Required practical: identifying ions

GCSE Chemistry, Chemical analysis, 25 minutes

Flame tests, metal hydroxide precipitates, and the tests for carbonate, halide and sulfate ions, put together to name an unknown salt.

What you will learn

Identify metal ions from flame colours and hydroxide precipitates
Identify carbonate, halide and sulfate ions
Deduce the name and formula of a compound from a results table

Positive ions: flames and precipitates

Flame test: clean a nichrome loop in hydrochloric acid and heat it until it gives no colour, dip it in the solid and hold it in a blue Bunsen flame.

lithium: crimson, sodium: yellow, potassium: lilac, calcium: orange-red, copper(II): green

Sodium hydroxide solution, a few drops then in excess:

aluminium, calcium, magnesium: white precipitate (only aluminium hydroxide dissolves in excess)
copper(II): blue, iron(II): green, iron(III): brown

In a mixture one flame colour can hide another: a trace of sodium's yellow swamps potassium's lilac.

The loop is nichrome (or platinum) because it does not colour the flame itself and does not react in it, and it is cleaned between tests so no ions from an earlier sample colour the flame.

To tell the three white precipitates apart, add more sodium hydroxide, then use a flame test: aluminium's dissolves in excess; calcium's does not, and calcium gives an orange-red flame; magnesium's does not dissolve either, and magnesium gives no flame colour.

The rest of this lesson and its 11 practice questions are locked.

In this lesson

Positive ions: flames and precipitates
Negative ions
Putting the results together
Try it: Flame colours

11 practice questions, marked as you go; about 25 minutes in all.

Key terms

Crimson, Yellow, Lilac, Green, Sodium hydroxide solution, White, Blue, Brown, Nitric, Cream

In this unit: Chemical analysis

Purity and formulations, chromatography and Rf values, tests for gases, and for separate Chemistry the tests for ions and flame emission spectroscopy.

All of Chemical analysis

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