The diagram shows a heating curve for a substance that begins as a solid. A graph titled heating curve of Substance It shows heat added in joules on the x-axis, and temperature in degrees celsius on the y-axis on a scale of 0 to 120. The graph has a diagonal line from 30 degrees up to 60 degrees. A horizontal line segment at 60 degrees. A diagonal line segment from 60 degrees up to 90 degrees. A horizontal line segment at 90 degrees and then another diagonal segment.
How many changes of state are shown?

0
1
2
3