Question

‘Alum’ is an example of:
(a) Single salt
(b) Double salt
(c) Acids
(d) None of the above

Answer 1

Hint: A salt is a compound formed as a result of an acid base reaction. This ‘product’ is then distinguished on the basis of cations and anions present in it. Single salt is the one which has one anion and one cation. Double salt contains more than one cation or anion. Acidic salt has an ionizable proton present in it.

Complete step by step answer:
Alum has the general formula \[XAl{{(S{{O}_{4}})}_{2}}.12{{H}_{2}}O\], where X is a monovalent cation such as potassium or ammonium.
"Alum" generally refers to potassium alum, i.e. \[KAl{{(S{{O}_{4}})}_{2}}.12{{H}_{2}}O\].
As we can see alum (or potassium alum) has one kind of anion (sulphate –\[SO_{4}^{2-}\]) and two cations (potassium –\[{{K}^{+}}\] and aluminium –\[A{{l}^{3+}}\]).
“The salts that contain more than one cation or anion are known as double salt”. Therefore, alum is a double salt.
Also, the metal ions in alum are surrounded by six water molecules each, to give the complete formula \[KAl{{(S{{O}_{4}})}_{2}}.12{{H}_{2}}O\], and hence, also called ‘dodecahydrate’.
Therefore, the answer is – option (b) – ‘Alum’ is an example of Double salt.

Additional information:
There are many types of alum, such as – Potassium alum, Soda alum, Ammonium alum, Chrome alum, Selenate alum, etc.

Note: Salt is an ionic compound that is formed as a result of the neutralization reaction of an acid and a base. It is composed of cations and anions. Salt is classified as –
Single salt –
It is formed by the neutralization of strong acids and strong bases.
For example – NaCl.
Acidic Salt –
“The salt formed by partial neutralization of a diprotic or a polyprotic acid is known as an acidic salt. These salts have ionizable \[{{H}^{+}}\]ion along with another cation”.
For example – \[NaHS{{O}_{4}}\]
Basic Salt –
“The salt formed by the partial neutralization of a strong base by a weak acid is known as a basic salt. They hydrolyze to form a basic solution.”
For example – \[N{{a}_{2}}C{{O}_{3}}\]
Double Salt –“The salts that contain more than one cation or anion are known as double salt. They are obtained by the combination of two different salts crystallized in the same ionic lattice.”
For example – \[FeS{{O}_{4}}.{{(N{{H}_{4}})}_{2}}S{{O}_{4}}.6{{H}_{2}}O\]
Mixed Salt –
“The salt that consists of a fixed proportion of two salts, often sharing either a common cation or common anion is known as mixed salt.”
For example – \[{{K}_{4}}[Fe{{(CN)}_{6}}]\]