This graph appears frequently on regents exams and is directly related to the "properties of water".
From left to right heat energy (in Joules) is being added to water. From right to left that same energy is being removed and lost into the environment.
Facts (from the Specific Heat table, ESRT page
1):
To change the temperature of 1g of liquid water by 1°C requires 4.18 Joules.
To change the temperature of 1g of solid water (ice) by 1°C requires only 2.11
Joules.
To change the temperature of 1g of water vapor (gas) by 1°C requires only 2.00
Joules.
Interpreting the graph:
A to B: solid ice is warming (energy added). B to
A: Solid ice is cooling (energy lost). Energy = 2.11 J/g·°C
[temperature change]
B to C: Solid ice is melting (energy added). C to B: Liquid water is freezing
(energy lost). Energy = 334 J/g [phase change]
C to D: Liquid water is warming (energy added). D to C: Liquid water is cooling
(energy lost). Energy = 4.18 J/g·°C
[temperature change]
D to E: Liquid water is evaporating or vaporizing (energy added). E to D: Water
vapor is condensing. Energy = 2260 J/g [phase
change]
Notice that where the line is sloped (A to B and C to D) there is a temperature change. Where the line is flat (B to C and D to E) there is a phase change but no temperature change.