### Correct Option: A **Explanation of Why Option A is Correct:** The Bronsted-Lowry theory of acids and bases, proposed by Johannes Nicolaus Bronsted and Thomas Martin Lowry in 1923, defines acids and bases in terms of proton (H⁺) transfer. According to this theory: - **Acids** are defined as substances that **donate protons (H⁺)** to other substances. - **Bases** are defined as substances that **accept protons (H⁺)** from other substances. This definition is broader than the Arrhenius definition, which limits acids to substances that produce H⁺ ions in water and bases to those that produce OH⁻ ions. The Bronsted-Lowry theory applies to reactions in any solvent, not just water, making it more versatile. For example, in the reaction between hydrochloric acid (HCl) and ammonia (NH₃): \[ \text{HCl} \, (aq) + \text{NH}_3 \, (aq) \rightarrow \text{Cl}^- \, (aq) + \text{NH}_4^+ \, (aq) \] In this reaction: - HCl donates a proton (H⁺) to NH₃, making HCl the acid. - NH₃ accepts the proton, making it the base. This proton transfer is the essence of the Bronsted-Lowry theory. ### Why the Other Options are Incorrect: **Option B: Acids are substances that accept protons (H⁺) while bases donate protons.** - This statement is incorrect because it reverses the definitions of acids and bases according to the Bronsted-Lowry theory. Acids donate protons, while bases accept them. Therefore, this option is fundamentally flawed. **Option C: Acids and bases only exist in aqueous solutions.** - This statement is misleading. While many common acids and bases (like HCl and NaOH) are often discussed in the context of aqueous solutions, the Bronsted-Lowry theory applies to reactions in any solvent. Acids and bases can exist in gaseous or solid states as well. For example, hydrogen chloride (HCl) can exist as a gas and still act as an acid by donating protons. **Option D: Acids always have a sour taste while bases always have a bitter taste.** - While it is true that many acids (like citric acid) have a sour taste and many bases (like sodium hydroxide) have a bitter taste, this statement is not a defining characteristic of acids and bases. Taste is not a reliable or universal property for identifying acids and bases, as not all acids or bases will exhibit these tastes, and some can be harmful or toxic. ### Summary of Key Points: - **Bronsted-Lowry Theory**: Acids donate protons (H⁺), and bases accept protons. - **Versatility**: This theory applies to reactions in any solvent, not just water. - **Common Misconceptions**: Taste is not a reliable indicator of acidity or basicity; acids and bases can exist in various states beyond aqueous solutions. - **Example Reaction**: HCl (acid) donates a proton to NH₃ (base), illustrating the theory in action. This understanding of acids and bases is crucial for further studies in chemistry, particularly in organic chemistry and biochemistry, where proton transfer plays a significant role in reaction mechanisms.