Constant volume or constant pressure?? In a question I tried to solved: "A house has well-insulated walls. It contains a volume of 100 m3 of air at 300 K. Calculate the energy required to increase the temperature of this diatomic ideal gas by 1.00°C." The solution start with the assumption by saying "consider heating it at constant pressure....." But why? wouldn't be a more accurate assumption if we said the volume is constant but not the pressure as -albeit slight- all temperature increase will result an increase in pressure as well due to P=nRT/V? Obviously house's volume is not changing! Results is highly effected as which way you go either by Cp or Cv.