Acid

Summary: A substance that donates hydrogen ions (H⁺) when dissolved in water. A proton donor. Tags: igcse chemistry definition acids-bases Created: 2026-07-14 Last Updated: 2026-07-14

An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. In the Brønsted-Lowry definition used at IGCSE, an acid is a proton donor.

Key properties of acids:

• Turn blue litmus red
• Have pH less than 7
• React with metals (above hydrogen in reactivity series) to produce salt + hydrogen gas
• React with bases and carbonates
• Conduct electricity in aqueous solution (due to presence of mobile ions)

Strong vs weak acids: Strong acids (HCl, H₂SO₄, HNO₃) completely dissociate in water. Weak acids (CH₃COOH, H₂CO₃) only partially dissociate.

See also: Acids and Bases, Indicators and pH, Neutralization

This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.