Suppose we heat a pure substance in solid phase at a constant pressure corresponding to it's triple point data up to it's triple point temperature where all the three phases of the substance are known to exist in equilibrium. Will further addition of heat at the same pressure result in departure from triple point temperature? If not will there be transition among the three phases in equilibrium such that the relative quantities of solid,liquid and vapor phases varies with further addition of heat? From the PVT surface of a pure substance such as water it can be observed at an exact temperature of 273.16 K(0.01 °C) and a partial vapor pressure of 611.73 pascals (ca. 6.1173 millibars, 0.0060373 atm) water will exist as ice, liquid water and water vapor in equilibrium.At the corresponding pressure and temperature we have the triple point line along the P-V plot which suggests addition of heat results in a change in specific volume of the mixture which is only possible if a phase transition is in operation.What would happen if we go on heating the mixture along it's triple point line?