First slide

Hardness of water

Question

Permanent hardness due to ${Mg}^{2 +}$ ions is best removed by

Moderate

A

$Ca {(OH)}_{2}$
B

${Na}_{2} {CO}_{3}$
C

${Na}_{2} {CO}_{3}$ + $Ca {(OH)}_{2}$
D

None of these

Solution

$Ca {(OH)}_{2}$ removes the permanent hardness due to ${Mg}^{2 +}$ ion, but it produces ${Ca}^{2 +}$ ions which are removed by ${Na}_{2} {CO}_{3}$ .
${Mg}^{2 +} + Ca {(OH)}_{2} \to Mg {(OH)}_{2} ↓ + {Ca}^{2 +}$
${Ca}^{2 +} + {Na}_{2} {CO}_{3} \to {CaCO}_{3} ↓ + 2 {Na}^{+}$
$Ca {(OH)}_{2}$ or ${Na}_{2} {CO}_{3}$ alone cannot remove the permanent hardness.

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