International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which statement correctly describes the solubility of carbonates?

1. All carbonates are soluble

2. All carbonates are insoluble, except potassium carbonate

3. Only sodium carbonates are soluble

4. Most carbonates are soluble

The correct answer is: All carbonates are insoluble, except potassium carbonate

The statement that all carbonates are insoluble except potassium carbonate is correct because it reflects the general solubility rules for carbonates. Most carbonate compounds are indeed insoluble in water; however, there are notable exceptions which primarily include the carbonates of alkali metals such as potassium and sodium. These alkali metal carbonates, including potassium carbonate, are soluble in water. To give some context, carbonates that typically fall under the insoluble category include those of transition metals and some heavier alkaline earth metals. However, carbonates formed with metals from Group 1 (alkali metals) and ammonium (NH4+) are generally soluble. It’s also important to note that while potassium carbonate is soluble, the statement mistakenly suggests it as the sole soluble carbonate when other alkali carbonates exist that also display similar solubility properties, such as sodium carbonate. This highlights the nuanced nature of solubility rules, where general trends are observed but with specific exceptions that need to be understood in a broader chemical context.