This question appeared in my IB Physics HL mock exam and I am stumped by the answer on the mark sheet. It really should be a trivial question but it's open to interpretation which is why I find it unreasonable. It was in Paper 1 from November 2014, 1. The problem statement, all variables and given/known data? Which of the following is true when thermal energy is converted into work in a single process and a cyclical process: Single process: / Cyclical process: A: complete conversion of thermal energy into work can occur / energy must be transferred from system B: complete conversion of thermal energy into work can never occur / energy must be transferred from system C: complete conversion of thermal energy into work can occur / energy need not be transferred from system D: complete conversion of thermal energy into work can never occur / energy need not be transferred from system 3. The attempt at a solution I was totally happy eliminating answers C and D which left A and B. I answered B. The answer on the mark sheet was A. The only way I can rationalize the answer comes from the possibility of a slow isothermal expansion of an ideal gas with a piston moving into a vacuum (so with a heat reservoir so the gas in the piston is at thermal equilibrium). However, an ideal gas is exactly that. If they had specified an ideal gas in the question I would have less trouble agreeing with this answer, however, I think the reality is, is that I am pulling in some heat from the surroundings to overcome the attraction between molecules, even if such an energy transfer is very minute. So while according to the ideal gas model A is correct, what is true is B? Would anyone be able to clarify as to why the above argument is wrong?