time and position dependent reaction

Hello everyone,

i am very new to this and not sure which moduls of the comsol software are needed to solve my problem.

there will be a phase change with heat production(white box-> black box in the attached file), but for the phase change the contact to the "black" phase is provided and its depending on the temperature where the next step of the transition is going on. for sure both phases have different physical properties.

i thought to use the heat transfer module and the reaction module, but there i couldnt find a function to localize the species A and B. is there any possibility to define position of the species A and B in the begining?

or should i use another modul of the software?

of course in the end i wanna see step by step where is phase is changing