Reactions of ions in aqueous solution (A-level only) — AQA A-Level Chemistry
Test yourself on Reactions of ions in aqueous solution (A-level only) with AQA A-Level practice questions.
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Reactions of ions in aqueous solution (A-level only) explained
The practical is a fixed sequence run on each unknown: add sodium hydroxide solution dropwise and then in excess, add ammonia solution dropwise and then in excess, and add sodium carbonate solution.
Read the full explanation
With sodium hydroxide, copper(II) gives a blue precipitate, iron(II) a green one that darkens in air, iron(III) a brown one, and aluminium and chromium(III) give white and green precipitates that redissolve in excess. With ammonia, the copper precipitate redissolves in excess to a deep blue solution while the aluminium one does not. With sodium carbonate, 2⁺ ions give a carbonate precipitate but 3⁺ ions fizz, releasing CO₂. Name reagents fully, because AQA gives no credit for OH⁻ where sodium hydroxide is meant.
Your focus
- Identify Cu²⁺, Fe²⁺, Fe³⁺, Al³⁺ and Cr³⁺ from their reactions with sodium hydroxide and with ammonia.
- Record separate observations for dropwise addition and for addition of excess reagent.
- Name reagents completely, giving sodium hydroxide rather than hydroxide ion in every answer.
Reactions of ions in aqueous solution (A-level only) exam tips
Marking Points
- one mark for naming a complete reagent, such as sodium hydroxide solution rather than the hydroxide ion
- one mark for the colour of the precipitate formed with a named metal ion
- one mark for stating whether that precipitate dissolves in excess reagent and the colour of the resulting solution
- one mark for a balanced ionic equation for the precipitation, or for the effervescence with sodium carbonate
Examiner Tips
- 💡Add reagent dropwise first and record that observation before adding excess: two observations, two marks.
- 💡Write the full name or full formula of the reagent every time.
- 💡Say precipitate and give its colour, then say separately what happens in excess.
Common Mistakes
- writing OH⁻ or CN⁻ as the reagent, which earns nothing because a reagent must be a chemical taken from a bottle
- confusing the ammonia result with the sodium hydroxide result for aluminium, which redissolves only in excess sodium hydroxide
- describing a precipitate colour as a solution colour, such as calling the iron(III) result an orange solution
- adding all the reagent at once, so missing the initial precipitate that later dissolves