Which statement about pH and magnesium hydroxide precipitation is true?

Which statement about pH and magnesium hydroxide precipitation is true?

• A. Higher pH favors precipitation
• B. Lower pH favors precipitation
• C. pH has no effect
• D. Magnesium hydroxide precipitates at any pH

Answer: (A)

Precipitation of magnesium hydroxide depends on pH because the solid forms only when there is enough hydroxide to drive the reaction Mg2+ + 2 OH- ⇌ Mg(OH)2(s). The key is the solubility product: precipitation occurs when the ion product [Mg2+][OH-]^2 exceeds Ksp. As pH rises, [OH-] increases, pushing the ion product above Ksp and forming solid Mg(OH)2. At lower pH, OH- is scarce, the ion product stays below Ksp, and Mg(OH)2 stays dissolved. So higher pH promotes precipitation. The idea that pH has no effect or that precipitation happens at any pH isn’t correct.

Precipitation of magnesium hydroxide depends on pH because the solid forms only when there is enough hydroxide to drive the reaction Mg2+ + 2 OH- ⇌ Mg(OH)2(s). The key is the solubility product: precipitation occurs when the ion product [Mg2+][OH-]^2 exceeds Ksp. As pH rises, [OH-] increases, pushing the ion product above Ksp and forming solid Mg(OH)2. At lower pH, OH- is scarce, the ion product stays below Ksp, and Mg(OH)2 stays dissolved. So higher pH promotes precipitation. The idea that pH has no effect or that precipitation happens at any pH isn’t correct.