Flame Tests and Metal Hydroxide Tests

Hold a little sodium in a Bunsen flame and it glows bright yellow; hold copper and the flame turns green. These vivid colours are the basis of the flame test, one of two simple ways to identify the metal ion (cation) in a compound. The other is the metal hydroxide test, in which adding sodium hydroxide solution produces a coloured precipitate whose colour reveals the metal. This lesson, part of Topic C4 of OCR Gateway Science A, sets out both methods, the colours you must learn, and how to tell apart ions that behave similarly.

By the end of this lesson you should be able to carry out a flame test and recall the flame colours, carry out the sodium hydroxide precipitate test and recall the precipitate colours, and distinguish ions that give the same colour using the behaviour in excess sodium hydroxide.

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Flame Tests for Metal Cations

In a flame test, a small sample of the compound is heated in a Bunsen flame and the colour the flame turns identifies the metal ion present. The method is:

1. Dip a clean nichrome (or platinum) wire into concentrated hydrochloric acid to clean it, then hold it in a blue flame until there is no colour — this removes any contamination.
2. Dip the clean wire into the sample.
3. Hold the wire in the edge of a roaring (blue) Bunsen flame and observe the colour.

The flame colours you must know are:

Metal ion	Flame colour
Lithium, $\text{Li}^+$Li+	Crimson (red)
Sodium, $\text{Na}^+$Na+	Yellow
Potassium, $\text{K}^+$K+	Lilac (purple)
Calcium, $\text{Ca}^{2+}$Ca2+	Orange-red
Copper, $\text{Cu}^{2+}$Cu2+	Green

Exam Tip: Learn the five flame colours exactly — lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green. Sodium (yellow) and potassium (lilac) are different colours; do not confuse them.

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Why a Clean Wire Matters

The wire must be cleaned before each test because any leftover compound — especially a sodium salt — would give its own colour and mask the one you are trying to see. The intense yellow of sodium is so strong that even a trace contaminates the result, which is why the wire is dipped in acid and burned until colourless first.

For the same reason, flame tests are unreliable for mixtures: if two metal ions are present, the brighter colour (often sodium's yellow) can hide a weaker one, so you cannot be sure which ions are there. A flame test works best on a single compound.

Exam Tip: If a flame test gives an unexpected or no result, the most likely fault is a dirty wire. Always clean the wire in acid and burn it colourless first; the persistent yellow of sodium contamination is the usual culprit.

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Metal Hydroxide Precipitate Tests

The second method for identifying a metal cation is to add a few drops of sodium hydroxide solution to a solution of the compound. Many metal ions form an insoluble metal hydroxide, which appears as a coloured precipitate, and the colour identifies the metal:

Metal ion	Colour of hydroxide precipitate	Note
Copper(II), $\text{Cu}^{2+}$Cu2+	Blue
Iron(II), $\text{Fe}^{2+}$Fe2+	Green	Turns brown on standing in air
Iron(III), $\text{Fe}^{3+}$Fe3+	Brown
Aluminium, $\text{Al}^{3+}$Al3+	White	Redissolves in excess sodium hydroxide
Calcium, $\text{Ca}^{2+}$Ca2+	White	Does not redissolve
Magnesium, $\text{Mg}^{2+}$Mg2+	White	Does not redissolve

For example, copper(II) ions give a blue precipitate of copper(II) hydroxide:

$\text{Cu}^{2+} + 2\text{OH}^- \rightarrow \text{Cu(OH)}_2$Cu2++2OH−→Cu(OH)2​

and iron(III) ions give a brown precipitate, $\text{Fe}^{3+} + 3\text{OH}^- \rightarrow \text{Fe(OH)}_3$Fe3++3OH−→Fe(OH)3​.

The precipitate forms because the metal hydroxide is insoluble in water, so it drops out of solution as a solid. The number of hydroxide ions in the formula matches the charge on the metal ion: a $+2$+2 ion such as copper(II) needs two hydroxide ions, while a $+3$+3 ion such as iron(III) needs three. Only a few drops of sodium hydroxide are needed to see the colour, and the precipitate is best judged against a white background.

Exam Tip: Learn the four coloured/white results: copper(II) blue, iron(II) green, iron(III) brown, and aluminium, calcium and magnesium all white. The coloured ones (blue, green, brown) are easy to tell apart; the white ones need an extra step.

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Telling Apart the White Precipitates

Three ions — aluminium ($\text{Al}^{3+}$Al3+), calcium ($\text{Ca}^{2+}$Ca2+) and magnesium ($\text{Mg}^{2+}$Mg2+) — all give a white precipitate, so the colour alone cannot distinguish them. The trick is to add excess sodium hydroxide:

• Aluminium hydroxide redissolves in excess sodium hydroxide, so the white precipitate disappears and the solution goes clear again.
• Calcium and magnesium hydroxides do not redissolve — the white precipitate stays.

So if a white precipitate dissolves in excess NaOH, the ion is aluminium; if it stays, it is calcium or magnesium. (A flame test then separates those two: calcium gives an orange-red flame, whereas magnesium gives no characteristic flame colour.)

flowchart TD
  A["Add NaOH: white precipitate forms"] --> B{"Add excess NaOH —<br/>does it redissolve?"}
  B -->|"Yes, dissolves"| C["Aluminium, Al³⁺"]
  B -->|"No, stays"| D{"Flame test"}
  D -->|"Orange-red flame"| E["Calcium, Ca²⁺"]
  D -->|"No flame colour"| F["Magnesium, Mg²⁺"]

Exam Tip: To separate the three white precipitates, add excess sodium hydroxide: only aluminium hydroxide redissolves. Calcium and magnesium stay white and are then told apart by a flame test (calcium = orange-red).

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Why the Two Tests Complement Each Other

The flame test and the hydroxide test do not detect the same set of ions, which is why having both is so useful. The flame test is the only one of the two that distinguishes the Group 1 metals — lithium, sodium and potassium — because none of these forms a precipitate with sodium hydroxide (their hydroxides are soluble alkalis, so nothing visible happens). The hydroxide test, on the other hand, is the only way to identify iron(II) and iron(III), which do not have characteristic flame colours, and it is what separates them by colour (green vs brown).

For ions that both tests can detect — such as copper(II) (green flame and blue precipitate) or calcium (orange-red flame and a white precipitate that stays in excess) — running both gives a double confirmation, which is far more reliable than a single result. This is especially valuable for the white-precipitate ions: a flame test that gives an orange-red colour tells you a white precipitate is calcium rather than magnesium or aluminium, settling the ambiguity in one extra step.

Exam Tip: Choose the test to suit the ion. Group 1 metals (Li, Na, K) need a flame test; iron(II)/iron(III) need the hydroxide test; for ions detectable by both, run both for a more certain answer.

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A Note on Iron(II) Turning Brown