Now, we are ready to learn about a second definition of acids and bases, the Brønsted-Lowry definition. According to the Brønsted-Lowry definition, an acid is a substance that can donate a proton (H+ ion) to another molecule. A base is a substance that can accept that donated H+.
After the Brønsted-Lowry acid donates its proton, it becomes the conjugate base of the acid. After the Brønsted-Lowry base accepts a proton, it becomes the conjugate acid of the base. We think of Brønsted-Lowry acids and bases in terms of their reactions with other molecules rather than just their structure. This is a more general definition and broadens the compounds that can be considered acids or bases.
Watch this video, which shows examples of how we can interpret a chemical reaction to determine, which substance is the acid, base, conjugate acid, and conjugate base. Water acts as a base because it is reacting with a strong acid. Water is an amphiprotic compound, which means it can act as an acid or a base. Hydrogen ions do not exist in water on their own, but immediately get grabbed by water molecules to form hydronium ions. A conjugate pair is always one acid and one base.
Source: Khan Academy, https://youtu.be/XVOU9c3crbc
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