Page 111 - Chemistry ICSE Class X

Study of Acids, Bases and Salts 97

— Salt. The compound formed when the hydrogen ion of an acid is replaced by a metal ion or ammonium ion, is called a
salt.
 A salt may be normal, acid or basic salt – Normal salt: NaCl, Na SO , KNO , Acid salt: NaHCO , NaH PO , Basic salt:
3
2
4
2
3
4
Ca(OH)Cl, Pb(OH)Cl
 A double salt when dissolved in water breaks up completely into the constituent ions.
 The salts which consist of more than one acid or basic radical other than H and OH are called mixed salts. Examples:
–
+
NaKCO , Ca(OCl)Cl are mixed salts.
3
 Some complex salts are Na[Ag(CN) ], K [Fe(CN) ], etc.
2
4
6
 All salts containing a common ion are said to belong to a family.
 Salts of sodium, potassium and some salts of calcium, magnesium, barium, aluminium are highly soluble.
— Preparation of soluble salts. Soluble salts can be prepared by the following methods.
 By synthesis
2Fe(s) + 3Cl (g) o 2FeCl (s)
2
3
red hot
 By simple displacement
Zn(s) + H SO (dil.) o ZnSO (aq) + H (g)
2
4
4
2
Mg(s) + H SO (dil.) o MgSO (aq) + H (g)
4
2
4
2
 By decomposing the salts of more volatile acids with less volatile acids
Na CO (s) + 2HCl o 2NaCl + CO 2 + H O
2
3
2
< 200°C
NaCl(s) + H SO (conc.) o NaHSO + HCl(g)
2 4 4
< 200°C
KNO (s) + H SO (conc.) o KHSO 4 + HNO (g)
2
4
3
3
 By neutralisation reaction
NaOH + HCl o NaCl + H O
2
Al(OH) 3 + 3H SO o Al (SO ) + 6H O
2
4 3
2
2
4
ZnO + H SO (dil.) o ZnSO (aq) + H O
2
2
4
4
— Preparation of insoluble salt. Insoluble salts can be prepared by the following methods:
 By synthesis
'
Pb(s) + S o PbS
'
Fe + S o FeS
'
2Fe + 3Cl o 2FeCl (s)
2 3
 By precipitation
Pb(NO ) (ag) + HCl(aq) o PbCl (s)p + 2HNO (aq)
2
3
3 2
Pb(NO ) (aq) + H SO (aq) o PbSO (s)p + 2HNO (aq)
3 2 2 4 4 3
AgNO (aq) + NaX (X = Cl, Br, I) o AgX(s)p + NaNO (aq)
3 3
— Hydrated salts. The salts which contain a definite amount of water held in their structure are called hydrated salts.
.
.
.
.
CuSO 5H O, FeSO 7H O, MgSO 7H O, Na CO 10H O are some hydrated salts.
4
2
2
4
2
4
2
2
3
— Water of crystallisation. The definite number of water molecules per molecule of salt present as bound water in its
structure is called the water of crystallisation. For example, CuSO . 5H O contains 5 molecules of water of crystallisation.
2
4
— Efflorescence. The property by which a hydrated salt loses its water of crystallisation when exposed to dry atmosphere
.
is called efflorescence. Salts showing this property are called efflorescent. Washing soda (Na CO 10H O), Epsom salt
3
2
2
.
.
.
(MgSO 7H O), Glauber’s salt (Na SO 10H O) and Copper sulphate crystals (CuSO 5H O) are efflorescents.
4 2 2 4 2 4 2