Above, we’ve shown the phase change diagram for some substance at some
pressure. You might very well see a phase change diagram on the SAT
Chemistry Subject Test, so you should understand what information they might
give you. Starting from the lower left, we see that, as heat is added to the
substance, its temperature rises. Moving left to right, we reach the first plateau;
that’s the substance’s freezing/melting point. Notice that the curve is flat for a
little while as the substance passes its melting point. In other words, to move
from a solid to liquid phase, we add heat—but for a while the temperature of the
substance doesn’t change. The heat energy absorbed is used to move the
substance from one phase to the next. The amount of heat that it takes a
substance to just move from solid to liquid phase—to just pass through its
melting point—is called the heat of fusion. For H 2 O at 1 atm, the heat of fusion
is 80 cal/g; this means that it takes 80 calories to change 1 g of H 2 O from 0°C in
the solid phase (ice) to 0°C in the liquid phase (water).
Golden Rule of
Phase Change
Adding heat to a substance
can change kinetic
energy or potential energy,
but never both.After the substance melts, if we continue to add heat, the temperature increases
until the substance reaches its boiling point. At the boiling point, the substance
doesn’t change temperature despite the continued addition of heat. The absorbed
heat is used to move the substance from the liquid phase to the gaseous phase.
The amount of energy that must be added to move the substance from liquid to
aseous phase is called the substance’s heat of vaporization. For H 2 O at 1 atm,
the heat of vaporization is 540 cal/g; it takes 540 calories to change 1 g of H 2 O
from 100°C in the liquid phase (water) to 100°C in the gaseous phase (steam).
Phase Change and Pressure
You know that if we add heat to a solid, the temperature of the solid moves
toward the melting point. If we add heat to a liquid, the temperature of the liquid
increases until it reaches the boiling point. One interesting phenomenon that you
should be aware of for the SAT Chemistry Subject Test is that, under higher
pressure, it’s harder for solids to melt, and it’s harder for liquids to vaporize.
However, if we reduce the pressure of the surrounding environment, we lower a