Which chemical is primarily responsible for precipitating magnesium hydroxide during lime-softening?

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Multiple Choice

Which chemical is primarily responsible for precipitating magnesium hydroxide during lime-softening?

Explanation:
Magnesium hardness is removed when hydroxide ions are added to the water. In lime-softening, lime (calcium hydroxide) dissolves and releases OH-, raising the pH. The Mg2+ ions in the water then react with these hydroxide ions to form magnesium hydroxide, Mg(OH)2, which is sparingly soluble and precipitates out: Mg2+ + 2 OH− → Mg(OH)2(s). This precipitation is the primary way magnesium is removed in the lime-softening process. Soda ash can influence hardness by forming carbonates, but the specific precipitation of magnesium as Mg(OH)2 is driven by the hydroxide from lime. Silicate and phosphate serve other roles like sequestering or control of scale, not the main precipitation of magnesium hydroxide in this process.

Magnesium hardness is removed when hydroxide ions are added to the water. In lime-softening, lime (calcium hydroxide) dissolves and releases OH-, raising the pH. The Mg2+ ions in the water then react with these hydroxide ions to form magnesium hydroxide, Mg(OH)2, which is sparingly soluble and precipitates out: Mg2+ + 2 OH− → Mg(OH)2(s). This precipitation is the primary way magnesium is removed in the lime-softening process.

Soda ash can influence hardness by forming carbonates, but the specific precipitation of magnesium as Mg(OH)2 is driven by the hydroxide from lime. Silicate and phosphate serve other roles like sequestering or control of scale, not the main precipitation of magnesium hydroxide in this process.

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