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HOW TO Read the Phase Diagram of Water
6
1
1
1
Solidus
elting curve)
Triple point
ation curve
Whenever you use a pressure canner, concentrate or distill under
vacuum, or freeze-dry food, you exploit the energetic phenomena that
occur when water changes from one state of matter to another. To
better understand these transitions, it helps to look at a phase diagram.
A phase diagram is a map that shows what form a substance will
assume at a range of pressures and temperatures. The phase diagram at the melting and boiling points, which shift as the pressure changes. (The
Temperature (˚F)
the right illustrates the behavior of pure 32water. By convention, colder “normal” freezing and boiling points of water at 0 °C / 32 °F and 100 °C /
212 705.2˚F
temperatures are on the left, hotter temperatures on the right, low 212 °F actually apply only when the ambient pressure is 1 bar / 14.5 psi.)
Pressure (bar)
SOLID
221
Triple point
Temperature (˚F)
32 212 705.2˚F
1
0.0061
Sublimation curve
SOLID
LIQUID
Solidus
(melting curve)
GAS
0 100 374
Temperature (˚C)
Triple point
Sublimation curve
GAS
Critical point 221
Pressure (bar)
1
Saturation curve
pressures (such as vacuums) near the bottom, and high pressures (such
as those in a pressure cooker) toward the top.
Solid lines delineate the boundaries between the realms of solid,
liquid, and gas. At temperature/pressure combinations beneath the solid
lines, the phases on either side can exist together—that is, the lines trace
LIQUID
SUPER-
The triple point is the unique combination of temperature and
The critical point marks the region on the phase diagram where
1 pressure at which all three phases of a substance exist in 2 CRITICAL
liquid and gas become indistinguishable. Beyond the critical point,
Critical point FLUID
equilibrium—that is, with no further melting or freezing taking place. the material exists as a supercritical fluid that displays features of
For water, the triple point occurs at 0.01 °C / 32 °F and 6.1 mbar /
both a liquid and a gas. Like a gas, a supercritical fluid is compressible
0.089 psi. The triple point of water is so reliable that it’s been used for
and expands to fill its container, but substances dissolve in it as if it
centuries to calibrate thermometers. If you put ice and water LIQUID into a
were liquid. The critical point of water falls at 221 bar / 3,205 psi and
closed container and let them come to equilibrium, the water vapor
374 °C / 705 °F. Supercritical water is unlikely to be found in the
will automatically assume its triple point pressure. (Note that this is the kitchen, but supercritical carbon dioxide is used in industrial
pressure of the water vapor alone, regardless of what other gases may
processes with food. Unlike the boiling and freezing points, the triple
be present in the container. Solidus It’s called the partial pressure.)
Temperature
and
(˚F)
critical points are fixed; they do not vary with pressure.
(melting curve)
32 212 705.2˚F
Saturation curve
SUPER-
CRITICAL
FLUID
SOLID
GAS
Solidus
(melting curve)
Triple point
Critical point
Saturation curve
0.00610 100 374
Temperature (˚C)
The so-called “boiling point” is not a pointit depends on pressure,
The conditions for freezing also depend on pressure, so there is no
10
3 so on a phase diagram it is a curve, called the saturation curve, that 4 “freezing point”instead there are two curves. Above the triple
Triple point
runs from the triple point to the critical point. On one side is liquid,
point there is the solidus or melting curve. On one side is solid, on the
−100 0 100 200 300 400 500
on the other, gas.
other, liquid. Water Sublimation has an unusual curve solidus that curves back to the left.
Temperature (°C)
This means you can freeze water by lowering the pressure, a property
exploited by pressure-shift freezing (see page 309). Below the triple
Alcohol (ethanol)
point is the sublimation curve that separates 0 solid from gas.
100 Triple point: 374−114 °C / −173 °F and 8.8 × 10 −6 mbar / 1.3 × 10 −7 psi
Temperature (˚C)
0 100 374
Critical point: 241 °C / 466 °F and 61 bar / 885 psi
Temperature (˚C)
SUPER-
CRITICAL
FLUID
Pressure (bar)
221
1
0.0061
Temperature (˚F)
32 212 705.2˚F
LIQUID
GAS
SOLID
Solidus
(melting curve)
Triple point
Sublimation curve
Temperature (˚F)
32 212 705.2˚F
LIQUID
GAS
Critical point
Saturation curve
0 100 374
Temperature (˚C)
Water SUPER-
Triple point: 0.01
CRITICAL
°C / 32 °F and 6.1 mbar / 0.089 psi. Critical point: 374 °C / 705 °F and 221 bar / 3,205 psi
Critical point FLUID
Saturation curve
Pressure (bar)
10 3 −100 0 100 300 500 700 900
10 1
10 −1
10 −3
10 −5
LIQUID
Critical point
Saturation curve
Temperature (°F)
SUPERCRITICAL REGION
GAS
Pressure (bar)
−400 −300 −200 −100 0 100 200
Melting curve
10 3
10 1
SOLID
LIQUID
Critical point
Saturation curve
Temperature (°F)
SUPERCRITICAL REGION
SUPER-
CRITICAL
FLUID
10 −1 Triple point
GAS
Sublimation curve
10 −3
−200 −100 0 100
Temperature (°C)
Nitrogen
Triple point: −210 °C / −346 °F and 127 mbar / 1.8 psi
Critical point: −147 °C / −233 °F and 34 bar / 493 psi
302 VOLUME 1 ·· HISTORY AND FUNDAMENTALS THE PHYSICS OF FOOD AND WATER 303