Testing Cations and Anions

Section 1 of 8The flame test for cations

1

Clean a loop of unreactive wire, such as nichrome or platinum, by dipping it in dilute acid.

2

Hold the loop in the hot blue part of a Bunsen flame until it gives no colour.

3

Dip the clean loop into the solid sample or its solution.

4

Place the loop at the edge of the blue flame and note the colour.

5

Compare the colour with known results to name the metal ion.

Section 2 of 8Flame colours to learn

Metal ion	Flame colour
Lithium, Li+	Red
Sodium, Na+	Yellow-orange
Potassium, K+	Lilac
Calcium, Ca2+	Brick-red
Barium, Ba2+	Apple-green

Section 3 of 8Why the method matters

Cleaning in acid

• Cleaning in acid Any ions left on the wire from an earlier test would add their own colour.
• A mixture of colours can hide the true ion, so the wire is cleaned until it gives no colour at all.

Using the blue flame

• Using the blue flame The blue Bunsen flame is hottest and gives little colour of its own, so the colour from the sample is easier to see.

Precise wording

• Precise wording Marks are only given for the exact colour, so write apple-green for barium rather than just green.

Section 4 of 8Testing for halide ions

1Add silver nitrate

• Add silver nitrate Add a few drops of silver nitrate solution, AgNO₃, to a solution of the sample.

2Silver ions meet halide ions

• Silver ions meet halide ions Ag⁺ ions combine with Cl^-, Br^- or I^- ions in solution.

3Precipitate forms

• Precipitate forms An insoluble silver halide is made, so the mixture turns cloudy.

4Colour identifies the halide

• Colour identifies the halide The colour of the solid shows which halide ion was present.

Section 5 of 8Halide results and equations

Halide ion	Precipitate	Ionic equation
Chloride	White silver chloride	Ag+(aq) + Cl-(aq) → AgCl(s)
Bromide	Cream silver bromide	Ag+(aq) + Br-(aq) → AgBr(s)
Iodide	Yellow silver iodide	Ag+(aq) + I-(aq) → AgI(s)

Section 6 of 8Worked equation with potassium chloride

Full equation

• Full equation potassium chloride + silver nitrate → potassium nitrate + silver chloride.
• KCl(aq) + AgNO₃(aq) → KNO₃(aq) + AgCl(s)

Ionic equation

• Ionic equation Only the silver and chloride ions change, so the ionic equation is Ag⁺(aq) + Cl^-(aq) → AgCl(s).
• The general form for any halide X^- is Ag⁺(aq) + X^-(aq) → AgX(s).

Spectator ions

• Spectator ions Potassium and nitrate ions stay dissolved and unchanged, so they are spectator ions and are left out of the ionic equation.
• State symbols are needed to show which substance is the solid precipitate, (s).

Section 7 of 8Key terms

Term	Meaning
Cation	A positively charged ion, such as a metal ion.
Anion	A negatively charged ion, such as a halide ion.
Flame test	A method that identifies certain metal ions by the colour they give to a flame.
Precipitate	An insoluble solid formed when two solutions react.
Spectator ion	An ion present in a reaction mixture that does not take part in the reaction.

Section 8 of 8Exam technique

1

Describing the barium flame as just green - the required answer is apple-green.

2

Skipping the acid-cleaning step, so leftover ions give a mixed or misleading colour.

3

Leaving out state symbols, especially (s) for the precipitate, in ionic equations.

4

Including the potassium and nitrate ions in the ionic equation instead of treating them as spectators.

5

Mixing up the precipitate colours - chloride is white, bromide is cream and iodide is yellow.