ACS Chemistry Practice Exam 2025 – The All-in-One Guide to Exam Success!

Hydroxides and peroxides are generally categorized based on their solubility in water. Most transition metal hydroxides and certain alkaline earth metal hydroxides exhibit low solubility, while those of alkali metals, such as sodium hydroxide and potassium hydroxide, are highly soluble.

The statement that they are insoluble except when paired with alkali metals (group 1 ions) and the heavier alkaline earth metals (calcium, strontium, and barium) is correct. These specific cations form soluble hydroxides, while other heavier metals tend to yield insoluble hydroxides.

For instance, magnesium hydroxide is considered sparingly soluble, whereas barium hydroxide is highly soluble. This solubility behavior is critical in various chemical applications and reactions, including pH regulation and precipitation reactions. Additionally, most metal peroxides are also classified as insoluble, with exceptions occurring for certain alkali metal peroxides. Thus, the categorization reflects the underlying principles of solubility rules in chemistry.