Acid

An acid has a pH value lower than 7. It is a substance that will react with water to produce H₃O⁺ and with a base to neutralize to H₂O and a salt. On a universal indicator, strong acids show up as a red or dark orange colour and weaker acids show up as an orange or yellow colour.

Brønsted Lowry Acid

A molecule that donates H^+.

Lewis Acid

A molecule that can accept a pair of electrons.