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Unit 8: Rates of reaction

Forming a precipitate — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 10 2 quizzes

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Forming a precipitate — HIA INTERNATIONAL notes: Define a precipitate, recognise precipitation reactions and describe how a precipitate is separated and collected.Precipitate = an insoluble solid formed when two solutions are mixed and react, or when a gas reacts with a solution.…

  • The mixture turns cloudy or coloured
  • A precipitate is a sign of a chemical reaction
  • Silver nitrate + calcium chloride → silver chloride + calcium nitrate
  • Iron(II) salts + sodium hydroxide
  • The colour of a precipitate
  • Calcium hydroxide + carbon dioxide → calcium carbonate + water

Section 1 of 8What a precipitate is

1Precipitate
Precipitate an insoluble solid formed when two solutions are mixed and react, or when a gas reacts with a solution.
2The mixture turns cloudy or coloured
  • The mixture turns cloudy or coloured and the solid slowly settles.
3A precipitate is a sign of a chemical reaction
  • A precipitate is a sign of a chemical reaction a new substance has been made.
4Precipitation reaction
  • Precipitation reaction two soluble salts swap partners to give an insoluble salt and a soluble salt.
    • (the precipitate)

Section 2 of 8Examples of precipitation reactions

Examples of precipitation reactions
Silver nitrate + calcium chloride → silver chloride + calcium nitrate
  • Silver nitrate + calcium chloride → silver chloride + calcium nitrate silver chloride is the white precipitate.
Silver nitrate + sodium chloride → silver chloride (white precipitate) + sodium nitrate.
Copper sulfate + sodium hydroxide → copper hydroxide (blue precipitate) + sodium sulfate.
Iron(II) salts + sodium hydroxide
  • Iron(II) salts + sodium hydroxide green precipitate; iron(III) salts give an orange-brown precipitate.
The colour of a precipitate
  • The colour of a precipitate can help to identify metal ions.

Section 3 of 8Limewater and carbon dioxide

Limewater and carbon dioxide
Limewater
Limewater a solution of calcium hydroxide.
Calcium hydroxide + carbon dioxide → calcium carbonate + water
  • Calcium hydroxide + carbon dioxide → calcium carbonate + water a gas is added to a liquid and a solid forms.
The white precipitate of calcium carbonate
  • The white precipitate of calcium carbonate makes the limewater turn cloudy or milky, which is the test for carbon dioxide.

Section 4 of 8Solubility rules

Soluble
  • Soluble all sodium, potassium and ammonium salts, and all nitrates.
Insoluble
  • Insoluble most hydroxides and carbonates of other metals.
Insoluble
  • Insoluble silver chloride and barium sulfate.

Section 5 of 8Making and collecting a precipitate

1
Mix the two solutions in a beaker and stir.
2
Observe the cloudy precipitate forming.
3
Filter the mixture through filter paper in a funnel; the precipitate stays as the residue and the solution passes through as the filtrate.
4
Wash the residue with distilled water.
5
Dry the precipitate in a warm oven.
6
Weigh the dry precipitate.

Section 6 of 8Uses of precipitation

Testing for ions
  • Testing for ions for example the chloride test uses acidified silver nitrate.
Removing pollutants from water.
Making insoluble pigments.
Mass is conserved
  • Mass is conserved the total mass before mixing equals the total mass after, in a closed system.

Section 7 of 8Key terms

TermMeaning
PrecipitateAn insoluble solid formed when solutions react.
Precipitation reactionA reaction in which an insoluble solid forms from solutions.
ResidueThe solid left on the filter paper.
FiltrateThe liquid that passes through the filter paper.
LimewaterA solution of calcium hydroxide that turns cloudy with carbon dioxide.
SolubleAble to dissolve in a liquid.
InsolubleUnable to dissolve in a liquid.
FiltrationSeparating an insoluble solid from a liquid using filter paper.

Section 8 of 8Exam technique

1
Saying a precipitate is a gas — a precipitate is an insoluble solid.
2
Saying the precipitate passes through the filter paper — it stays on the paper as the residue.
3
Forgetting that a precipitate shows a reaction — a new insoluble substance has been made.
4
Thinking cloudy limewater shows oxygen — it shows carbon dioxide.

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