Acids have hydrogen ions (H+) as their defining characteristic.

Here's why:

* Arrhenius Definition: This classic definition states that an acid is a substance that produces hydrogen ions (H+) when dissolved in water.

* Brønsted-Lowry Definition: This broader definition says an acid is a substance that donates a proton (H+).

Examples:

* Hydrochloric acid (HCl): When dissolved in water, HCl dissociates into H+ and Cl- ions.

* Sulfuric acid (H2SO4): This strong acid donates two protons, forming 2H+ and SO4^2- ions.

Important Note: While acids always have H+ ions, they may also have other ions depending on the specific acid. For instance, sulfuric acid (H2SO4) also has sulfate ions (SO4^2-) in addition to hydrogen ions.