To determine which of the given substances is a Brønsted-Lowry acid, we need to understand the definition of a Brønsted-Lowry acid. According to the Brønsted-Lowry theory, an acid is a substance that can donate a proton (H⁺), while a base is a substance that can accept a proton.
Let's go through each of the options:
A. NH₃ (Ammonia):
- Ammonia tends to accept a proton to form NH₄⁺ (ammonium ion), which means it behaves as a Brønsted-Lowry base.
B. F⁻ (Fluoride ion):
- The fluoride ion can accept a proton to become HF (hydrofluoric acid), meaning it acts as a Brønsted-Lowry base.
C. NH₄⁺ (Ammonium ion):
- The ammonium ion can donate a proton to form NH₃ (ammonia), indicating that it behaves as a Brønsted-Lowry acid.
D. BF₃ (Boron trifluoride):
- Boron trifluoride tends to accept pairs of electrons rather than protons, which is more aligned with the Lewis theory of acids and bases, not the Brønsted-Lowry theory.
Given this information, the correct example of a Brønsted-Lowry acid from the provided options is:
C. NH₄⁺ (Ammonium ion).
