, while zinc gives a white precipitate that dissolves in excess to a colourless solution. That is what tells them apart.
The other memorable result is copper(II): the light blue precipitate dissolves in excess ammonia to give a striking dark blue solution. Nothing else on the list does this, so it is close to a fingerprint.
Calcium gives no precipitate with aqueous ammonia (at most a very slight cloudiness), which distinguishes it from aluminium and zinc a second way.
Iron(II) green and iron(III) red-brown behave the same with both reagents — insoluble in excess. Their colours are distinctive enough that they need no tie-breaker.
Colour first, then solubility in excess — the same two questions every time.

Part 4 of 6

Anions: five tests, and why the order matters

Carbonate. Add dilute acid. You see effervescence, and the gas turns limewater milky, confirming carbon dioxide.
$CO_{3}^{2 -} + 2 H^{+} \overset{}{\to} H_{2} O + CO_{2}$
Halides. Acidify with dilute nitric acid, then add aqueous silver nitrate. Chloride gives a white precipitate, bromide a cream one, iodide a yellow one — a colour sequence that deepens as you go down the group.
Why acidify first? Because carbonate and sulfite ions would also give a white precipitate with silver nitrate. The acid destroys them first, so a white precipitate can only mean chloride.
Why nitric acid and not hydrochloric? Because hydrochloric acid would add chloride ions of its own, and you would get a white precipitate from every sample. This is a favourite exam question — be ready to say it in one sentence.
Sulfate. Acidify with dilute nitric acid, then add aqueous barium nitrate: a white precipitate. Same logic for the acid — it removes carbonate and sulfite, which would otherwise give white precipitates too.
Sulfite. Add acidified aqueous potassium manganate(VII). The purple colour disappears, because the sulfite reduces the manganate(VII). Watch the wording: 'decolourised', not 'goes clear' — a solution can be clear and still purple.
Nitrate. Add aqueous sodium hydroxide and a piece of aluminium foil, then warm. The aluminium reduces the nitrate to ammonia, which you identify with damp red litmus turning blue.
The nitrate test is really a gas test wearing a disguise, so an ammonium salt would give the same positive result. Test for ammonium first, before you add the aluminium.

Part 5 of 6

Six gas tests

Ammonia turns damp red litmus paper blue. It is the only common alkaline gas, so this test is decisive.
Carbon dioxide turns limewater milky — a white precipitate of calcium carbonate.
Chlorine bleaches damp litmus paper, turning it white. Note the difference from ammonia: red litmus, turns blue for ammonia; any litmus, turns white for chlorine. Writing 'turns litmus red' scores nothing.
Hydrogen gives a squeaky pop with a lighted splint.
Oxygen relights a glowing splint. Lighted splint for hydrogen, glowing splint for oxygen — swapping them is the commonest slip in this list.
Sulfur dioxide turns acidified aqueous potassium manganate(VII) from purple to colourless, the same decolourising you saw with sulfite. That is not a coincidence: acidifying a sulfite releases sulfur dioxide.
The word 'damp' in these tests is doing real work — dry litmus paper does not respond, because the gas has to dissolve to act as an acid or an alkali. If the question says 'damp', your answer should too.

Part 6 of 6

Flame tests

Method: clean a nichrome or platinum wire by dipping it in concentrated hydrochloric acid and holding it in the flame until it burns without colour. Then dip it in the sample and hold it in the edge of a hot blue Bunsen flame.
Cleaning is not a formality — a trace of sodium from a previous sample gives a yellow flame that drowns out everything else. Marks are given for the cleaning step.
The six colours to know: lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green.
Flame tests identify the metal ion only. A flame test cannot tell you the anion, so it is one half of an identification, never the whole thing.
Calcium and copper(II) appear on both the flame-test list and the sodium-hydroxide list, so you can confirm either of them two independent ways. When a question gives you two results, use both — agreement is what turns a guess into a deduction.

Worked examples

Example 1. A colourless solution gives a white precipitate with a few drops of aqueous sodium hydroxide. The precipitate dissolves when excess sodium hydroxide is added. With aqueous ammonia it gives a white precipitate that does not dissolve in excess. Identify the cation, explaining your reasoning.

Show the working

Start with the first result. A white precipitate with sodium hydroxide narrows it to aluminium, calcium or zinc.
The precipitate dissolves in excess sodium hydroxide. Calcium's does not, so calcium is out. That leaves aluminium and zinc.
Now use the ammonia result, which is exactly the test that separates those two. Insoluble in excess ammonia means aluminium; soluble would have meant zinc.
Check the result is consistent with the solution being colourless — aluminium salts are, so nothing contradicts it.

Answer: Aluminium, $Al^{3 +}$ . White precipitate soluble in excess $NaOH$ but insoluble in excess $NH_{3} (aq)$ is unique to aluminium among the ions listed.

Example 2. A student has a solution of an unknown sodium salt. Adding dilute nitric acid then aqueous silver nitrate gives a cream precipitate. Adding dilute hydrochloric acid to a fresh portion gives no effervescence. Identify the anion and explain what the second test rules out.

Show the working

Take the silver nitrate result first. A precipitate with acidified silver nitrate points to a halide, and the colour decides which one.
White would be chloride, cream is bromide, yellow is iodide. Cream gives bromide.
Now the second test. Acid on a carbonate produces carbon dioxide and visible effervescence; no effervescence means no carbonate is present.
This matters because carbonate would have given a white precipitate with silver nitrate too, had the sample not been acidified first — so the second test confirms the reasoning rather than just adding information.

Answer: The anion is bromide, $Br^{-}$ . The absence of effervescence with acid rules out carbonate, $CO_{3}^{2 -}$ .

Example 3. A white solid gives a lilac flame test. When dilute hydrochloric acid is added, a gas is produced that turns limewater milky. Identify the compound and state one further test you would do to confirm your conclusion.

Show the working

The flame colour identifies the metal ion. Lilac is potassium.
The gas turning limewater milky is carbon dioxide, and acid produces carbon dioxide from a carbonate.
Put the two halves together: a potassium ion and a carbonate ion, giving potassium carbonate, $K_{2} CO_{3}$ .
For confirmation, choose a test on the half you have identified least directly. The flame test rests on one observation, so repeat it with a cleaned wire, or check that no precipitate forms with aqueous sodium hydroxide, as expected for a Group I ion.

Answer: Potassium carbonate, $K_{2} CO_{3}$ . Confirm by repeating the flame test with a freshly cleaned wire, and by checking that aqueous sodium hydroxide gives no precipitate, as expected for potassium.

Where people go wrong

Mistake: Naming the ion without saying what you would see.

Instead: Every test answer needs reagent, observation and conclusion.

The command word is usually Describe or Identify, and the marking points are the observations. 'Add silver nitrate' on its own earns nothing.

Mistake: Using dilute hydrochloric acid to acidify before adding silver nitrate.

Instead: Use dilute nitric acid.

Hydrochloric acid introduces chloride ions, so every sample would give a white precipitate. This is one of the most frequently examined 'why' questions in the topic.

Mistake: Writing that a decolourised solution 'goes clear'.

Instead: Say the purple colour disappears, or that the solution is decolourised.

Clear means transparent, not colourless — purple potassium manganate(VII) is already clear. Examiners treat 'goes clear' as not answering the question.

Mistake: Confusing the lighted splint and the glowing splint.

Instead: Lighted splint plus a squeaky pop means hydrogen; a glowing splint relighting means oxygen.

They are different observations testing different gases, and swapping them turns a correct identification into a wrong one.

Mistake: Assuming a white precipitate with sodium hydroxide means calcium.

Instead: White could be aluminium, calcium or zinc — you must add excess to tell them apart.

The first observation narrows the field; only the excess test resolves it. Stopping halfway loses the second mark every time.

Check yourself

What do you see when aqueous sodium hydroxide is added to a solution containing $Fe^{3 +}$ , and does it change with excess?
Answer
A red-brown precipitate forms, and it is insoluble in excess sodium hydroxide.
Which test distinguishes $Al^{3 +}$ from $Zn^{2 +}$ , and what is the difference in the result?
Answer
Aqueous ammonia. Both give a white precipitate, but zinc's dissolves in excess to a colourless solution while aluminium's does not.
Describe the test for sulfate ions.
Answer
Acidify with dilute nitric acid, then add aqueous barium nitrate. A white precipitate confirms sulfate.
Why must a sample be acidified with nitric acid before the silver nitrate test?
Answer
To remove carbonate and sulfite ions, which would also give a white precipitate. Nitric acid is used rather than hydrochloric because hydrochloric acid would add chloride ions.
How would you distinguish chlorine from ammonia using litmus paper?
Answer
Ammonia turns damp red litmus blue; chlorine bleaches damp litmus paper white.
Which two species both decolourise acidified aqueous potassium manganate(VII), and why is that unsurprising?
Answer
Sulfite ions in solution and sulfur dioxide gas. Acidifying a sulfite releases sulfur dioxide, so they are chemically the same test.
Give the flame colours for lithium, potassium and barium.
Answer
Lithium red, potassium lilac, barium light green.
Why is the wire cleaned in concentrated hydrochloric acid before a flame test?
Answer
To remove residues from previous samples. Sodium in particular gives a strong yellow flame that would mask the colour being tested.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A student is given an aqueous solution of a single salt, labelled solution X. The results of three tests are shown. 1. A flame test gives an orange-red colour. 2. Adding a few drops of aqueous sodium hydroxide gives a white precipitate, which does not dissolve when excess is added. 3. Acidifying a fresh sample with dilute nitric acid and adding aqueous barium nitrate gives a white precipitate.

(a) Identify the cation present in solution X, using two of the results. 2 marks No calculator

Mark scheme

B1 calcium / Ca2+
B1 orange-red flame AND white precipitate with NaOH insoluble in excess, both quoted

(b) Identify the anion present in solution X. 1 mark No calculator

Mark scheme

B1 sulfate / SO4 2-

(c) Explain why the sample in test 3 is acidified with dilute nitric acid rather than dilute hydrochloric acid. 2 marks No calculator

Mark scheme

B1 acid removes carbonate (and sulfite) ions which would also give a white precipitate
B1 hydrochloric acid would add chloride ions, giving a white precipitate in any sample, owtte

(d) Give the formula of solution X. 1 mark No calculator

Mark scheme

B1 CaSO4; ecf from (a) and (b)

Question 2. Two colourless solutions, P and Q, each contain one cation from the list Al³⁺, NH₄⁺, Zn²⁺.

(a) Describe a single test that would show which solution contains ammonium ions, including the result you would expect. 3 marks No calculator

Mark scheme

B1 add aqueous sodium hydroxide and warm
B1 a gas is given off which turns damp red litmus paper blue
B1 no precipitate forms, unlike the other two ions, ORA

(b) Describe how you would distinguish between the remaining two cations. 3 marks No calculator

Mark scheme

B1 add aqueous ammonia until in excess
B1 zinc: white precipitate soluble in excess giving a colourless solution
B1 aluminium: white precipitate insoluble in excess

(c) Suggest why aqueous sodium hydroxide alone cannot distinguish between these two cations. 1 mark No calculator

Mark scheme

B1 both give a white precipitate that dissolves in excess sodium hydroxide, owtte

Question 3. A gas is collected from a reaction.

(a) State the test and result for sulfur dioxide. 2 marks No calculator

Mark scheme

B1 add / bubble through acidified aqueous potassium manganate(VII)
B1 turns from purple to colourless / is decolourised. CON if 'goes clear' is the only wording

(b) A student tests the same gas with a glowing splint and reports that it relights. Predict what this tells you, and comment on whether both results can be correct. 2 marks No calculator

Mark scheme

B1 a glowing splint relighting indicates oxygen
B1 the two results contradict each other, so one test must have been carried out or observed incorrectly, owtte

Words to know

Qualitative analysis: Identifying which ions or gases are present in a substance, rather than how much of each.
Precipitate: An insoluble solid formed when two solutions react.
In excess: Continuing to add a reagent after the first change, to see whether a precipitate dissolves.
Decolourised: Having lost its colour. Not the same as 'clear', which means transparent.
Damp litmus paper: Litmus moistened with water so that a gas can dissolve in it and act as an acid or alkali.

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