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CONCEPTS Applied Energy released
Decreased molecular potential energy
Candle Wax, Bubbles, and a Lightbulb Freezing Condensation
Here are three experiments you can do on heat flow: releases energy releases energy
Conduction: Use melted candle wax to stick thumbtack
heads to a long metal rod. Heat one end of the rod
with a flame. Record any evidence you observe that
heat moves across the rod by conduction.
Convection: Choose a calm, warm day with plenty
of strong sunlight. Make soap bubbles to study
convection currents between a grass field and
an adjacent asphalt parking lot. Find other adjacent
areas where you think you might find convection
currents and study them with soap bubbles, too.
Record your experiments, findings, and explanations
for what you observed.
Radiation: Hold your hand under an unlighted electric Melting Evaporation
lightbulb and then turn on the bulb. Describe evidence absorbs energy absorbs energy
that what you feel traveled to you by radiation, not Increased molecular potential energy
conduction or convection. Describe any experiments
you can think of to prove you felt radiant energy. Energy absorbed
FIGURE 4.17 Each phase change absorbs or releases a quan-
tity of latent heat, which goes into or is released from molecular
potential energy.

Today, an understanding of this particle structure is basic to
physics, chemistry, biology, geology, and practically every other its two directions. Both occur at a temperature called the freez-

science subject. This understanding has also resulted in present- ing point or the melting point, depending on the direction of the
day technology. phase change. In either case, however, the freezing and melting
points are the same temperature.
The liquid-gas phase change also occurs in two diff erent

PHASE CHANGE directions. The temperature at which a liquid boils and changes

Solids, liquids, and gases are the three common phases of mat- to a gas (or vapor) is called the boiling point. Th e temperature
ter, and each phase is characterized by diff erent molecular at which a gas or vapor changes back to a liquid is called the
arrangements. The motion of the molecules in any of the three condensation point. The boiling and condensation points are

common phases can be increased by (1) adding heat through a the same temperature. There are conditions other than boiling
temperature diff erence or (2) the absorption of one of the fi ve under which liquids may undergo liquid-gas phase changes,
forms of energy, which results in heating. In either case, the and these conditions are discussed in the next section, “Evapo-
temperature of the solid, liquid, or gas increases according to ration and Condensation.”

the specific heat of the substance, and more heating generally You probably are not as familiar with solid-gas phase
means higher temperatures. changes, but they are common. A phase change that takes a
More heating, however, does not always result in solid directly to a gas or vapor is called sublimation. Moth-
increased temperatures. When a solid, liquid, or gas changes balls and dry ice (solid CO 2 ) are common examples of mate-
from one phase to another, the transition is called a phase rials that undergo sublimation, but frozen water, meaning
change. A phase change always absorbs or releases a quantity common ice, also sublimates under certain conditions. Per-
of heat that is not associated with a temperature change. Since haps you have noticed ice cubes in a freezer become smaller
the quantity of heat associated with a phase change is not with time as a result of sublimation. The frost that forms in a

associated with a temperature change, it is called latent heat. freezer, on the other hand, is an example of a solid-gas phase
Latent heat refers to the “hidden” energy of phase changes, change that takes place in the other direction. In this case,
which is energy (heat) that goes into or comes out of internal water vapor forms the frost without going through the liquid
potential energy (Figure 4.17). state, a solid-gas phase change that takes place in an opposite
There are three kinds of major phase changes that can occur: direction to sublimation.

(1) solid-liquid, (2) liquid-gas, and (3) solid-gas. In each case, the For a specific example, consider the changes that occur
phase change can go in either direction. For example, the solid- when ice is subjected to a constant source of heat (Figure 4.18).

liquid phase change occurs when a solid melts to a liquid or Starting at the left side of the graph, you can see that the tem-
when a liquid freezes to a solid. Ice melting to water and water perature of the ice increases from the constant input of heat.

freezing to ice are common examples of this phase change and The ice warms according to Q = mcΔT, where c is the specifi c
4-15 CHAPTER 4 Heat and Temperature 99