sblock Elements - Result 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) $Mg\left(HCO _3\right) _2$ and $Mg(OH) _2$
(b) $Ca\left(HCO _3\right) _2$ and $Ca(OH) _2$
(c) $Mg\left(HCO _3\right) _2$ and $MgCO _3$
(d) $Ca\left(HCO _3\right) _2$ and $CaO$
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Answer:
Correct Answer: 1. (d)
Solution:
- The temporary hardness of a water sample is due to compound $X$ [i.e. $\operatorname{Mg}\left(HCO _3\right) _2$ ]. Boiling of this sample converts $X$ [i.e. $\left.Mg\left(HCO _3\right) _2\right]$ to compound $Y\left[\right.$ i.e. $\left.\operatorname{Mg}(OH) _2\right]$. Generally, temporary hardness is due to presence of magnesium and calcium hydrogen carbonates. It can be removed by boiling. During boiling, the soluble $Mg\left(HCO _3\right) _2$ is converted into insoluble $Mg(OH) _2$ and $Ca\left(HCO _3\right) _2$ changed to insoluble $CaCO _3$. These precipitates can be removed by filteration.
$Mg\left(HCO _3\right) _2 \xrightarrow{\text { Heating }} Mg(OH) _2 \downarrow+2 CO _2 \uparrow$
$Ca\left(HCO _3\right) _2 \xrightarrow{\text { Heating }} CaCO _3 \downarrow+H _2 O+CO _2 \uparrow$
