Environmental Engineering Reference
In-Depth Information
LATENT HEAT
The concept of latent heat can best be understood by conducting a small experiment. Start
with a large block of ice out of a freezer and measure its temperature; perhaps it may be
−15° C. Then place it in a Pyrex glass beaker and heat the beaker at a constant rate,
monitoring the temperature of the ice continuously. Keep heating the beaker until all the
ice has melted into water; eventually it will reach boiling point and vaporize as steam. If
the temperature values are plotted against time, we find a steady increase in temperature
(representing heat input from the heater and some heat flow from the air, which will be
warmer than the ice) until melting starts. Despite the steady addition of heat, there is no
increase in temperature until the ice melts completely (Figure 3.9). A similar effect is
found on vaporization. Where has the heat that was being added continuously gone? It
was being used not to raise the temperature during melting or vaporization but to change
the physical state of the water, either from solid to liquid or from liquid to vapour. As the
heat appears to be hidden, it is known as latent heat.
Figure 3.9
The pattern of temperature and phase changes for
water. The temperature remains constant during each phase
change as long as pressure remains constant. Differences in
specific heat of ice and water give different gradients for the
lines A-B and C-D.
HEAT CONSUMPTION
A change of state, from solid to liquid, or from liquid to vapour, involves a considerable
use of energy. In the first case we need 3·33 × 10
5
J kg
-1
; this quantity of heat is called
the latent heat of fusion. In the second, much more energy is needed. At 10° C the latent
heat of vaporization is 2·48 × 10
6
J kg
-1
but it falls slightly with increasing temperature.
To get a better idea of this large quantity of energy needed for evaporation, the amount
consumed in evaporating only 10 g of water is about the same as that needed to raise the
temperature of 60 g of water from 0° C to boiling point (100° C). We tend to be most
aware of evaporational cooling after swimming. The effect of evaporation leads to the
extraction of heat from the skin surface; sweating works in a similar way.
