What is a spectator ion?

An ion that is in the solution before and after and takes no part in the reaction. In the lead iodide example, K+ and NO3- are spectators.

How do I write the ionic equation?

Write every soluble ionic substance as separate ions, keep the precipitate as one formula, then delete anything identical on both sides.

Why did nothing happen?

Because both possible products were soluble. No precipitate means no visible reaction, which is a perfectly good observation to record.

Skip to the experiment

MrChemCoach

Double Displacement Reactions

IGCSE 0620 · CBSE Class 10 · Precipitation

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Step 1 of 4Pour the first solution into the beaker

Double displacement in brief

Two soluble salts swap partners in solution. If one of the new pairings is insoluble, it drops out as a precipitate and you see it. If both stay dissolved, nothing seems to happen, and that is a result too.

AB(aq) + CD(aq) → AD(s)↓ + CB(aq)

Spectator ions are in the solution before and after, and take no part. Cross them out to get the ionic equation.
Writing the ionic equation: split every soluble ionic substance into ions, keep the precipitate as one formula, then delete anything identical on both sides.
Where marks go: the colour of the precipitate, and state symbols: (s) for the solid and (aq) for everything else.

The six reactions in this lab
Mix	Precipitate	What it looks like
BaCl₂ + Na₂SO₄	BaSO₄	Thick white. The test for sulfate ions.
Pb(NO₃)₂ + KI	PbI₂	Bright yellow, the “golden rain”.
CuSO₄ + NaOH	Cu(OH)₂	Pale blue and jelly-like. The test for Cu²⁺.
FeCl₃ + NaOH	Fe(OH)₃	Reddish-brown, the colour of rust.
AgNO₃ + NaCl	AgCl	White and curdy. The test for chloride ions.
Na₂CO₃ + CaCl₂	CaCO₃	Fine white, like chalk.

For revisionTheory, method and exam tips

What this experiment shows

In a double displacement reaction two soluble salts swap partners in solution. If one of the possible new pairings is insoluble, it drops out as a precipitate and you see the change; if both are soluble, nothing appears to happen at all. Knowing the solubility rules is therefore the whole skill: they tell you in advance whether you will get a precipitate, and what colour it will be.

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)

What you need

Test tubes and a rack
Dilute solutions of the two salts
A teat pipette
A white tile, so a pale precipitate still shows

How it is done

Put about 2 cm depth of the first solution in a clean test tube.
Add the second solution a few drops at a time.
Watch against a white background and note any colour and any solid that forms.
Add more of the second solution to check whether the precipitate redissolves.
Record the colour of the precipitate, not just the fact that there was one.

What you should see

Lead(II) nitrate with potassium iodide gives a bright yellow precipitate of lead(II) iodide.
Silver nitrate with sodium chloride gives a white precipitate of silver chloride.
Copper(II) sulfate with sodium hydroxide gives a pale blue precipitate of copper(II) hydroxide.
If both possible products are soluble, the mixture stays clear - and that is a result too.

Where marks are lost

Writing "a precipitate forms" with no colour. The colour is usually the mark.
Forgetting state symbols. The precipitate is (s); everything else is (aq).
Leaving the spectator ions in an ionic equation. They appear unchanged on both sides, so cross them out.
Adding so much of the second solution at once that a colour change is missed.

Questions students ask

What is a spectator ion?: An ion that is in the solution before and after and takes no part in the reaction. In the lead iodide example, K⁺ and NO₃^- are spectators.
How do I write the ionic equation?: Write every soluble ionic substance as separate ions, keep the precipitate as one formula, then delete anything identical on both sides.
Why did nothing happen?: Because both possible products were soluble. No precipitate means no visible reaction, which is a perfectly good observation to record.

Try these next

Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.