Page 100 - Chemistry_Form_2

Chemistry
for Secondary Schools

This type of reaction is also referred to as a double displacement reaction. An
example of a precipitation reaction is the reaction between an aqueous solution
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of silver nitrate and an aqueous solution of sodium chloride to form white
precipitates of silver chloride.

Another example is when an aqueous solution of sodium sulfate is mixed with an
aqueous solution of barium chloride to form solid barium sulfate.

Precipitation reactions are common in various everyday activities. Some of the
activities that relate to precipitation reactions include:

(a) Formation of soap scum: When soap is used in hard water (which contains
calcium and magnesium ions), it reacts with these ions to form insoluble
soap scum.
(b) Curdling of milk: When lemon juice or vinegar (acid) is added to milk, the
casein proteins in the milk precipitate out, causing it to curdle.

(c) Rainwater formation: In cloud seeding, silver iodide (AgI) is introduced
into clouds, where it reacts with water droplets to form solid ice crystals,
leading to rainfall.

(d) Treating wastewater: In water treatment, chemicals like aluminium sulfate
(Al (SO ) ) and lime (Ca(OH) ) are added to wastewater to precipitate out
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harmful substances.
(e) Formation of kidney stones (calcium oxalate crystals): In some individuals,
excess calcium and oxalate in the urine combine to form calcium oxalate
precipitates, which develop into kidney stones.

Redox reactions

A redox reaction is a chemical reaction in which the oxidation number or
oxidation state of the participating chemical species changes by losing or gaining
one or more electron(s). Redox is a short form for reduction-oxidation reaction.
Redox reactions are common and important to some of the basic functions of

Student’s Book Form Two 93

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