Standard Enthalpies of Formation

[MCracchiolo 1C]

When trying to calculate the standard enthalpy of formation for a compound, I understand you use the reactants in their most stable forms, but how do I know whether the product formed is in the right state?

[sahiltelang-Discussion 1J]

In most problems they usually provide you the phase they are in (usually in the subscript of the chemical equation). If not, then you can look at the temperature of the reaction to determine the state. If temperature is not given, assume it is at 25 degrees C (standard conditions).