A Level Chemistry OCR Practice Exam

Which definition describes an acid?

1. A species that accepts protons

2. A species that donates protons

3. A species that contains hydroxide ions

4. A species that produces salts

The correct answer is: A species that donates protons

The definition describing an acid is that it is a species that donates protons. This aligns with the Brønsted-Lowry theory of acids and bases, where an acid is characterized as a proton donor. When an acid donates a proton (H⁺ ion), it decreases the pH of a solution, making it acidic. Common examples include hydrochloric acid (HCl), which dissociates in water to donate a proton and form hydronium ions (H₃O⁺). The other definitions do not accurately represent what an acid is. A species that accepts protons describes a base according to the Brønsted-Lowry definition. Species containing hydroxide ions typically refer to bases, which are associated with increasing the pH of a solution. Finally, a species that produces salts relates to the reaction between acids and bases but does not encapsulate the fundamental definition of an acid itself, as not all acids are involved in producing salts in every reaction context.