The main objective of this experiment is to introduce the idea of frequency analysis response. Chemical engineering students are seldom familiar with frequency domain ideas, in contrast with mechanical or electric engineers. Hence, the experiment reproduces the idea of sinusoidal response in time domain, and in a physical system which is more familiar in the field of chemical engineer as a continuously mixed tank. Acid and base are alternatively added to an Erlenmeyer flask with an acid base indicator (blue thymol).The video shows how the colours can be neatly differentiated at low frequencies: yellow= acid and blue=base. At frequencies higher than 0.1 Hz the yellow and blue are less and less distinguished, which is equivalent to a decrease in the process gain.