A responding variable is the component of an experiment that responds to change. For example, if salt is added to water to see how the pH level changes, the water is the responding variable because it is the component of the experiment that changes and reacts to the salt.
Every experiment has three components: manipulated variables, responding variables and constant variables. In the example, the manipulated variable is the salt because different amounts need to be added to the water to record changes in pH. Salt is not the responding variable because the makeup of the substance remains unchanged; only the quantity changes. The constant variable is the unchanged aspects of an experiment, such as the temperature of the water. Ultimately, the responding variable is the part of an experiment that changes or reacts to a variable.