What is the name of the region (on the graph) where the temperature remains constant while heat is being absorbed (or released)?

A heating curve graphically represents the phase transitions that a substance undergoes as heat is added to it. The plateaus on the curve mark the phase changes. The temperature remains constant during these phase transitions.

> b takes place in the gas phase in an isothermal tubular reactor. the feed is one mole of a per one mole of c, an inert. the entering temperature and pressure are 427°c and 10 atm, respectively. the gas constant r = 0.08206 atm · l/mol · k.