Salts: types, solubility and uses: WASSCE Chemistry

"Salt" to most people means the white crystals on the dinner table. To a chemist, sodium chloride is just one member of a huge family that includes copper(II) sulphate for spraying crops, ammonium nitrate fertiliser, baking soda and the alum used to clear muddy water. This lesson classifies salts, gives the rules for predicting whether they dissolve, and describes their uses.

Learning objectives

By the end of this lesson you should be able to:

define a salt and name salts from their parent acid and base
classify salts as normal, acid, basic, double or complex
use the solubility rules to predict whether a salt is soluble
state the uses of important salts

What a salt is

Definition

A salt is a compound formed when the replaceable hydrogen ions of an acid are wholly or partly replaced by a metal ion or an ammonium ion.

A salt's name comes from its metal (or ammonium) and its parent acid:

Acid	Salt name ends in	Example
Hydrochloric acid, HCl	-chloride	Sodium chloride, NaCl
Sulphuric acid, H₂SO₄	-sulphate (or hydrogensulphate)	Copper(II) sulphate, CuSO₄
Nitric acid, HNO₃	-nitrate	Potassium nitrate, KNO₃
Carbonic acid, H₂CO₃	-carbonate (or hydrogencarbonate)	Calcium carbonate, CaCO₃
Ethanoic acid, CH₃COOH	-ethanoate	Sodium ethanoate, CH₃COONa
Phosphoric(V) acid, H₃PO₄	-phosphate	Calcium phosphate, Ca₃(PO₄)₂

Types of salts

Type	Description	Examples
Normal salt	All the replaceable hydrogen of the acid has been replaced	NaCl, Na₂SO₄, CaCO₃, K₂CO₃
Acid salt	Only part of the replaceable hydrogen has been replaced; it still contains replaceable H	NaHSO₄ (sodium hydrogensulphate), NaHCO₃ (sodium hydrogencarbonate), Ca(HCO₃)₂
Basic salt	Contains hydroxide (or oxide) as well as the usual anion; formed when a base is only partly neutralised	Zn(OH)Cl, Mg(OH)Cl, basic lead(II) carbonate
Double salt	Two normal salts crystallised together in fixed proportions; gives the ions of both in solution	Potash alum, K₂SO₄·Al₂(SO₄)₃·24H₂O; ammonium iron(II) sulphate
Complex salt	Contains a complex ion, a central metal ion bonded to molecules or ions	Na[Al(OH)₄] (sodium aluminate); [Cu(NH₃)₄]SO₄

Only dibasic or tribasic acids can form acid salts. Sulphuric acid forms both NaHSO₄ (acid salt) and Na₂SO₄ (normal salt); monobasic HCl can form only NaCl.

Solubility rules

Salts	Solubility in water
All sodium, potassium and ammonium salts	Soluble
All nitrates	Soluble
Chlorides	Soluble, except silver chloride (AgCl) and lead(II) chloride (PbCl₂, which dissolves in hot water)
Sulphates	Soluble, except barium sulphate (BaSO₄) and lead(II) sulphate (PbSO₄); calcium sulphate is only slightly soluble
Carbonates	Insoluble, except those of sodium, potassium and ammonium
Hydroxides	Insoluble, except those of sodium, potassium and ammonium; calcium hydroxide is slightly soluble

Worked example

Predicting solubility. Is each salt soluble?

Salt	Rule applied	Soluble?
Lead(II) nitrate	All nitrates are soluble	Yes
Barium sulphate	Exception among sulphates	No
Ammonium carbonate	All ammonium salts are soluble	Yes
Calcium carbonate	Carbonates are insoluble (Ca is not Na, K or NH₄)	No
Silver chloride	Exception among chlorides	No

Whether a salt is soluble decides how it is prepared (next lesson).

Uses of salts

Salt	Uses
Sodium chloride (common salt)	Seasoning and preserving food (salted fish); raw material for sodium hydroxide and chlorine; oral rehydration salts
Sodium carbonate (washing soda)	Softening hard water; making glass, soap and paper
Baking soda, NaHCO₃	Baking powder (gives CO₂ to raise bread and cakes); antacid; fire extinguishers
Calcium carbonate (limestone)	Making cement, lime and glass; antacid tablets
Calcium sulphate (plaster of Paris)	Plaster casts for broken bones; building
Copper(II) sulphate	Fungicide (Bordeaux mixture) for crops; test for water; electroplating
Ammonium nitrate, ammonium sulphate, potassium nitrate	Fertilisers; KNO₃ and NH₄NO₃ also in explosives
Ammonium chloride	Electrolyte in dry cells; soldering flux
Potash alum	Clearing muddy water (coagulation); dyeing; stopping bleeding from small cuts
Magnesium sulphate (Epsom salt)	Laxative
Silver bromide	Photographic film
Iron(II) sulphate	Iron tablets for anaemia

In Sierra Leone

Along the coast of Sierra Leone, families produce sea salt by boiling or sun-drying seawater, a traditional source of income. Potash alum is sold in markets and used to clear muddy well and river water before use: it coagulates the fine clay particles so they settle out. The water still needs boiling or chlorinating to kill germs.

Common mistake

Sodium hydrogencarbonate, NaHCO₃, is an acid salt, even though its solution is slightly alkaline. "Acid salt" means it still has a replaceable hydrogen, not that its solution is acidic.

Key points

A salt forms when the replaceable hydrogen of an acid is wholly or partly replaced by a metal or ammonium ion.
Types: normal (all H replaced), acid (some H left: NaHSO₄, NaHCO₃), basic (contains OH⁻), double (alum) and complex (complex ion).
Solubility rules: all Na, K, NH₄ salts and all nitrates dissolve; chlorides except AgCl, PbCl₂; sulphates except BaSO₄, PbSO₄ (CaSO₄ slightly); carbonates are insoluble except Na, K, NH₄.
Salts are used in food, farming, medicine, water treatment and industry.

WASSCE examination focus

WAEC tests this

Expect to define a salt, to classify given salts (normal, acid, basic, double), to use solubility rules to predict soluble and insoluble salts, and to state uses of named salts such as NaHCO₃, CuSO₄, alum and plaster of Paris.

Quick check

Define an acid salt and give an example.
Which of these is insoluble in water: KCl, PbSO₄, NH₄NO₃, Na₂CO₃?
Give one use of potash alum.
Why can hydrochloric acid not form an acid salt?

Answers:

A salt in which only part of the replaceable hydrogen of an acid has been replaced, e.g. NaHSO₄ or NaHCO₃.
PbSO₄.
Any one: clearing (coagulating) muddy water; dyeing; stopping bleeding from small cuts.
It is monobasic: it has only one replaceable hydrogen, so replacing it gives a normal salt.

Salts: types, solubility and uses