According to Arrhenius, an acid is a substance with hydrogen in its formula which releases hydrogen ions in water. By that definition hydrogen chloride is an acid because:

HCl(g) → H+(aq) + Cl-(aq)

Similarly, sulfuric acid is acidic because:

H2SO4(aq) → 2 H+(aq) + SO42-(aq)

Arrhenius also stated that bases are substances with OH in their formulas which release hydroxide ions in water. Clearly sodium hydroxide illustrates this definition:

NaOH(s) → Na+(aq) + OH-(aq)

This early model describes many of the common acids and bases but it runs into trouble with simple ammonia, a weak base with no oxygen in its formula (hence no OH).