Heat is an
extensive quantity, and heat required or released for a particular temperature change
depends on the
amount of substance undergoing change. The
graph above gives data for changes involving
1 g of substance.
heating curve 1 g water 400 by 250 no colours
The graph shows that at temperatures where
addition of heat causes
an increase in temperature, the extent of the temperature increase also
depends on the
phase of water.
In these regions of the graph the lines slope to the right because as heat is added, the temperature increases. The line for heating liquid water slopes to the right to a greater extent that the ones for heating ice or water vapour.
This means that for liquid water, more heat must be added to change the temperature by 1 °C than for either of the other phases.
The heat required to cause a 1 °C temperature change for 1 g of a substance is known as its
specific heat capacity. The unit for specific heat capacity is
J °C–1 g–1
(read as Joules per degree C per gram).
The specific heat capacities for ice, water and water vapour can be calculated from the graph above by dividing (per means "divide by") the heat in joules added over a particular temperature range by the temperature range in °C
–1.