Question

Give equations to show what happens when temporary hard water is boiled.

Answer 1

Temporary hardness of water is due to the presence of calcium hydrogen carbonate $Ca{(HC{O_3})_2}$ $(aq)$ and magnesium hydrogen carbonate $Mg{(HC{O_3})_2}$ $(aq)$ are bicarbonates of calcium and magnesium. Both calcium hydrogen carbonate and magnesium hydrogen carbonate decompose when heated. The original insoluble carbonate is reformed. This happens when water is boiled.

Complete answer: Temporary hardness of water is due to the presence of bicarbonates of magnesium and calcium. They react with calcium hydroxide to form carbonates of magnesium and calcium. Thus, the temporary hardness of water is removed.
$Mg{(HC{O_3})_2}\mathop \to \limits^\Delta MgC{O_3} + C{O_2} + {H_2}O$
$Ca{(HC{O_3})_2}\mathop \to \limits^\Delta CaC{O_3} + C{O_2} + {H_2}O$
In Clark’s process, the temporary hardness of water is removed by adding calcium hydroxide. It removes the hardness of water by converting bicarbonates into carbonate.
$Ca{(HC{O_3})_2} + Ca{\left( {OH} \right)_2} \to 2CaC{O_3} \downarrow + 2{H_2}O$.

Additional information: The solubility of gases decreases when the temperature is raised and when the boiling point of water is reached ${100^o}C$ (or $212$ degrees Fahrenheit), water vapor starts to form inside the liquid in the form of bubbles.

Note:
Water that contains bicarbonate of calcium and magnesium or of both is called temporary hard water and water that contains chlorides or sulphates of calcium or magnesium or of both is called Permanent hard water. Also note that the methods used to remove hardness from water are Chemical Process of Boiling Hard Water, Adding Slaked Lime (Clark's Process), Adding Washing Soda, Calgon Process, Ion Exchange Process, Using Ion Exchange Resins, and etc.