Explanation:
a. The melting point is at 20C and the boiling point is at 80C. The melting point is when a solid is going through a phase change to become a liquid, that is the 1st plateau in the graph. The boiling point is when the liquid is going through vaporization to become a gas. That is the second plateau.
b. At 30C, it is a liquid, since our melting point is at 20C.
c. The substance before 20C is a solid, as we add energy, it will melt and become a liquid.
The graph is different for each substance but in this same configuration where a change in the graph is a different phase or a phase change.
Where that first incline = solid
1st plateau= melting or freezing
2nd incline= liquid
2nd plateau= vaporization or condensation
3rd incline= gas
