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16.5Unit 8: Chemical reactions and neutralisation

Forming a precipitate — Cambridge Checkpoint Science 0893 Revision Notes & Topical Questions

5 min read 7 parts Lesson 5 of 5 1 quiz

📚 Cambridge Checkpoint Science 0893 Hub / Topical Revision
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Lesson summary

16.5 Forming a precipitate — CHECKPOINT (CHEMISTRY) notes: Describe how a precipitate forms, how it is separated, and how precipitate colours identify ions.Precipitate — an insoluble solid formed when two solutions react.The mixture turns cloudy.Solubility — soluble substances di…

  • Metal ions
  • Chloride ions
  • Sulfate ions
  • Filtration
  • Solubility — soluble substances dissolve in water and insoluble substances do not
  • Mix two clear solutions

Part 1 of 5What a precipitate is

Precipitate
  • Precipitate an insoluble solid formed when two solutions react.
  • The mixture turns cloudy.
Solubility
  • Solubility soluble substances dissolve in water and insoluble substances do not.

Part 2 of 5Making and separating a precipitate

1
Mix two clear solutions.
2
An insoluble solid appears and the mixture turns cloudy.
3
Filter the mixture.
4
The precipitate stays in the filter paper as the residue.
5
Wash the residue and dry it.
  • Filtrate
    • the liquid that passes through the filter paper.

Part 3 of 5Common precipitation reactions

Solution ASolution BPrecipitateColour
Copper sulfateSodium hydroxideCopper hydroxideBlue
Iron(II) sulfateSodium hydroxideIron(II) hydroxideGreen
Iron(III) chlorideSodium hydroxideIron(III) hydroxideOrange-brown
Silver nitrateSodium chlorideSilver chlorideWhite
Barium chlorideSodium sulfateBarium sulfateWhite

Part 4 of 5Using precipitates to test for ions

Metal ions
  • Metal ions sodium hydroxide gives coloured hydroxide precipitates that identify copper and iron.
Chloride ions
  • Chloride ions silver nitrate gives a white precipitate.
Sulfate ions
  • Sulfate ions barium chloride gives a white precipitate.

Part 5 of 5Limewater test

  • Reaction
    • carbon dioxide reacts with limewater to form insoluble white calcium carbonate: Ca(OH)2 + CO2 → CaCO3 + H2O.
    • This makes the limewater milky.

Key terms

TermMeaning
PrecipitateInsoluble solid formed when two solutions react.
SolubleAble to dissolve in water.
InsolubleUnable to dissolve in water.
FiltrationSeparating an insoluble solid from a liquid.
ResidueSolid left on the filter paper.
FiltrateLiquid that passes through the filter paper.

Avoid these mistakes

1
Saying the precipitate passes through the filter paper — it stays as the residue.
2
Mixing up residue and filtrate.
3
Saying a precipitate is a gas — it is an insoluble solid.
4
Saying all precipitates are white — colours differ.

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