Acid
Definition and meaning of Acid in chemistry.
An acid is a substance that donates hydrogen ions, or protons, when dissolved in water. Acidic solutions have a pH below 7 and share properties like a sour taste and reactivity with many metals. Chemists also use broader definitions, including the Bronsted-Lowry proton donor and the Lewis electron-pair acceptor.
In more detail
The Arrhenius definition is the everyday one. It says an acid increases the concentration of hydrogen ions when it dissolves in water. The Bronsted-Lowry definition is broader.
It defines an acid as any substance that donates a proton to another substance, called a base. This donation can happen even outside of water. The Lewis definition is broader still.
It defines an acid as any species that accepts a pair of electrons. This covers reactions that do not involve protons at all. Acids share recognizable properties.
They taste sour, turn blue litmus paper red, and conduct electricity in solution. They also react with active metals to release hydrogen gas. Strong acids, like hydrochloric acid and sulfuric acid, ionize almost completely in water.
Nearly every molecule releases its proton. Weak acids, like acetic acid, ionize only partially, leaving most molecules intact in solution. This difference in ionization, not concentration, is what makes an acid strong or weak.
A common misconception is that a strong acid is simply a very concentrated one. Concentration describes how much acid is dissolved. Strength describes what fraction of it ionizes.
A dilute strong acid can still be more corrosive than a concentrated weak acid. When an acid reacts with a base, the two neutralize each other, forming a salt and water.
Key facts
| pH | Below 7 in aqueous solution |
|---|---|
| Bronsted-Lowry definition | Proton (H+) donor |
| Lewis definition | Electron-pair acceptor |
| Strong acid example | HCl, ionizes almost completely |
| Weak acid example | Acetic acid, ionizes partially |
| Field | General Chemistry |
Hydrochloric acid, HCl, is a strong acid. When dissolved in water it ionizes almost completely to form hydrogen ions and chloride ions. This is why it conducts electricity well and reacts vigorously with metals like zinc.
Frequently asked questions
What is the difference between a strong and a weak acid?
A strong acid ionizes almost completely in water. A weak acid ionizes only partially, leaving most of its molecules undissociated in solution.
What happens when an acid reacts with a base?
They neutralize each other, producing a salt and water. For example, hydrochloric acid and sodium hydroxide react to form sodium chloride and water.
Is acid strength the same as acid concentration?
No. Strength describes what fraction of acid molecules ionize in water. Concentration describes how much acid is dissolved, so a dilute strong acid can still be more reactive than a concentrated weak one.