Question

What is the cause of temporary hardness? How it is removed by Clark's process and why the reagent used in it must be calculated?

Answer 1

Temporary hardness is caused by dissolved calcium hydrogen carbonate($Ca(HC{O}_3{)}_2$)

In Clark's method, calculated amount of slaked lime $Ca(OH{)}_2$ is added to temporary hard water which precipitates $Ca(HC{O}_3{)}_2$ out and removes temporary hardness.\

$Ca(HC{O}_3{)}_2+Ca(OH{)}_2\to 2CaC{O}_3+2H_{2}O$

(slaked lime) (insoluble

calcium

carbonate)

The calculated or controlled amount of slaked lime is added because slaked lime is itself a source of calcium ions(and hence hardness). So, care must be taken to avoid adding an excess amount of it.