Hi, I'm doing a high school physics project and am trying to figure out if a certain setup that I'm using is adiabatic or isothermal, in order to determine what equation I can use to calculate the work that my setup does-- the threads I've come across so far only explain the difference, but not how to tell whether a certain setup is one or the other. Setup: I have an ordinary empty water bottle, which has a small hole on its bottom made by a heated paperclip. This hole is small enough such that a thumb tack can fit tightly through it as a 'seal' of sorts. Additionally, the bottle has a valve on its top, through which compressed air can be sent and contained with very marginal loss over relatively short periods of time such as a few minutes or less (which is what I'm dealing with for my setup). I pump a certain pressure of compressed air into the bottle (say, 30 psi). I then attach it to a toy car and remove the thumb tack, and a stream of pressurized air is expelled, propelling the car. This experiment is performed for an ordinary water bottle (0.5 L), a 1 L bottle, and a 2 L bottle that typically carries pop. I understand the definitions of 'isothermal' and 'adiabatic', but I still don't understand how to tell which (if either) my project is? This is (unfortunately) far above anything I have or likely will be covering in my high school physics class. Isothermal indicates that the temperature of the gas in the system is constant throughout. Adiabatic indicates that there is no heat transfer between the fluid (air, in this case), and the surroundings, which may or may not be isentropic if the process is internally reversible. If someone could clarify this for me, along with a citable source such as a university or formal research paper (Wikipedia doesn't count), that would be much appreciated. Thank you.