A substance which ionizes to form hydrogen ions (H+) when dissolved in water is called an acid.

Here's why:

* Acids are substances that donate protons (H+) when dissolved in water.

* Hydrogen ions (H+) are what give acids their characteristic properties, such as a sour taste and the ability to react with bases.

Examples of acids include:

* Hydrochloric acid (HCl)

* Sulfuric acid (H2SO4)

* Acetic acid (CH3COOH)