Omitted from the statement of the question is the boiling point of hydrogen at 3 atm.Without that information,or knowledge of boiling points for similar gases,students would not be able to judge whether condensation of the gaseous hydrogen is likely or unlikely to occur when decreasing the temperature to-20C(6).Sanger et al.cite this misidentification of the conditions under which changes in state occur as one threat to the question's validity(12).Using the normal boiling point(20K)and the enthalpy of vaporization(0.90kJ/mol) (27,28),the Clausius-Clapeyron equation predicts the boiling point of hydrogen at 3 atm to be25.8K(-247C).While a student choosing a response consistent with a condensed phase (e.g.,choice C)would be in error by nearly an order of magnitude in absolute temperature,the error is similar to the factor of