Question

Which salt does not cause hardness in water?

• A. Calcium
• B. Sodium
• C. Magnesium
• D. Both 1 & 3

Answer 1

The correct option is B

Sodium

The explanation of the correct option:

Option B:

The hardness of water:

• The hardness of water is due to the presence of soluble bicarbonates $\left({{\mathrm{CO}}_3}^{-}\right)$, chlorides $\left({\mathrm{Cl}}^{-}\right)$, and sulfates $\left({{\mathrm{SO}}_4}^{-}\right)$of calcium $\left(\mathrm{Ca}\right)$ and magnesium $\left(\mathrm{Mg}\right)$.
• When water does not form lather with soap it is considered as hard water.
• Sodium stearate (${\mathrm{C}}_{18}{\mathrm{H}}_{35}{\mathrm{NaO}}_2$) is soluble in water. sodium ions don't form scum.

The explanation of the incorrect option :

• Option A: Calcium salt is soluble in water it will react with soap to form calcium stearate (${\mathrm{C}}_{18}{\mathrm{H}}_{35}{\mathrm{CaO}}_2$) that's why it causes the hardness of the water.
• Option C: Magnesium salt is soluble in water it will react with soap to form magnesium stearate(${\mathrm{C}}_{18}{\mathrm{H}}_{35}{\mathrm{MgO}}_2$) that's why it causes the hardness of water
• Option D: Both magnesium and calcium salt causes the hardness of the water.

Hence, the correct option is (B). Sodium salt does not cause hardness in water.