CHEMISTRY · Stage 7 · Chapter 4: Chemical Reactions 1
Chemistry 4.4 Precipitation reactions — Insoluble Solids from Solutions
5 min read 8 parts Lesson 4 of 6
Lesson summary
4.4 Precipitation reactions — Insoluble Solids from Solutions — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe and explain precipitation reactions, predict whether a precipitate forms and explain how precipitation reactions are useful.Precipitation reacti…
- Soluble and insoluble
- Other pairs
- Potassium iodide and lead nitrate
- Silver nitrate and sodium chloride
- Cobalt sulfate and sodium hydrogencarbonate solutions
- Change in properties
Part 1 of 8What a precipitation reaction is
- Precipitation reaction = a reaction in which two reactants in solution react to make a precipitate.
- Precipitate = a suspension of tiny solid particles in a liquid, which forms when one product is insoluble.
- It appears as a cloudy suspension that gradually settles at the bottom.
- Soluble and insoluble — a soluble substance dissolves and an insoluble substance does not.
- Cause — particles in the two solutions combine to make a compound that cannot stay dissolved.
- Other pairs — other pairs of soluble substances react to make a precipitate if one product is insoluble.
Part 2 of 8Examples
- Potassium iodide and lead nitrate — colourless solutions make tiny pieces of a yellow lead iodide precipitate, and potassium nitrate stays dissolved.
- potassium iodide + lead nitrate → lead iodide + potassium nitrate.
- Silver nitrate and sodium chloride — a white precipitate of silver chloride forms, and sodium nitrate stays dissolved.
- Cobalt sulfate and sodium hydrogencarbonate solutions — they make a cobalt carbonate precipitate.
- Change in properties — colour and solubility differ between the reactants and the products.
Part 3 of 8Other types of reaction
- Combustion reactiona substance reacts quickly with oxygen and gives out light and heat, for example burning magnesium.
- Precipitation reactiontwo solutions react to form an insoluble solid.
- All typesatoms are rearranged and join together differently to make new substances.
Part 4 of 8Predicting precipitates
- Sodium, potassium and ammonium salts are all soluble.
- All nitrates are soluble.
- Most chlorides are soluble, except silver chloride and lead chloride.
- Most sulfates are soluble, except barium sulfate, lead sulfate and calcium sulfate (slightly).
- Most carbonates and hydroxides are insoluble, except those of sodium, potassium and ammonium.
- Method — swap the partners of the two mixed solutions and check whether either new compound is insoluble.
- Barium chloride and sodium sulfate give a white precipitate of barium sulfate.
- Copper sulfate and sodium hydroxide give a pale blue precipitate of copper hydroxide.
- Sodium carbonate and calcium chloride give a white precipitate of calcium carbonate.
- Potassium nitrate and sodium chloride give two soluble salts, so no precipitate forms.
Part 5 of 8Identifying metals
- Testadd a few drops of sodium hydroxide solution to a solution of an unknown compound.
- Blue precipitatea copper compound, as in copper chloride + sodium hydroxide → copper hydroxide + sodium chloride.
- Dark green or brown precipitatean iron compound, with green for iron(II) and brown for iron(III).
- Light green precipitatea nickel compound.
| Reactants | Precipitate | Colour |
|---|---|---|
| Silver nitrate + sodium chloride | Silver chloride | White |
| Lead nitrate + potassium iodide | Lead iodide | Yellow |
| Copper sulfate + sodium hydroxide | Copper hydroxide | Blue |
Part 6 of 8Using precipitates
- Separation — filter the precipitate, wash it with distilled water to remove soluble impurities and dry it, which is a way to make insoluble salts.
- Waste water — precipitation removes harmful metal ions in industry.
Part 7 of 8Spectator ions
- Reacting ionsthe metal particles and hydroxide particles find each other and stick together as a solid.
- Spectator ions ions that stay dissolved and do not take part, such as sodium and sulfate in copper sulfate with sodium hydroxide.
- Amountsadding more of the same solution gives more precipitate until one reactant is used up.
Part 8 of 8Safety
- Lead nitrate carries hazard symbols, so wear gloves and eye protection and do not pour the solution down the sink.
- Hazard symbolsthey show that a substance can help other substances to burn (oxidising), is a health hazard, can burn skin and eyes (corrosive) or damages things that live in water.
- Lead and silver compounds are toxic or stain skin, so dispose of waste as instructed.
- Evidencethe cloudy solid in a clear liquid is evidence of a chemical change, and total mass is unchanged.
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