Hydroxides are insoluble in water except when paired with which cation?

Hydroxides are insoluble in water except when paired with which cation?

• A. Sodium
• B. Calcium
• C. Magnesium
• D. Iron

Answer: (A)

Most metal hydroxides are insoluble in water, but there’s an important exception: hydroxides of alkali metals dissolve readily. When sodium pairs with hydroxide, it forms sodium hydroxide, which dissolves in water to give a clear solution—NaOH is highly soluble. The other cations in the list form hydroxides that are only sparingly soluble or insoluble (magnesium and calcium hydroxides are not very soluble, and iron hydroxide is insoluble), so they don’t dissolve to the same extent. So the cation that makes the hydroxide soluble is sodium.

Most metal hydroxides are insoluble in water, but there’s an important exception: hydroxides of alkali metals dissolve readily. When sodium pairs with hydroxide, it forms sodium hydroxide, which dissolves in water to give a clear solution—NaOH is highly soluble. The other cations in the list form hydroxides that are only sparingly soluble or insoluble (magnesium and calcium hydroxides are not very soluble, and iron hydroxide is insoluble), so they don’t dissolve to the same extent. So the cation that makes the hydroxide soluble is sodium.