The quantity of heat required for conversion of a particular mass of substance at one temperature to a different temperature depends on the initial state of the substance and the final state of the substance, not on the pathway between them.
| Direct pathway: | (1) 1 g H2O(s) |  | 1 g H2O(l) |
| or | at -25 °C | at 0 °C | |
| Indirect pathway: |  1 g H2O(s)  | ||
| at 0 °C | |||
This is useful because commonly an overall process can be broken down to a series of steps for which the heat requirements are known, and these can be summed to calculate the unknown heat requirement.
EXAMPLE: The heat required for conversion of 1 g ice at -25 °C to 1 g of liquid H2O at 0 °C can be calculated in this way from the data in the heating curve (hovering over the points give their x,y coordinates)blah
Step 1:
H2O(s)at -25 °C
q = qStep1 = 51 J
plus
Step 2:
H2O(l) at 0 °C q = qStep2 = 334 J equals(1)
(1) H2O(s) at -25 °C
H2O(l) at 0 °Cq1= qStep1 + qStep2
The two steps in the example correspond to the first two stages of the heating curve and q1 is the sum of the heat required for these two steps (385 J).
The heat requirements for any set of consecutive states in the heating curve add to give the overall heat requirement for conversion of the specified initial state to the specified final state.