Final answer:
The independent variable in the experiment that asks if changing the volume of a gas affects the pressure is the volume of the gas. Volume and pressure are inversely related, as explained by the kinetic-molecular theory and Boyle's Law.
Step-by-step explanation:
In the research question, "Does changing the volume of a gas affect the pressure?", the independent variable is the volume of the gas. This is because the independent variable is the one the experimenter changes or manipulates to observe the effect on a dependent variable, which in this case would be the pressure. According to the kinetic-molecular theory, as the volume of a gas decreases, the gas molecules have less space to move around, leading to more frequent collisions with the walls of the container, and consequently, an increase in pressure. Similarly, if the volume is increased, there is more space for the molecules, resulting in fewer collisions and a decrease in pressure. These relationships align with one of the gas laws which states that at constant temperature and amount of gas (number of moles), the pressure of a gas is inversely proportional to its volume (Boyle's Law).