Heating curve below shows the temperature change that occurs as a solid is heated. what is occurring at segment bc of the graph? a line graph showing the heating curve of a substance being heated from the solid phase all the way to the gas phase. from left to right across the x-axis, the different regions of the heating curve are labeled with variables a through f. first, there is an incline with a positive slope labeled ab, followed by a horizontal region labeled bc. then there is another incline with a positive slope labeled cd, followed by another horizontal region labeled de. the end of the heating curve shows another incline with a positive slope labeled ef.