The (molar) enthalpy of formation is the heat released ()or absorbed ( ) in a chemical reaction at constant pressure when simple substances combine into a more complex substance. At standard conditions of pressure and temperature (1 atm and 298 K), it is denoted . For elements in their standard states, . For gases, the standard state is the atom or molecule existing at standard state conditions. For solids with different phases, the assignment is arbitrary. The hydrogen ion is also defined to have H⁺ = 0.