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

What is the formula for the blue precipitate formed when sodium hydroxide is added to Cu²⁺ ions?

1. CuO

2. CuCl₂

3. Cu(OH)₂

4. CuSO₄

The correct answer is: Cu(OH)₂

When sodium hydroxide (NaOH) is added to copper(II) ions (Cu²⁺), a blue precipitate is formed due to the reaction between hydroxide ions (OH⁻) from the sodium hydroxide and copper ions. The correct formula for this precipitate is copper(II) hydroxide, which is represented as Cu(OH)₂. In this reaction, the copper(II) ions react with the hydroxide ions to form an insoluble compound, Cu(OH)₂, which appears as a blue precipitate. The formation of this blue precipitate is a characteristic test for the presence of copper(II) ions in a solution. Other compounds mentioned in the options serve different purposes or result from different reactions. For example, CuO is copper(II) oxide, which is a solid product that doesn't typically form in a reaction with hydroxide but rather through thermal decomposition or reaction with oxygen. CuCl₂ is copper(II) chloride, which is a soluble salt and would not form a precipitate in this scenario. Likewise, CuSO₄, or copper(II) sulfate, is also a soluble compound in water and does not participate in the formation of a precipitate when sodium hydroxide