Acids or bases can be tested by chemical WHAT

Acids or bases can be tested by chemical indicators.

Explanation:

A chemical indicator can be described as a paper substance which changes colour when dipped in an acidic or basic solution. It can be described as a chemical indicator for hydrogen ions (H+) and hydronium ions (H3O+).

Acidic solutions normally dive red-orange colours depending on the acidicity of the substance. The basic solutions give blue-violets colours depending on the basicity of the substance.