**Correct Option: C. Acids are substances that donate protons, while bases accept protons.** ### Detailed Explanation: The Bronsted-Lowry theory of acids and bases, proposed by Johannes Nicolaus Bronsted and Thomas Martin Lowry in 1923, defines acids and bases based on their ability to donate or accept protons (H⁺ ions). 1. **Understanding Protons:** - A proton (H⁺) is essentially a hydrogen atom that has lost its electron. In chemical reactions, the transfer of protons is a key aspect of acid-base chemistry. 2. **Definition of Acids and Bases:** - According to the Bronsted-Lowry theory: - **Acids** are defined as proton donors. This means that when an acid is present in a reaction, it will release a proton to another species. - **Bases** are defined as proton acceptors. This means that a base will take up a proton from another species during a reaction. 3. **Example Reaction:** - Consider the reaction between hydrochloric acid (HCl) and water (H₂O): \[ \text{HCl} + \text{H}_2\text{O} \rightarrow \text{Cl}^- + \text{H}_3\text{O}^+ \] - In this reaction: - HCl donates a proton (H⁺) to water, making it an acid. - Water accepts the proton, becoming hydronium ion (H₃O⁺), making it a base in this context. 4. **Why Option C is Correct:** - Option C accurately reflects the definitions provided by the Bronsted-Lowry theory: acids donate protons, and bases accept protons. This is the fundamental principle of this theory and is widely accepted in the study of acid-base chemistry. ### Why the Other Options are Incorrect: - **Option A: Acids are substances that donate electrons, while bases accept protons.** - This statement is incorrect because it misrepresents the definition of acids. Acids do not donate electrons; they donate protons. The donation of electrons is more relevant to Lewis acids and bases, which is a different theory. - **Option B: Acids are substances that accept protons, while bases donate protons.** - This option is incorrect because it reverses the definitions. Acids are proton donors, not acceptors. This confusion is a common pitfall for students, so it’s important to remember the correct definitions. - **Option D: Acids and bases are solely defined by their pH levels.** - While pH is a measure of the acidity or basicity of a solution, it does not define what acids and bases are. The Bronsted-Lowry theory provides a more fundamental understanding of their behavior in chemical reactions. pH is a result of the concentration of H⁺ ions in a solution, but it does not encompass the broader definitions of acids and bases. ### Revision Summary: - **Bronsted-Lowry Theory:** Acids donate protons (H⁺), and bases accept protons. - **Acid Example:** HCl donates a proton to water, forming H₃O⁺. - **Common Pitfall:** Confusing acids with electron donors or misidentifying their roles in reactions. - **pH Levels:** While important, pH does not define acids and bases; it is a measure of their strength in solution.