High School
SAT
ACT
University
MCAT
LSAT
DAT
Wize University Chemistry Textbook > Phase Transitions and Phase Diagrams

Phase Diagrams

Heating CurvesPrevious Section
Intro to Equilibrium + characteristics of dynamic equilibriumNext Section
Popular Courses
Find My Course
0:00 / 0:00

Phase Diagram

A phase diagram is a plot of pressure vs temperature which describes the thermodynamically favored phase(s) for a substance.

The phase diagram below is one for compounds that have a higher density in their solid form than in their liquid form!
This describes most compounds in nature. A commonly seen example is CO2.

Photo by OpenStax / CC BY


PAGE BREAK

Photo by OpenStax / CC BY

Triple Point: where solid, liquid and gas are at equilibrium

Critical Point: The point on a phase diagram at which the substance is indistinguishable between liquid and gaseous states

Supercritical Fluid: Matter which has been pushed to temperatures and pressures beyond those of its critical point.
  • Label this in the diagram!
  • This results in a fluid that displays properties of both a liquid and a gas (low viscosity of a gas, but high density of a liquid).
  • No amount of pressure can force the substance back into the liquid phase

PAGE BREAK

General Notes

Photo by OpenStax / CC BY

Solids are favored at
high
pressures and
low
temperatures

Gases are favoured at
low
pressures and
high
temperatures

The normal boiling and melting points can be identified by drawing a horizontal line across P = 1 atm and observing where that line intersects the melting and vaporization curves.

PAGE BREAK

How Phase Diagrams Relate to Density and Enthalpy


The phase diagram above is very similar to the one we just looked at for CO2 where the solid phase has a higher density than the liquid phase.
Based on the density relationships shown above we can say that:

Left: When a melting curve has a positive slope (most compounds) the solid is denser than the liquid.
Right: When a melting curve has a negative slope (more uncommon) the liquid is denser than the solid.

Water's melting curve has a negative slope in the phase diagram that we will look at next!
PAGE BREAK

Water's Phase Diagram



Phase diagrams that have negative slopes for the melting curve are for compounds that have a higher density in their liquid form than in their solid form!

PAGE BREAK

We can remember this because we know that ice floats on water!



Wize Concept
Know that water's phase diagram has a negative slope for the melting curve, whereas most other compounds' phase diagrams have a positive slope.


0:00 / 0:00

Extra Low Yield Information for Phase Diagrams

How the Freezing Point of Water Changes With Increasing Pressure


Above we have a phase diagram with a positive slope, and one with a negative slope. If we chose a point in the solid phase and increased the pressure (leaving temperature constant), what would happen?
  • For the phase diagram with a positive slope, the substance would be in the
    solid
    phase
  • For the phase diagram with a negative slope (like water), the substance may now be in the
    liquid
    phase

What should happen to the freezing point in water's phase diagram when we increase pressure? Should it increase, decrease, or stay the same?
Decrease because of the negative slope line, the freezing point of water goes down as pressure increases

PAGE BREAK

Understanding How a Phase Diagram and Heat Curve are Related

If we are given a phase diagram, we can construct a heat curve at a given pressure. To do this we must draw a horizontal line at P=1atm and remember that phase changes take energy and happen at a constant temperature.


Translating Phase Diagrams to Heating Curves

0:00 / 0:00

Example: Phase Diagrams


a) Using the phase diagram, can you determine whether the solid or liquid phase is more dense?

As the slope for the solid-liquid line is positive, it is expected that the solid will be more dense

b) What is the phase at 5 atm and 35 K?

At 35 K and 5 atm, we would expect to find a supercritical fluid (gas/liquid hybrid) since we are beyond the critical point.

c) At a pressure of 3 atm, what temperature range would you expect to find a gas?

At 3 atm, we would expect to find a gas at 20 K or higher.

d) At what approximate temperature and pressure would we expect to find the triple point?

We would expect to find the triple point at approximately 2 atm and 10 K

Practice: Identifying a Phase Change

Considering the following phase diagram


The process that describes the transition from a to b is
0:00 / 0:00

Example: Phase Diagrams

The phase diagram of Argon is shown below. Argon has two distinct solid phases α and β.
When the solid-α is more dense than solid-β.

a) Label the phases on the diagram below.







PAGE BREAK

b) Does argon have a normal fusion curve?

Argon has a normal fusion curve because the slope is positive. This means that solid is denser than liquid which makes it a normal fusion curve.

c) Can solid-β undergo sublimation? Explain your answer.

Solid – β can sublime because it shares a phase boundary with gas

d) Label the triple point(s) on the graph and describe what phases are present at those points

The triple points are indicated above. T1 contains solid – β, liquid and gas phases in equilibrium. T2 contains solid – α, solid – β, and liquid in equilibrium

Practice: Phase Diagrams

Refer to the phase diagram below for the following questions.


Which statements about this diagram are true?
Extra Practice
Phase diagrams
The following phase diagram is of argon gas:
Temperature
Phase diagrams
The following phase diagram is of argon gas
(a) Which has a higher density? Solid alpha or beta?
(b) Can solid alpha undergo sublimation?
Consider the following phase diagram for an unknown compound X.
What is the normal boiling point of compound X?
Phase Diagrams
The phase diagram of Argon is shown below. Argon has two distinct solid phases α and β.
When the solid-α is more dense than solid-β.
a) Label the phases on the diagram below.
The figure below is a phase diagram for iodine (I2). Use the figure to answer the following two questions
The phase diagram of Argon is shown below. Argon has two distinct solid phases α and β. When the solid-α to solid-β phase transition occurs, heat is given off.
a) Label the sections of the phase diagram with the appropriate phases
b) Which has a higher density solid-α or solid β?
Phase Diagrams: N2
Shown below is a phase diagram forN2{\small \text{N}_2}
Label the phases of the phase diagram, the triple point and the critical point
What is the normal boiling point of N2{\small \text{N}_2}?
Practice: Phase Diagrams
Refer to the phase diagram below for the following questions.
Phase Diagrams
The phase diagram of Argon is shown below. Argon has two distinct solid phases α and β.
When the solid-α is more dense than solid-β.
a) Label the phases on the diagram below.
Shown below is a phase diagram for N2
a) What is the normal boiling point of N2N_2 ? (enter answer in K)
b) What is the normal freezing point of N2N_2? (enter answer in K)
Consider the following phase diagram:
Which of the above locations shows the gas'liquid phase transition line?
Practice: Phase Diagrams
Refer to the phase diagram below for the following questions.
Phase Diagrams
The phase diagram of Argon is shown below. Argon has two distinct solid phases α and β.
When the solid-α is more dense than solid-β.
a) Label the phases on the diagram below.
Intermolecular forces: Phase Diagrams
Using the following phase diagram, can you determine whether the solid or liquid phase is more dense? What is the phase at 5 atm and 35 K? At a pressure of 3 atm, what temperature range would you expect to find a gas? At what approximate temperature and pressure would we expect to find the triple point?
Phase Diagrams
Choose which of the following statements are true:
The critical point on a phase diagram indicates where the liquid and gas phases become identical for a substance
The boiling point of a compound is defined as the temperature where the vapour pressure is equal to the surrounding pressure
Phase Diagram
The point on a phase diagram where all three phases of a substance are in equilibrium is known as:
Phase diagrams
The following phase diagram is of argon gas
(a) Which has a higher density? Solid alpha or beta?
(b) Can solid alpha undergo sublimation?
Phase Diagram: N2
Shown below is a phase diagram for N2
a) What is the normal boiling point of N2N_2 ?
b) What is the normal freezing point of N2N_2?
According to the phase diagram given for substance W, which of the following statements is incorrect?
In the phase diagram below, which is the gas to liquid transition?
Phase Diagrams
The heating Curve for Water is shown below
What is happening along the line labelled A?
The point on a phase diagram where all three phases of a substance are in equilibrium is known as:
Considering the following phase diagram
Which of the following statements is correct?
Considering the following phase diagram
The process that describes the transition from a to b is
Considering the following phase diagram
Which of the following statements is correct?
Shown below is a phase diagram for N2
a) What is the normal boiling point of N2N_2 ?
b) What is the normal freezing point of N2N_2?
Practice: Phase Diagrams
Refer to the phase diagram below for the following questions.
Phase Diagrams
In the phase diagram below, identify the process occurring going from point A to B.