Question

How can we remove permanent hardness from water? [3 marks]

Answer 1

1) The first process involves the addition of washing soda. Washing soda reacts with the soluble salts of calcium and magnesium and converts them into insoluble carbonates. [1.5 marks]
2) Next process is Calgon’s process.
Here we use Calgon or sodium-hexa-meta-phosphate which removes the hardness in water by the adsorption of ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ ions.
The addition of Calgon to hard water causes the calcium and magnesium ions of hard water to displace sodium ions from the anion of Calgon. This results in the removal of calcium and magnesium ions from hard water in the form of a complex with Calgon. [1.5 marks]