A Level Chemistry OCR Practice Exam

What is an alkali?

1. A compound that donates protons in reactions

2. A type of base that dissolves in water, forming hydroxide ions

3. A solution that contains acidic properties

4. A neutralization reaction between an acid and a base

The correct answer is: A type of base that dissolves in water, forming hydroxide ions

An alkali is defined specifically as a type of base that is soluble in water and produces hydroxide ions (OH⁻) in solution. This is a key characteristic that distinguishes alkalis from other bases, which may not dissolve in water. When an alkali dissolves in water, it increases the concentration of hydroxide ions, leading to a solution with a pH greater than 7, thereby exhibiting basic properties. In contrast, a compound that donates protons in reactions pertains to acids, which do not fit the definition of an alkali. A solution with acidic properties is indicative of an acidic solution, not an alkali. Lastly, the neutralization reaction between an acid and a base describes a chemical process rather than defining a substance type, which is not relevant to the question of what constitutes an alkali. Thus, the emphasis on solubility in water and the generation of hydroxide ions is what makes this definition of an alkali accurate.