Hydrogen ions (H+) are considered to be acids. In the Brønsted-Lowry theory of acids and bases, an acid is defined as a substance that donates protons (H+ ions) to another substance. Since hydrogen ions are essentially protons, they fit this definition perfectly.
When H+ ions are present in a solution, they increase the acidity of that solution. For instance, in water, when H+ ions are generated, they lead to the formation of hydronium ions (H3O+), which is a common way to represent the presence of hydrogen ions in aqueous solutions.
On the other hand, bases are substances that can accept H+ ions or donate hydroxide ions (OH–). Therefore, hydrogen ions themselves cannot be classified as bases. In summary, hydrogen ions act as acids in chemical reactions by donating protons.