What is the part of the experiment that is changed on purpose?
The part that is changed on purpose is called the independent variable.
The independent variable is the the part of the experiment that the experimenter has direct control over. This variable is usually plotted on the y axis.
Changes in the independent variable cause changes in other variables. Experiments are designed so the changes in all but one of the other variables are controlled or kept constant. The other variable that can change as a result of changes in the independent variable is called the dependent variable. The dependent variable is usually plotted on the x axis.
In an experiment on Charles Law the experimenter can control the temperature of the gas and measure the temperature. Temperature then becomes the independent variable.
The Volume of the gas can then be measured as a result of the temperature change. The volume becomes the dependent variable.