4.4Chapter 4: Chemical Reactions 1

Precipitation reactions — Alhussan Chemistry - Stage 7 Revision Notes & Topical Questions

5 min read 10 parts Lesson 4 of 6 3 quizzes

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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

Section 1 of 10What a precipitation reaction is

Precipitation reaction

Precipitation reaction a reaction in which two reactants in solution react to make a precipitate.

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

Soluble and insoluble a soluble substance dissolves and an insoluble substance does not.

Cause

Cause particles in the two solutions combine to make a compound that cannot stay dissolved.

Other pairs

Other pairs other pairs of soluble substances react to make a precipitate if one product is insoluble.

Section 2 of 10Examples

1Potassium iodide and lead nitrate

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.

2Silver nitrate and sodium chloride

Silver nitrate and sodium chloride a white precipitate of silver chloride forms, and sodium nitrate stays dissolved.

3Cobalt sulfate and sodium hydrogencarbonate solutions

Cobalt sulfate and sodium hydrogencarbonate solutions they make a cobalt carbonate precipitate.

4Change in properties

Change in properties colour and solubility differ between the reactants and the products.

Section 3 of 10Other types of reaction

Other types of reaction

Combustion reaction

Combustion reaction a substance reacts quickly with oxygen and gives out light and heat, for example burning magnesium.

Precipitation reaction

Precipitation reaction two solutions react to form an insoluble solid.

All types

All types atoms are rearranged and join together differently to make new substances.

Section 4 of 10Predicting 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

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.

Section 5 of 10Identifying metals

1Test

Test add a few drops of sodium hydroxide solution to a solution of an unknown compound.

2Blue precipitate

Blue precipitate a copper compound, as in copper chloride + sodium hydroxide → copper hydroxide + sodium chloride.

3Dark green or brown precipitate

Dark green or brown precipitate an iron compound, with green for iron(II) and brown for iron(III).

4Light green precipitate

Light green precipitate a 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

Section 6 of 10Using precipitates

Separation

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

Waste water precipitation removes harmful metal ions in industry.

Section 7 of 10Spectator ions

Reacting ions

Reacting ions the metal particles and hydroxide particles find each other and stick together as a solid.

Spectator ions

Spectator ions ions that stay dissolved and do not take part, such as sodium and sulfate in copper sulfate with sodium hydroxide.

Amounts

Amounts adding more of the same solution gives more precipitate until one reactant is used up.

Section 8 of 10Safety

Lead nitrate carries hazard symbols, so wear gloves and eye protection and do not pour the solution down the sink.

Hazard symbols

Hazard symbols they 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.

Evidence

Evidence the cloudy solid in a clear liquid is evidence of a chemical change, and total mass is unchanged.

Section 9 of 10Key terms

Term	Meaning
Precipitation reaction	A reaction between solutions that makes an insoluble solid.
Precipitate	A suspension of tiny solid particles formed in a reaction.
Soluble	Able to dissolve.
Insoluble	Not able to dissolve.
Spectator ion	An ion that stays dissolved and takes no part in a reaction.
Hazard symbol	A symbol showing how a substance can cause harm.