Question

Which material is suitable for water purification/softening?

• A. Silicones
• B. Zeolites
• C. Asbestos
• D. Quartz

Hint:

Think ion exchange, sodium ions swapped for calcium and magnesium.

Answer 1

The Correct Option is B

Step 1: Hard water contains dissolved calcium and magnesium salts that need to be removed for washing and industrial use, this process is called softening.
Step 2: Zeolites are hydrated sodium aluminosilicates with a porous, cage like structure, general formula close to Na₂Al₂Si₂O₈.xH₂O. When hard water is passed through a bed of zeolite (commercially called permutit), the loosely held Na⁺ ions in the zeolite framework exchange places with the Ca²⁺ and Mg²⁺ ions dissolved in the water. The hardness causing ions get trapped inside the zeolite lattice and soft water containing Na⁺ ions comes out. This ion exchange method is widely used in water softening plants, and the exhausted zeolite can be regenerated by washing with concentrated brine (NaCl solution).
Step 3: Silicones are synthetic polymers with a Si-O backbone used as water repellents, lubricants and sealants, they have no ion exchange capacity and cannot remove Ca²⁺ or Mg²⁺. Asbestos is a fibrous silicate mineral valued for heat resistance and insulation, it is not used in water treatment for softening. Quartz is simply crystalline silicon dioxide, a hard inert mineral with no ion exchange sites, so it cannot soften water either.
Step 4: Therefore zeolites, option B, is the material actually used for water softening.