Question

Boiling method can be used to remove............of water.

• A. Temporary hardness
• B. Permanent hardness
• C. Both temporary and permanent hardness
• D. Neither temporary nor permanent hardness

Hint:

Remember:
- Bicarbonates \(\rightarrow\) Temporary Hardness (Removed by boiling or Clark's method using lime).
- Chlorides/Sulfates \(\rightarrow\) Permanent Hardness (Cannot be removed by boiling; requires ion exchange or washing soda).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Water hardness is caused by the presence of multivalent metallic cations, primarily calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)).
Hardness is classified into two types: temporary hardness and permanent hardness, depending on the anions associated with these cations.

Step 2: Detailed Explanation:
- Temporary Hardness: This is caused by the presence of dissolved bicarbonates of calcium and magnesium, i.e., \(\text{Ca(HCO}_3)_2\) and \(\text{Mg(HCO}_3)_2\).
It is called "temporary" because it can be easily removed by physical methods like boiling.
During boiling, the soluble bicarbonates undergo thermal decomposition into insoluble carbonates:
\[ \text{Ca(HCO}_3)_2 \xrightarrow{\Delta} \text{CaCO}_3\downarrow + \text{H}_2\text{O} + \text{CO}_2\uparrow \] \[ \text{Mg(HCO}_3)_2 \xrightarrow{\Delta} \text{MgCO}_3\downarrow + \text{H}_2\text{O} + \text{CO}_2\uparrow \] (Further boiling can convert \(\text{MgCO}_3\) into the even less soluble magnesium hydroxide, \(\text{Mg(OH)}_2\)).
These insoluble precipitates settle down as scale or can be separated by filtration, thereby removing the hardness.
- Permanent Hardness: This is caused by the presence of sulfates, chlorides, and nitrates of calcium and magnesium (e.g., \(\text{CaCl}_2\), \(\text{MgSO}_4\)).
These salts are highly stable and do not decompose or precipitate upon heating or boiling.
Removing permanent hardness requires chemical treatment, such as the addition of washing soda (\(\text{Na}_2\text{CO}_3\)), ion exchange, or zeolites.
Therefore, boiling is only effective for removing temporary hardness.

Step 3: Final Answer:
Boiling method can be used to remove temporary hardness of water.