Question

Which of the following ions are responsible for unstable hardness of water?
A. $CaC{l_2},MgS{O_4}$
B. $C{a^{2 + }},M{g^{2 + }}$
C. ${K^ + },CaC{O_3}$
D. $Ca{\left( {HC{O_3}} \right)_2},Mg{\left( {HC{O_3}} \right)_2}$

Answer 1

Hint: Unstable hardness of water is also known as temporary hardness which can be removed from boiling. Hardness is due to the presence of the sulphides and salts of the chlorides. The instability of hard water can be removed by boiling.

Complete step by step answer:
We know that the water hardness is basically caused by the presence of salts of magnesium and calcium ions. Based on the type of salt present, hardness can be classified into two types, i.e., Temporary and permanent hardness respectively which is described as follows-
Temporary Hardness- This type of hardness results from the presence of bicarbonates of calcium and magnesium. This type of hardness is generally unstable as it can be removed easily by boiling the water. Due to this type of nature, this hardness is generally temporary.
Permanent Hardness- Permanent hardness on the other hand is more stable as it is not easily removed by boiling. This type of hardness results due to the presence of salts like sulphates and chlorides of calcium and magnesium.
Hence, the correct option will be D) $Ca{\left( {HC{O_3}} \right)_2},Mg{\left( {HC{O_3}} \right)_2}$ as the temporary hardness is due to presence of bicarbonates of magnesium and calcium.

Additional Information:
Temporary hardness which consists of salts of carbonates and bicarbonates is generally considered unstable because boiling can easily convert these bicarbonate salts into insoluble carbonates, which can then be easily removed and the water could turn into soft water. The bicarbonates present in hard water are soluble in water but boiling automatically results in separation of bicarbonates due to the formation of insoluble carbonates. On the other hand, permanent water cannot be softened by the process of boiling.

Note: While doing such questions, one should not be confused regarding which type of hardness is unstable. Since, temporary hardness persists for a shorter duration of time and can be easily reversed, hence it is considered as unstable hardness.