When studying gases, irish chemist robert boyle noticed that as he changed the pressure, the volume of a gas would also change. in this experiment, what is the dependent variable? explain your answer?

In Boyel's experiment he changed the pressure and noticed the change in volume with respect to pressure keeping temperature and moles of gas constant

So here we have three kinds of quantities

a) controlled variables : Temperature and moles as they have to be kept constant

b) independent variable : pressure as we are varying it and the volume is changing with respect to pressure

c) Dependent variable: Volume , as it depends upon the change in pressure.

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.