A Level Chemistry OCR Practice Exam

What defines an alkali in terms of its reaction in water?

1. A type of base that forms hydroxide ions

2. A base that does not dissolve in water

3. A neutral species in acid-base reactions

4. A chemical that donates protons

The correct answer is: A type of base that forms hydroxide ions

An alkali is defined as a type of base that dissolves in water to form hydroxide ions (OH⁻). This definition is crucial in understanding the behavior of alkalis in aqueous solutions. When an alkali dissolves in water, it increases the concentration of hydroxide ions in the solution, which is responsible for its basic properties, such as raising the pH and reacting with acids to form water and a salt. The ability to form hydroxide ions upon dissolution is what distinguishes alkalis from other bases that may not dissolve well in water (which can lead to confusion if considering only the general definition of a base). While not all bases are alkalis, all alkalis are bases that are soluble in water, and thus specifically contribute to the increase of hydroxide ions in a solution. Understanding this distinction is essential for chemistry students, as it allows for clearer comprehension of acid-base reactions, pH scales, and the behavior of different substances in chemical reactions.