PHASE CHANGES AND PHASE DIAGRAM
Phase changes are transformations of matter from one physical state to another. They occur when energy (usually in the form of heat) is added
or removed from a substance. They are characterized by changes in molecular order; molecules in the solid phase have the greatest order, while those
in the gas phase have the greatest randomness or disorder.
Types of Phase Changes
• The change from solid to liquid is melting, liquid to gas is vaporization, and solid to gas is
sublimation. These changes take place when heat is absorbed (heat gained). They are endothermic
processes.
• The reverse change from gas to liquid is condensation, gas to solid is deposition, and liquid to
solid is freezing. These changes give off heat (heat lost) and are exothermic processes.
Phase diagram is a graphical representation of the physical states of a substance under
different conditions of temperature and pressure. It gives the possible combinations of pressure and
temperature at which certain physical state or states a substance would be observed. Each substance
has its own phase diagram.
What are the features of a phase diagram?
Phase diagrams are plots of pressure (1 atm = 101.3 kpa = 760 mm Hg) versus temperature (usually in degrees Celsius or Kelvin).
A. The Three Areas
The three areas are marked solid, liquid, and vapor. Under a set of conditions in the
diagram, a substance can exist in a solid, liquid, or vapor (gas) phase. The labels on the
graph represent the stable states of a system in equilibrium.
Suppose a pure substance is found at three different sets of conditions of temperature and
pressure corresponding to A, B, and C as shown in the following diagram. Under the set of
conditions at A in the diagram, the substance would be a solid as it falls into that area of
the phase diagram. At B, it would be a liquid; and at C, it would be a vapor (gas).
B. Three Lines (Curves)
The lines that serve as boundaries between physical states represent the combinations of pressures and temperatures at which two phases can exist in
equilibrium. In other words, these lines define phase change points.
1. The green line divides the solid and liquid phases, and represents melting (solid to liquid) and freezing (liquid to solid) points.

Melting (or freezing) curve – the curve on a phase diagram which represents the
transition between liquid and solid states. It shows the effect of pressure on the melting
point of the solid. Anywhere on this line, there is equilibrium between the solid and the
liquid.
2. The blue line divides the liquid and gas phases, and represents vaporization (liquid to
gas) and condensation (gas to liquid) points.

Vaporization (or condensation) curve – the curve on a phase diagram which represents
the transition between gaseous and liquid states. It shows the effect of pressure on the
boiling point of the liquid. Anywhere along this line, there will be equilibrium between
the liquid and the vapor.
3. The red line divides the solid and gas phases, and represents sublimation (solid to gas)
and deposition (gas to solid) points.

Sublimation (or deposition) curve – the curve on a phase diagram which represents the
transition between gaseous and solid states. It represents the effect of increased
temperature on a solid at a very low constant pressure, lower than the triple point.
C. Two Important Points
1. The triple point

The triple point is the combination of pressure and temperature at which all three
phases of matter are at equilibrium. It is the point on a phase diagram at which the
three states of matter coexist. The lines that represent the conditions of solid-liquid,
liquid-vapor, and solid-vapor equilibrium meet at the triple point.
2. The critical point

The critical point terminates the liquid/gas phase line.

The critical point, the temperature above which the gas cannot be liquefied no
matter how much pressure is applied (the kinetic energy simply is too great for
attractive forces to overcome).

Beyond the temperature of the critical point, the merged single phase is known as
a supercritical fluid (indistinguishable between gas or liquid- neither one).

The temperature and pressure corresponding to this are known as the critical
temperature and critical pressure.
The Phase Diagram for Water

Notice that the triple point for water occurs at a very low pressure, 0.006 atm and at
273.2 K temperature. Also notice that the critical temperature is 647 K (374°C). It
would be impossible to convert water from a gas to a liquid by compressing it above
this temperature. The critical pressure is 218 atm.

The normal melting and boiling points of water are found in exactly the same way as
we have already discussed - by determining where the 1 atm pressure line crosses the
solid-liquid, and then the liquid-vapor equilibrium lines. The normal melting point of
water is 273 K (0 oC), and its normal boiling point is 373 K (100 oC).
The Phase Diagram for Carbon Dioxide

The only thing special about this phase diagram is the position of the triple point,
which is well above atmospheric pressure. It is impossible to get any liquid carbon
dioxide at pressures less than 5.2 atmospheres.

At 1 atm pressure, carbon dioxide will sublime at a temperature of 197.5 K (-75.5 °C).
This is the reason why solid carbon dioxide is often known as "dry ice." There is no
liquid carbon dioxide under normal conditions - only the solid or the vapor.
Name: _________________________________________
STEM 12- ___
Group No: _____
Score: ___________
Date: ____________
ACTIVITY 6- INTERPRETING PHASE DIAGRAM
Direction. Refer to the following phase diagram of a certain substance to answer the following questions.
Part A.
Part B.
1. What phases of water coexist at point C in water’s phase diagram?
1. In what phase is the substance at 50 °C and 1 atm pressure?
2. At what pressure and temperature conditions will all three phases of
the substance be present?
2. What two phase changes occur at point D in the phase diagram for
water?
3. What is the critical temperature of water?
3. What is the normal melting point of the substance?
4. What pressure is at water’s normal boiling point?
4. What phase(s) will exist at 1 atm and 70 °C?
5. What occurs at the triple point?
Part C.
Part D.
1. Look at the phase diagram for carbon dioxide. Above which pressure
and temperature is carbon dioxide unable to exist as a liquid?
1. What is the temperature at which the triple point occurs?
2. At which pressure and temperature do the solid, liquid, and gaseous
phases of carbon dioxide coexist?
2. What 2 phase changes occur at Point A?
3. What two phase changes occur at point E in the phase diagram for
carbon dioxide?
3. What phase change does the substance at 100 bars undergo as the
temperature decreases from 250 K to 200 K?
4. What phase change occurs as carbon dioxide moves from -78°C to
24°C at a pressure of 50 atm?
4. What is the pressure at which the critical point occurs?
5. What 2 phase changes occur at Point B?