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container. In the gas, or vapor, state, the molecules have even more energy, are free of one another, and
can fill up the full volume of a container.
To melt a solid means to change it from the solid state to the liquid state. The process required
energy because the molecules of the solid must be freed from the rigid structure. Melting an ice cube to
form liquid water is a common example. To freeze a liquid to form a solid is the reverse of melting and
requires that energy to be removed from the liquid, so that the molecules can settle into a rigid structure.
To vaporize a liquid means to change it form the liquid state to the vapor (gas) state. This process,
like melting, requires energy because the molecules must be freed from their clusters. Boiling liquid water
to transfer it to water vapor (or steam—a gas of individual water molecules) is a common example.
Condensing a gas to form a liquid is the
reverse of vaporizing: it requires that
energy be removed from the gas, so that
the molecules can cluster instead of
flying away from one another.
Suppose you start with a system
in its solid state and heat it at a steady
rate. Figure 1 shows how the system’s
temperature changes. At first, the
temperature increases linearly. This is
Figure 1 The temperature of a system that is heated at a steady state
not hard to understand since equation
(5) can be written as Slope of the T‐Versus‐Q graph ∆ 1 (7) The slope of the graph depends inversely on the system’s specific heat. A constant specific heat implies a
constant slope and thus a linear graph. In fact, you can measure c from such a graph. NOTE: The different slopes indicate that the solid, liquid, and gas phases of a
substance have different specific heats.
But there are times, shown as horizontal line segments, during which the heat is being transferred
to the system but the temperature isn’t changing. These are phase changes. The thermal energy goes into
breaking molecular bonds rather than speeding up the molecules. A phase change is characterized by a
change in the thermal energy without a change in temperature.
The amount of heat energy per unit mass that must be transferred as heat when a sample
completely undergoes a phase change is called the heat of transformation L. Thus when a sample of
mass m completely undergoes a phase change, the total energy transferred is (8)
When the phase change is from liquid to gas (then the sample must absorb heat) or from a gas to liquid
(then the sample must release heat), the heat of transformation is called the heat of vaporization LV. For
water at its normal boiling or condensation temperature, 539 cal/g 40.7 kJ/mol 2256 kJ/kg (9) When the phase change is from solid to liquid (then the sample must absorb heat) or from liquid to solid
(then the sample must release heat), the heat of transformation is called the heat of fusion LF. For water at 5 its normal freezing or melting temperature, 79.5 cal/g 6.01 kJ/mol 333 kJ/kg (10) 2.7 Calorimetry
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This document was uploaded on 03/20/2014 for the course PHYS 215 at Lafayette.
- Fall '09