SALTS
Lesson outline/Objectives
Salt in daily life
definition and examples of salt
Classification of salt (soluble and Insoluble salt)
Types of salt
preparation and properties of salt
Deliquescent ,efflorescent and hygroscopic properties of salt
drying agent
recovery of salt from solution
hydrolysis of salt.
colours of salt
Neutralization reaction
SALT IN DAILY LIFE
1. NH Cl is used as an electrolyte in dry cell (Leclanche cell)
4
2. CaCO is used as medicine to neutralise acidity in the stomach
3
3. CaCl is used as antifreeze while fused CaCl is used as a drying agent and also in dessicator.
2 2
4. CaSO is used for making plaster of Paris.
4
5. CuSO is used in dyeing and calico printing.
4
6. MgSO is used as a laxative.
4
7. KNO is used for making gunpowder, matches and soil fertilizer.
3
8. NaCl is used for preserving food and in glazing pottery.
9. ZnCl is used in petroleum refining
2
DEFINITION AND EXAMPLES OF SALT
A salt is referred to as the compound formed when all or part of the ionisable hydrogen ion in an
acid is replaced by a metallic or ammonium ion. Examples of salt are NaCl, ZnSO 4,KHSO4.
METHOD OF PREPARATION OF SALTS
The method of preparing a particular salt depends on its:
1. Solubility in water
2. Stability to heat.
SOLUBLE SALT
Soluble salts can be prepared by the following method:
1. Neutralization of an acid by an alkali
2. Action of dilute acid on a metal.
3. Action of dilute acid on an insoluble base.
4. Action of dilute acid on trioxocarbonate (IV)..
INSOLUBLE SALTS
Insoluble salts can be prepared by the following method:
1. Double decomposition or precipitation.
Pb (NO ) 3 2(aq) + 2NaCl (aq) 2NaNO 3 (aq) + PbCl 2(s)
AgNO 3 (aq) + NH Cl
4 (aq) NH NO 4 3 (aq) + AgCl (s)
2. Direct combination of 2 elements.
Fe (s) + S (s) FeS (s)
2Fe( + 3Cl s) 2(g) 2FeCl 3(s)
SALT SOLUBILITY RULE
S/NO SOLUBLE SALTS INSOLUBLE SALTS
1. All Na , K and NH salt
+ +
4
+
2. All trioxonitrate (v)
3. All chloride except PbCl , HgCl and AgCl are soluble in hot
2 2
water.
4. Trioxocarbonate (iv) of Na , K and + +
All other trioxocarbonate (iv).
NH 4
+
5. Trioxosulphate (vi) of Na , K , NH + +
4
+
All other trioxosulphate (IV)
& Cu 2+
6. Sulphide of Na , K and NH + +
4
+
All other sulphide.
7. All tetraoxosulphate (vi) except PbSO , BaSO and CaSO are slightly soluble
4 4 4
in H O2
8. All hydrogen trioxocarbonate (iv)
TYPES OF SALTS
There are five main types of salts namely:
1. Normal salt.
2. Acid salts
3. Basic salts
4. Double salts.
5. Complex salts.
1. Normal salts: are the salts formed when all the replaceable hydrogen ion in the acid has been
completely replaced by a metal ion e.g. NaCl, K SO , Na PO , NaNO etc. Normal salts are 2 4 3 4 3
neutral to litmus
HCl (aq) + NaOH (aq) NaCl (aq) + HO 2 (l)
H SO + KOH
2 4(aq) (aq) K SO + H O
2 4(aq) 2 (aq)
2. Acid salts: Acid salts are formed when the replaceable hydrogen ion in the acids are only
partially replaced by a metal e.g. NaHSO , Na HPO , NaH PO , NaHCO . They can be produce 4 2 4 2 4 3
from acids which contain more than one replaceable hydrogen ion. Acids with two replaceable
hydrogen ions can form only one acid salt while acid with three replaceable hydrogen ions can
form two different acid salts
H SO
2 4(aq) + NaOH (aq) NaHSO + H O 4(aq) 2 (l)
2H PO + 3NaOH
3 4(aq) (aq) NaH PO + Na HPO +3H O
2 4(aq) 2 4(aq) 2 (l)
Acid salts turn blue litmus red. Acid salts can be converted to normal salt if the remaining
replaceable hydrogen ions in the acid salt are replaced in with metallic ions.
KHSO + 4(aq) KOH (aq) K SO 2 4(aq) + HO 2 (l)
3. Basic salts: Basic salts are formed when only part of the hydroxide ions of a base are replaced
by the negative ions from an acid. It can occur when there is insufficient supply of acid for
complete neutralization of the base e.g Zn(OH)Cl , Mg(OH)Cl, Mg(OH)NO , Bi(OH) NO e. t .c. 3 2 3
Zn(OH) 2(aq) + HCl (aq) Zn(OH)Cl (aq) + HO 2 (l)
Because of the presence of hydroxide ion in the salt, it has basic properties. Basic salts turn red
litmus blue. Basic salts react with excess acid to form a normal salt and water only.
Mg(OH)NO + HNO 3(aq) 3(aq) Mg(NO ) 3 2(aq) + HO 2 (l)
4. Double salts: Double salts are salt which ionize to produce three different types of ions in
solution. Usually two of these are positively charged (metallic or NH4 ion) while the other is +
negatively charged e.g. (NH ) Fe(SO ) .6H O, KAl(SO ) .12H O, KCr(SO ) .12H O.
4 2 4 2 2 4 2 2 4 2 2
(NH ) Fe(SO ) .6H O: Ammonium iron (II) tetraoxosulphate (VI) hexahydrate.
4 2 4 2 2
KAl (SO ) .12H O: Aluminium Potassium tetraoxosulphate (V) dodecahydrate (Potash alum).
4 2 2
KCr (SO ) .12H O: Chromium (III) Potassium tetraoxosulphate (VI) dodecahydrate (Chrome
4 2 2
alum).
5. Complex salts: Complex salts contains complex ion i.e ion consisting of a charged group of
atom e.g. Na Zn(OH) , K Fe(CN) , NaAl(OH) .
2 4 4 6 4
Na Zn(OH) : Sodium tetrahydroxozincate (ii)
2 4
K Fe(CN) : Potassium hexacyanoferrate (iii)
4 6
NaAl(OH) : Sodium tetrahydroxoaluminate (iii)
4
Na Zn(OH)
2 4 2Na + [Zn(OH) ]
+
4
2-
K Fe(CN)
4 6 4K +
+ [Fe(CN) ] . 6
4-
EFFLORESCENCE, DELIQUESCENCE AND HYGROSCOPIC
When certain compound is exposed to the air, they either lose their water of crystallization or
they absorb moisture from their surroundings. The term efflorescent, deliquescent and
hygroscopic are used to describe such compound.
EFFLORESCENTS: are substances which on exposure to air, lose some or all of their water of
crystallization. The phenomenon or process is efflorescence. There is loss of weight or mass of
the substances.
e.g Na CO .10H O2 3 2 Na CO .H O
2 3 2 + 9H O 2
Other examples are Na SO .10H O, MgSO .7H O and CuSO .5H O e.t.c 2 4 2 4 2 4 2
DELIQUESCENTS: are substances that absorb so much water from air and form a solution e.g.
NaOH, CaCl , FeCl , MgCl , KOH and P O . There is a gain in weight.
2 3 2 4 10
HYGROSCOPIC: are substances which absorb moisture on exposure to the atmosphere
without forming a solution. If they are solids, no solution will be formed but if a liquid absorb
water, it gets diluted. There is little or no difference in mass e.g Conc. H SO , NaNO , CuO, CaO 2 4 3
and anhydrous Na CO . 2 3
DRYING AGENTS
These are substances which have high affinity for water or moisture. They are either
deliquescent or hygroscopic. They remove water molecules to effect physical change. Drying
agents are different from dehydrating agents which removes elements of water i.e hydrogen and
oxygen atoms or intra-molecular water.
Drying agents which react with gases are not used to dry the gas e.g conc. H SO is not used to 2 4
dry NH and H S gas. 3 2
NH 3(g) + H SO2 4(aq) (NH ) SO4 2 4(aq)
HS
2 (g) + H SO 2 4(aq) 2H O +
2 (l) SO + 2(g) S (s)
Drying agent Gases
Conc. H SO 2 4 All gases except NH & H S 3 2
Fused CaCl 2 All gases except NH 3
CaO or quicklime For ammonia
PO
2 5 All gases except ammonia
Silica gel All gases
Salts are usually dried in desiccator.
HYDROLYSIS OF SALT
Some salts undergoes hydrolysis in water to give an acidic or alkaline medium (solution) e.g.
Na CO , NaHCO , AlCl , Na S, NH Cl, CH COONa e.t.c.
2 3 3 3 2 4 3
Na CO + H O
2 3 2 NaOH + H CO . 2 3
AlCl + H O3 2 Al (OH) + HCl.
3
Na S 2 + HO 2 NaOH + HS 2
Hydrolysis of salt occurs when a salt react with water e.g salt of strong acid and weak base gives
acidic solution. The change in PH of solution is due to hydrolysis.
ANHYDROUS AND HYDRATED SALT
Anhydrous salts: are salts which do not contain water and cannot be crystallized out from
aqueous solution.
Hydrated salts/salts with water of crystallization: are salts which combine chemically with water.
The water molecule is loosely held to the salt molecule and when heated, such salt lose their
water of crystallization. The water attached is known as water of crystallization e.g
Cu(NO ) .3H O: Copper (ii) trioxonitrate (v) trihydrate.
3 2 2
MgSO .7H O: Magnesium tetraoxosulphate (vi) heptahydrate.
4 2
FeSO .7H O: Iron (ii) tetraoxosulphate (vi) heptahydrate.
4 2
Salts without water of crystallization Salts with water of crystallization
NaCl CuSO .5H O
4 2
KNO 3 FeSO .7H O
4 2
KMnO 4 ZnSO .7H O
4 2
(NH ) SO
4 2 4 Na CO .10H O
2 3 2
K SO
2 4 Cu(NO ) .3H O 3 2 2
Pb(NO ) 3 2 MgSO .7H O 4 2
AgNO 3 Zn(NO ) .6H O. 3 2 2
Recovery of salt from solution
Heating to dryness.
Preparation of crystalline zinc tetraoxosulphate(vi) from metallic zinc.
Preparation of sodium chloride crystal by Neutralization.
Preparation of copper(ii) crystals from insoluble base copper(ii)oxide.
Preparation of lead (ii) trioxonitrate (v) crystal from lead (ii)trioxocarbonate(iv).
