Here's why:

* Arrhenius Definition of Acids: An acid is a substance that increases the concentration of hydrogen ions (H+) when dissolved in water.

* Example: When hydrochloric acid (HCl) dissolves in water, it dissociates into hydrogen ions (H+) and chloride ions (Cl-):

HCl (aq) → H+ (aq) + Cl- (aq)

The presence of these hydrogen ions is what gives acids their characteristic properties, such as:

* Sour taste: The sour taste of acids is due to the interaction of hydrogen ions with taste buds.

* Reaction with bases: Acids react with bases to form salt and water, neutralizing each other.

* Ability to turn litmus paper red: Litmus paper is a pH indicator that turns red in acidic solutions due to the presence of hydrogen ions.