Flame Tests

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.45 — How to carry out a flame test
• 2.46 — Flame colours for Li⁺, Na⁺, K⁺, Ca²⁺ and Cu²⁺

Specification section 2(h) Chemical tests — also taught in Titrations, Preparing a Soluble Salt, Preparing an Insoluble Salt, Tests for Gases, and for Water.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Several metal ions give a characteristic colour when heated strongly in a flame, and a FLAME TEST uses that to identify them. It is the quickest identification in the whole specification: one wire, one flame, one colour.

Statement 2.46 — the five colours

1. LITHIUM, Li⁺ — a RED flame, sometimes described as crimson.
2. SODIUM, Na⁺ — a strong YELLOW or orange flame.
3. POTASSIUM, K⁺ — a LILAC flame, which is pale and easily masked.
4. CALCIUM, Ca²⁺ — an ORANGE-RED or brick-red flame.
5. COPPER, Cu²⁺ — a BLUE-GREEN flame.
6. These five are the ones the specification names, and they must simply be learned.

Two of the five need care. Calcium's orange-red and lithium's red are close enough that they can be confused, and sodium's yellow is so intense that even a trace of sodium contamination will mask everything else — potassium's pale lilac in particular disappears entirely if any sodium is present.

Think of it like a single loud voice drowning a room

In a room where several people are speaking quietly, one person shouting makes everyone else inaudible — not because they have stopped talking but because nothing else can be heard over it. Sodium is the shouter. Its yellow is so strong that a trace left on a wire from a previous test, or from a fingerprint, will swamp a lilac potassium flame completely. This is why the whole procedure below is built around obsessive cleaning: the test is not delicate because the colours are faint but because one of them is overwhelming.

Statement 2.45 — carrying out the test

1. Take a clean NICHROME or PLATINUM wire, chosen because these metals give no flame colour of their own.
2. CLEAN the wire by dipping it in concentrated hydrochloric acid and holding it in a hot Bunsen flame.
3. REPEAT the cleaning until the wire produces NO COLOUR in the flame. This is the step that matters most.
4. Dip the clean wire into the acid again and then into the solid sample, so that a little sticks to it.
5. Hold the wire in the edge of a hot BLUE Bunsen flame and observe the colour produced.
6. Use a blue flame rather than a yellow one, because a yellow safety flame has its own colour and would hide the result.

Two of those steps deserve their reasons stated. The WIRE is nichrome or platinum because most metals colour a flame themselves — an iron wire would contribute its own colour and ruin the test. And the FLAME must be the blue roaring flame, since the yellow safety flame is yellow for the same reason a sodium test is: it contains glowing carbon particles, and its colour would mask a faint result.

The hydrochloric acid has a purpose too. It converts the compound to a chloride, and chlorides are more volatile than most other salts, so more of the metal gets into the flame and the colour is stronger. It also helps clean the wire.

Reading a set of results, including a failed one

Four solids are flame-tested. A gives lilac, B gives yellow, C gives blue-green, D gives yellow on the first attempt and lilac when repeated with a freshly cleaned wire. Identify the metal ion in each.

1. A gives lilac, so A contains POTASSIUM.
2. B gives yellow, so B contains SODIUM.
3. C gives blue-green, so C contains COPPER.
4. D is the instructive one. The first result was yellow and the second, with a clean wire, was lilac.
5. Yellow from a contaminated wire is sodium left over from testing B, and it masked the true colour. D contains POTASSIUM.
6. The general rule: if a yellow flame appears, always repeat with a freshly cleaned wire before accepting it, because yellow is both a real result and the standard contamination.

Answer: A and D potassium, B sodium, C copper — with D's first result being sodium contamination.

That last point is worth carrying into any flame test question. A yellow flame is ambiguous in a way the other four are not: it might mean sodium is present, or it might mean the wire was dirty. Every other colour, being fainter, can only appear if the ion producing it is genuinely there.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 2.46 — the five flame colours

Statement 2.45 — carrying out a flame test

1. Take a nichrome or platinum wire, which gives no flame colour of its own.
2. Clean it by dipping in concentrated hydrochloric acid and holding it in a hot flame.
3. Repeat until the wire gives NO colour in the flame.
4. Dip the clean wire in the acid and then in the solid sample.
5. Hold it in the edge of a hot BLUE Bunsen flame and observe the colour.

Why each detail of the method is there

Nichrome or platinum wire

These give no flame colour of their own, so they add nothing to the result.

Cleaning until colourless

Any residue from a previous test would give its own colour, especially sodium.

Hydrochloric acid

Converts the compound to a more volatile chloride, so more metal reaches the flame and the colour is stronger.

Blue flame

A yellow safety flame has its own colour and would mask a faint result.

Why a flame test can go wrong

• SODIUM CONTAMINATION is the commonest problem — its yellow masks everything, especially potassium's lilac
• A DIRTY WIRE carries over the previous sample's colour
• Using the YELLOW safety flame hides faint colours behind the flame's own colour
• Using an iron or copper wire adds a colour of its own
• If a YELLOW flame appears, repeat with a freshly cleaned wire before accepting it

Model answer [4 marks]

Describe how to carry out a flame test on a solid sample. [4]

Take a clean nichrome or platinum wire, which does not give a flame colour of its own. Clean it by dipping it in concentrated hydrochloric acid and holding it in a hot Bunsen flame, repeating this until the wire produces no colour at all in the flame. Dip the cleaned wire into the hydrochloric acid and then into the solid sample so that a little sticks to it. Hold the wire in the edge of a hot blue Bunsen flame and observe the colour produced.

Model answer [4 marks]

Explain why the wire must be cleaned until it gives no colour, and why a blue flame is used rather than a yellow one. [4]

The wire must be cleaned until it gives no colour because any residue from a previous sample would produce its own flame colour and would be mistaken for the colour of the sample being tested. This matters particularly for sodium, whose yellow colour is so intense that even a trace of it masks other colours, especially the pale lilac of potassium. A blue Bunsen flame is used because the yellow safety flame has a strong colour of its own, which would hide the colour produced by the sample.

Not this: A yellow flame always means the sample contains sodium.

This: It may — or the wire may not have been cleaned properly, since sodium is the commonest contaminant and its colour is the most intense. A yellow result should always be repeated with a freshly cleaned wire before it is accepted.

Mark-losing trap. Clean the wire until it gives NO COLOUR. This is the step the marks are for.

Mark-losing trap. Use the BLUE flame, not the yellow safety flame, or the flame's own colour masks the result.

Mark-losing trap. Sodium's yellow masks potassium's lilac completely. Repeat any yellow result with a clean wire.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What flame colour does a copper(II) ion give?
2. Grade 7 · Explain [2 marks] — Explain why a nichrome or platinum wire is used for a flame test rather than an iron wire.
3. Grade 8 · Explain [3 marks] — A student flame-tests a solid and sees a yellow flame. Explain why they should not immediately conclude that the sample contains sodium.
4. Grade 9 · Describe [6 marks] — Select every statement that belongs in a full-mark description of how to carry out a flame test, with the reason for each precaution.
5. 9+ · Analyse [6 marks] — A student flame-tests a white solid and observes a yellow flame. They conclude the solid is sodium chloride. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who first saw the colours these ions give, and the one whose gaps were filled by exactly this kind of evidence. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Humphry Davy — the one who first saw the colours these ions give

Davy is the person who first saw several of these flame colours, because he was the first to have the metals at all. Potassium burning on water with a lilac flame is one of the earliest descriptions of a flame colour tied to a specific element, and his accounts of the new metals are full of such observations because colour was one of the few tools he had for characterising a substance nobody had seen. He is the right person to ask what you can learn about a material from how it burns.

• “What colour did the new metals burn with?”
• “How useful is a colour for identifying a substance?”
• “What else could you tell about a metal from its flame?”
• “Did you realise the colours were characteristic of the element?”
• “How did you describe a colour before there were standards for it?”

Dmitri Mendeleev — the one whose gaps were filled by exactly this kind of evidence

Flame colours are the crude ancestor of spectroscopy, and spectroscopy is what filled several of the gaps in Mendeleev's table. Caesium and rubidium were both discovered by their spectral lines and named for their colours — caesium from the Latin for sky-blue, rubidium for deep red. Mendeleev's predictions and the new technique arrived at nearly the same moment, and each strengthened the other. He is the right figure for how a new method of observation can suddenly make a theoretical scheme testable.

• “How did colours in a flame help identify new elements?”
• “Which of your predicted elements were found this way?”
• “Why are caesium and rubidium named after colours?”
• “What did a new observing technique do for your table?”
• “How did you decide a newly found element belonged in a particular gap?”

Carry on through the course

• Previous lesson: Tests for Gases, and for Water
• Next lesson: Tests for Ions
• Group 1 — the Alkali Metals — Metals soft enough to cut with a knife, which set fire to water — and get worse as you go down.
• Group 7 — the Halogens — A green gas, a brown liquid and a grey solid — and a trend that runs the opposite way to Group 1.
• The Gases in the Air — Four gases, one of which is doing almost all the chemistry — and how to measure it without ever touching it.
• Combustion and Carbonates — Three things burning in oxygen, one carbonate breaking apart, and the 0.04% gas that changes the climate.
• The Reactivity Series — Putting eleven metals in order by watching what each one will and will not do.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification