A Level Chemistry OCR Practice Exam

Which statement best describes a base?

1. A proton donor

2. A species that is a proton acceptor

3. A substance that donates electrons

4. A neutral substance in solution

The correct answer is: A species that is a proton acceptor

The statement that best describes a base is that it is a species that acts as a proton acceptor. In the Brønsted-Lowry acid-base theory, a base is defined specifically by its ability to accept protons (H⁺ ions) from acids. When a base encounters an acid, it effectively removes a proton from the acid, resulting in the formation of the conjugate acid of the base. This definition highlights the fundamental aspect of bases in chemical reactions, particularly in aqueous solutions, where they often react with acids to neutralize them. By accepting protons, bases play a crucial role in maintaining pH balance in various chemical and biological systems. In contrast to the other options, a proton donor is characteristic of acids rather than bases. While some bases can donate electrons, that property is not exclusive or defining for bases as a class. Additionally, referring to bases as neutral substances in solution is misleading because bases can significantly impact the pH of a solution, making it basic (or alkaline), rather than neutral. Thus, the correct descriptor of a base is its role as a proton acceptor.