sblock Elements 1 Question 1
1. The temporary hardness of a water sample is due to compound $X$. Boiling this sample converts $X$ to compound $Y . X$ and $Y$, respectively, are
(2019 Main, 12 April II)
(a) $\mathrm{Mg}\left(\mathrm{HCO} _{3}\right) _{2}$ and $\mathrm{Mg}(\mathrm{OH}) _{2}$
(b) $\mathrm{Ca}\left(\mathrm{HCO} _{3}\right) _{2}$ and $\mathrm{Ca}(\mathrm{OH}) _{2}$
(c) $\mathrm{Mg}\left(\mathrm{HCO} _{3}\right) _{2}$ and $\mathrm{MgCO} _{3}$
(d) $\mathrm{Ca}\left(\mathrm{HCO} _{3}\right) _{2}$ and $\mathrm{CaO}$
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Answer:
Correct Answer: 1. (a)
Solution:
- The temporary hardness of a water sample is due to compound $X$ [i.e. $\operatorname{Mg}\left(\mathrm{HCO} _{3}\right) _{2}$ ]. Boiling of this sample converts $X$ [i.e. $\left.\mathrm{Mg}\left(\mathrm{HCO} _{3}\right) _{2}\right]$ to compound $Y\left[\right.$ i.e. $\left.\operatorname{Mg}(\mathrm{OH}) _{2}\right]$. Generally, temporary hardness is due to presence of magnesium and calcium hydrogen carbonates. It can be removed by boiling. During boiling, the soluble $\mathrm{Mg}\left(\mathrm{HCO} _{3}\right) _{2}$ is converted into insoluble $\mathrm{Mg}(\mathrm{OH}) _{2}$ and $\mathrm{Ca}\left(\mathrm{HCO} _{3}\right) _{2}$ changed to insoluble $\mathrm{CaCO} _{3}$. These precipitates can be removed by filteration.
$\mathrm{Mg}\left(\mathrm{HCO} _{3}\right) _{2} \xrightarrow{\text { Heating }} \mathrm{Mg}(\mathrm{OH}) _{2} \downarrow+2 \mathrm{CO} _{2} \uparrow$
$\mathrm{Ca}\left(\mathrm{HCO} _{3}\right) _{2} \xrightarrow{\text { Heating }} \mathrm{CaCO} _{3} \downarrow+\mathrm{H} _{2} \mathrm{O}+\mathrm{CO} _{2} \uparrow$
