1. The problem statement, all variables and given/known data Two equal masses of an ideal gas initially at the same temperature and pressure are compressed to half of their initial volumes, one of them isothermally, and the other while thermally isolated from its surroundings. Which one of the following is the same for both samples of the gas? A) the heat given during compression B) the internal energy of the compressed gas C) the density of the compressed gas D) the work done on the gas during compression 2. Relevant equations Nil 3. The attempt at a solution My initial thought was "is there a diff between 'isothermally' and 'thermally isolated from surroundings' ???". Or is it just a trick statement? i gave it a try and chose B as the answer cause i thought internal energy of ideal gas depends on its temperature. Since both are (i assumed) isothermally compressed, internal energy at the end must be the same, regardless of change in pressure or volume. Alas, my physics teacher posted answers on the net and it came out as C. WHY??????