Acid
Definition and meaning of Acid in chemistry.
An acid is a substance that donates hydrogen ions (protons) to other substances and produces a solution with a pH below 7. More broadly, an acid can be defined as a proton donor (Brønsted–Lowry) or an electron-pair acceptor (Lewis).
In more detail
Acids taste sour, turn blue litmus paper red, and react with many metals and with carbonates. Strong acids such as hydrochloric acid ionize almost completely in water, while weak acids such as acetic acid ionize only partially. When an acid reacts with a base, the two neutralize each other to form a salt and water.
Key facts
| pH | Below 7 in aqueous solution |
|---|---|
| Brønsted–Lowry | Proton (H⁺) donor |
| Lewis | Electron-pair acceptor |
| Field | General Chemistry |
Hydrochloric acid (HCl) is a strong acid; in water it donates its proton almost completely to form hydrogen ions and chloride ions.
Frequently asked questions
What is the difference between a strong and a weak acid?
A strong acid ionizes almost completely in water, while a weak acid ionizes only partially, leaving most of its molecules undissociated.
What happens when an acid reacts with a base?
They neutralize each other, producing a salt and water. For example, HCl and NaOH react to form sodium chloride and water.