The graph below shows how the temperature changes when energy in the form of heat is added to 1 g of water.
heating curve 1 g water 400 by 250 no colours
Heat is an
extensive quantity, and the amount of heat required or released
depends on the
amount of substance undergoing change.
The
graph above gives data for changes involving
1 g of substance.
The heat required from the surroundings or released to the surroundings for different masses of water will be different, but these can be calculated from the data for 1 g of substance.
Hovering over the points at either end of the line at 0 °C shows that 334 J is required to melt 1 g of ice. This can be expressed as 334 J g–1 (read 334 joules per gram).
Provided there is no temperature change, the heat required to melt any mass of ice can be calculated by multiplying +334 J g–1 by the mass in grams. The sign is positive because heat is added to the ice.
334 J g–1 of heat is released on freezing water at 0 °C (–334 J g–1). The sign is negative because heat flows from the water.
EXAMPLES:Calculate the heat required to melt 10 g of ice at 0 °C to 10 g water at 0 °C:
heat = 334 J g–1 × 10 g = +3340 J
Note that the gram unit cancels because g–1 × g = g0 =1
Calculate the heat released in freezing of 0.5 g liquid water at 0 °C to ice at 0 °C.
heat = –334 J g–1 × 0.5 g = –167 J
Note that the gram unit cancels because g–1 × g = g0 =1