6.1: What is an acid and a base? - Chemistry LibreTexts Arrhenius's definition of acids and bases. The earliest definition of acids and bases is Arrhenius's definition which states that: An acid is a substance that forms hydrogen ions H + when dissolved in water, and; A base is a substance that forms hydroxide ions OH-when dissolved in water.; or example, hydrochloric acid is an acid because it forms H + when it dissolves in water.