The Arrhenius Theory explains what acids and bases are based on how they behave in water.

Acids:

• What Are They? Acids are substances that, when mixed with water, create more hydrogen ions ($H^+$).
• Example: A good example is hydrochloric acid ($HCl$). When it mixes with water, it breaks apart and produces hydrogen ions and chloride ions ($Cl^-$): $HCl \rightarrow H^+ + Cl^-$

Bases:

• What Are They? Bases are substances that, when mixed with water, create more hydroxide ions ($OH^-$).
• Example: Sodium hydroxide ($NaOH$) is a good example. When it dissolves in water, it breaks apart into sodium ions ($Na^+$) and hydroxide ions: $NaOH \rightarrow Na^+ + OH^-$

To sum it up, according to the Arrhenius Theory, acids give off $H^+$ ions, and bases give off $OH^-$ ions. This idea is key to understanding how acids and bases react with each other.