A compound that increases the hydrogen ion concentration (H+) in a solution is called an acid.

Here's why:

* Acids donate protons (H+) to a solution. This directly increases the concentration of H+ ions.

* Bases, on the other hand, accept protons, decreasing the concentration of H+ ions.

The strength of an acid is determined by how readily it donates protons. Strong acids donate protons easily, leading to a higher concentration of H+ ions. Weak acids donate protons less readily, resulting in a lower concentration of H+ ions.