Question

A sample of water contains $\text{Mg}(\text{HCO}_3)_2$ and $\text{Ca}(\text{HCO}_3)_2$. On boiling this water, these hydrogen carbonates are removed as precipitates. The precipitates are

• A. $\text{MgCO}_3, \text{CaCO}_3$
• B. $\text{Mg}(\text{OH})_2, \text{Ca}(\text{OH})_2$
• C. $\text{Mg}(\text{OH})_2, \text{CaCO}_3$
• D. $\text{MgCO}_3, \text{Ca}(\text{OH})_2$

Hint:

Boiling removes temporary hardness (caused by bicarbonates). The key difference is that $\text{Ca}(\text{HCO}_3)_2$ precipitates as $\text{CaCO}_3$, but $\text{Mg}(\text{HCO}_3)_2$ ultimately precipitates as $\text{Mg}(\text{OH})_2$ because magnesium carbonate undergoes hydrolysis in hot water.

Answer 1

The Correct Option is C

Step 1: Identify the type of water hardness.
The presence of soluble bicarbonates of calcium ($\text{Ca}(\text{HCO}_3)_2$) and magnesium ($\text{Mg}(\text{HCO}_3)_2$) causes temporary hardness in water.

Step 2: Write the reaction for the decomposition of Calcium bicarbonate upon boiling.
When water containing $\text{Ca}(\text{HCO}_3)_2$ is boiled, the soluble bicarbonate decomposes to form insoluble calcium carbonate ($\text{CaCO}_3$) precipitate. \[ \text{Ca}(\text{HCO}_3)_2(\text{aq}) \xrightarrow{\text{Boiling}} \text{CaCO}_3(\text{s}) \downarrow + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g}) \uparrow \] The precipitate from $\text{Ca}(\text{HCO}_3)_2$ is $\text{CaCO}_3$.

Step 3: Write the reaction for the decomposition of Magnesium bicarbonate upon boiling.
When water containing $\text{Mg}(\text{HCO}_3)_2$ is boiled, the soluble bicarbonate decomposes to form magnesium carbonate ($\text{MgCO}_3$). \[ \text{Mg}(\text{HCO}_3)_2(\text{aq}) \xrightarrow{\text{Boiling}} \text{MgCO}_3(\text{s}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g}) \uparrow \] However, unlike calcium carbonate, $\text{MgCO}_3$ is slightly soluble in hot water and reacts further with water via hydrolysis (or decomposition under high temperature) to form the less soluble magnesium hydroxide ($\text{Mg}(\text{OH})_2$) precipitate. \[ \text{MgCO}_3(\text{s}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{Mg}(\text{OH})_2(\text{s}) \downarrow + \text{CO}_2(\text{g}) \uparrow \] The final precipitate from $\text{Mg}(\text{HCO}_3)_2$ is $\text{Mg}(\text{OH})_2$.

Step 4: Conclude the final precipitates.
The precipitates formed are $\text{Mg}(\text{OH})_2$ and $\text{CaCO}_3$.