Phase Diagram

Phase Diagrams show what effect temperature and pressure have on a pure substance in a closed system without any air.

The Phase Diagram

Concept: The Properties of the 3 Phases of Matter 

6m
Video Transcript

Welcome back, guys. In this new video, we’re going to take a look at Phase Diagrams. Now,
We’ve talked about this way back in the beginning of the semester. We’re going to say under appropriate conditions of pressure and temperature, matter exists in three different forms. Matter can exist as a gas, a liquid or a solid.
Now, when it comes to each of these different states or phases of matter, they have different properties that are specific to them. So if we take a look here, we're going to have a microscopic explanation for the behaviour of all three phases.
We’re going to say gases, they assume the shape and volume of their container. If you have a large container, you input gas in there; the gases will spread out as far as possible to take up as much space as they can inside the container. We say that this is the gas behaving ideally.
Liquids assume the shape of the portion of its container it occupies, but not the volume, not always. Because let’s say we have a container that can hold a gallon of water and the gallon is shaped a specific way, but let’s say we don’t have a gallon of water. Let’s say we have only half a gallon of water and we pour it into the container. The water will take up half of the volume and take up the shape but it won’t be able to take up the rest of the volume because there’s not enough water. So water, liquids, always take up the shape but not necessarily the volume. If there’s not enough liquid, it won’t be able to fill up the entire container.
Solids are very stubborn. Solids maintain a fixed volume and shape. So you have a container, you dump a rock in there. The rock does not conform. It doesn’t change its shape to fit the container. It doesn’t try to fill up the container. It is what it is.
Now, compressibility, we’ve talked about this as well. Compressibility has to do with us being able to push down on the molecules that make up that phase.
Gases are highly spread out inside a container, but if I apply pressure, I can compress them, squeeze down so that those gas particles, gas molecules, get closer and closer together. So we’re going to say gases are highly compressible. I can increase the pressure and squeeze their molecules closer.
Now, liquids are not far apart. They’re right on top of each other but they’re fluid. They can move around each other. There’s not much space, not much free space between them. They’re rubbing up against each other. So we’re going to say that liquids are not really compressible. We might be able to squeeze a little bit but overall they’re not compressible because the molecules are already as close as they’re going to be. They’re just rubbing up against each other as they’re sliding around and moving around.
Solids, their molecules are frozen in place. They’re already as close as they’re going to be. They’re stacked on top of each other. So we’re going to say that solids are not compressible, not at all.
Then we’re going to say viscosity. Viscosity is just the resistance to flow. That means it doesn’t want to move. So we talked about this earlier too. If you make a comparison between a bucket of water and bucket of honey. So if you take the bucket of water and turn it upside down on someone’s head, the water is going to come right down and drench that person. But let’s say we have a bucket of honey. Honey moves a lot slower than water. Honey is more viscous, it doesn’t want to move as fast. So if I take that bucket and dump it over someone’s head, some of it might come out immediately but a good majority of it is going to take time to come out.
How do we decrease viscosity? If you increase the temperature, you’re going to excite the molecules. The molecules get faster, that honey will come flowing out faster. So if you increase temperature, you decrease viscosity.
So we’re going to say gases, gases are bouncing all over the place. They move incredibly fast, so they’re going to have low viscosity. They don’t have a resistance to flow. They want to move as fast as they can. Liquids have moderate viscosity. Depending on the liquid, they can flow fast or slow. And then solids really don’t flow, they’re just what they are. So solids have a high viscosity.
So these are each of the microscopic explanations of each of these phases of matter and remember especially the terms viscosity because these are the terms that you’re going to be associating with intermolecular forces and all the other things that we talked about when drawing a compound.

Concept: Understanding the Phase Diagram

7m
Video Transcript

Associated with the three different phases are these properties but also what we call a phase diagram. We’re going to say this shows the effect that temperature, temperature down here, and pressure have on a pure substance in a closed system. So there’s no air entering the system. We’re going to say any substance, when we do a phase diagram we can look at its three phases. We can have its solid phase, its liquid phase. and its gas phase. And the phase diagram basically explains at such a temperature and such pressure, what phase will that matter exist as. Will it exist as a solid, a liquid or a gas at this particular temperature and this particular pressure? That’s all the phase diagram is really useful for.
And what we’re going to say is this phase diagram represents a compound and the only compound it doesn’t represent is water. Water is a very unique substance. If you think about it, if you freeze water, it becomes ice, right? Let’s say you take that ice cube and you dunk it into a cup of water. What usually happens is the ice cube actually floats on top of the water. This means that the ice, the solid, is actually less dense than the liquid. The solid ice weighs less, that’s why it’s able to float on the surface of the liquid. This is a weird phenomenon that happens only with water.
All other substances, when you freeze them to a solid and you drop them in their liquid form, the solid naturally just sinks to the bottom, which is what we expect, but water isn’t that way. And, again, why does water behave this way? Because of the H bonding that exists in water. The H bonding that exists between the molecules give them a certain orientation and when you freeze them, this orientation gets changed even further giving us a special shape. This special shape allows the solid to float on its liquid form.
So the phase diagram for water is different in this way. You see this line here? This line points towards the right. That signifies that this phase diagram is any compound except water. Water’s line points to the left. It’s the only one where that line is going to move towards the left. We’re going to say here, as the temperature and pressure increase, we’re going to say that the liquid portion becomes more dense. That’s what moving that line over to the left really signifies.
And we’re going to say how do we read this phase diagram? Well, we’re going to say a phase diagram again shows all three states. We’re going to say here, if we’re going straight from solid to gas, that phase change is called sublimation and that can happen. Carbon dioxide can go straight from a solid to a gas skipping the liquid phase altogether.
If we’re going from a gas to a solid, we’re depositing a solid so that’s deposition. If we’re going from a solid to a liquid, then we’re going to say that that is melting or what we call fusion. So remember fusion or melting mean the same thing. If we’re going from a liquid to solid, that’s freezing. If we’re going from liquid to gas, that’s vaporization. If we’re going from gas to liquid, that’s condensation. We should know all of these different types of phase changes.
Next, we’re going to say that let’s say that this is point A, point B, point C and point D. We’re going to say at point B, we have something called the triple point. At the triple point, the substance exists in all three phases. At that point, exists as a liquid, a solid and a gas, all at once, the triple point.
We’re going to say at point D, point D is called the critical point. The critical point is the last point on our phase diagram where our substance can exist as a liquid or a gas. If we go beyond the critical point, the substance becomes something entirely different. It becomes a substance that we can’t recognize. It becomes a substance called liquid plasma. The temperature is so high, the pressure is so high that the compound doesn’t look like a solid or a gas anymore. We can’t recognize it. It looks like liquid plasma. If you wanted to realize what does liquid plasma looks like just think of lava or maybe the sun. If you could like physically reach the sun and grab a chunk of it, that’s what liquid plasma would look like or lava will probably be the closest thing on earth that we can say liquid plasma looks like.
So again, the critical point is the last point at which we can still recognize the substance. Once we go a little bit beyond the critical point, we can’t tell if it’s a liquid or a gas anymore and it looks like something called liquid plasma.
And then we’re going to say that these other major portions of the phase diagram. So one, remember, that water has a different phase diagram from all other substances, and then to recognize the different points associated with the phase diagram. We have three phases of matter and the phase diagram just explains how temperature and pressure play a role on going from one phase to another phase.

The phase diagram looks at the transition of matter between the phases of solid, liquid and gas as temperature and pressure are affected.

Problem:

Answer the following questions based on the image above: 

a. At what temperature can we no longer tell the difference between the liquid and the gas? _____________

 b. Which point represents an equilibrium between the solid, liquid and gas phase?    _____________

 c. Which line segment represents an equilibrium between fusion and freezing? _____________

 d. Which line segment represents an equilibrium between sublimation and deposition? _____________

 e. Which line segment represents an equilibrium between condensation and vaporization? _____________

 f. What is the normal freezing point of this unknown substance? _____________

 g. What is the normal boiling point of this unknown substance? _____________

7m

Phase Diagram Additional Practice Problems

Evaporation is: 

Select one:

a. a cooling process for humans when they sweat. 

b. All of these 

c. increased by increasing temperature. 

d. an endothermic process. 

e. the opposite process as condensation.

Watch Solution

A gaseous sample of a substance is boiled at constant pressure. Which of the following diagrams best represents the situation if the final temperature is (a) above the boiling point of the substance and (b) below the boiling point but above the freezing point of the substance?

Watch Solution

Use these data to draw a qualitative phase diagram for H2. Does H2 sublime at 0.05 atm? Explain.

Watch Solution

Use these data to draw a qualitative phase diagram for ethylene (C2H4). Is C2H4(s) more or less dense than C2H4(l)?

Watch Solution

The phase diagram for substance A has a solid-liquid line with a positive slope, and that for substance B has a solid-liquid line with a negative slope. What macroscopic property can distinguish A from B?

Watch Solution

Match each numbered point in the phase diagram for compound Q with the correct molecular depiction below:

Watch Solution

Many heat-sensitive and oxygen-sensitive solids, such as camphor, are purified by warming under vacuum. The solid vaporizes directly, and the vapor crystallizes on a cool surface. What phase changes are involved in this method?

Watch Solution

Name the phase change in each of these events:

(a) A diamond film forms on a surface from gaseous carbon atoms in a vacuum.

(b) Mothballs in a bureau drawer disappear over time.

(c) Molten iron from a blast furnace is cast into ingots (“pigs”).

Watch Solution

Name the phase change in each of these events:

(a) Dew appears on a lawn in the morning.

(b) Icicles change into liquid water.

(c) Wet clothes dry on a summer day.

Watch Solution

How does the energy of attraction between particles compare with their energy of motion in a gas and in a solid? As part of your answer, identify two macroscopic properties that differ between a gas and a solid.

Watch Solution

Complete this table describing the shape and volume of each phase.

                             Shape           Volume

fixed            Solid    ____            _____

variable       Liquid   ____           _____

                    Gas      ____           _____

Watch Solution

Enter the critical temperature of water. 

Watch Solution

What factors cause changes between the solid and liquid state? Check all that apply.


a. A solid can be converted to a liquid by cooling.
b. A liquid can be converted to a solid by cooling.
c. A solid can be converted to a liquid by heating.
d. A liquid can be converted to a solid by heating.

 

Watch Solution

What factors cause changes between the liquid and gas state? Check all that apply.
 

a. A gas can be converted into a liquid by decreasing the pressure of a gas sample.
b. A liquid can be converted to a gas by heating.
c. A gas can be converted into a liquid by increasing the pressure of a gas sample.
d. A liquid can be converted to a gas by cooling.
e. A gas can be converted into a liquid by cooling.
f. A gas can be converted into a liquid by heating.

Watch Solution

At 100oC and 1 atm, water is in which phase?

a. solid

b. liquid

c. gas

d. supercritical fluid

e. solid-liquid equilibrium

f. liquid-gas equilibrium

g. solid-gas equilibrium

Watch Solution

What is the normal boiling point of this compound?

 

Watch Solution

The phase diagram for sulfur is shown below. The rhombic and monoclinic phases are two solid phases.

Below what pressure will solid sulfur sublime?

Express your answer using two significant figures.

Watch Solution

Which of the following statements is NOT correct?

a. As a liquid is heated at a constant pressure above the critical pressure, it eventually changes to the gas phase.
b. It is possible to find a path such that a liquid can be converted to a gas without a phase change.
c. Increasing the pressure of a fluid at a constant temperature above the critical temperature produces no phase change no matter how great the pressure becomes.
d. Below the critical temperature both the liquid and gas phase can exist. Higher pressure favors the liquid phase.

Watch Solution

A phase diagram is a pressure-temperature graph that shows the ranges of temperature and pressure over which each phase is stable.

The phase diagram is divided into three regions, each region representing one stable phase: solid, liquid, or gas. There are three boundary lines that separate the phases from one another. These boundary lines represent the equilibrium between two phases. The point at which the three boundary lines intersect is called the triple point. All three phases can exist in equilibrium with each other at this point.

Use the phase diagram for CO 2 and determine which of the following statements are correct.

SELECT ALL THAT APPLY.

  • All three phases of CO2 exist simultaneously at the triple point.
  • When the pressure is 1 atm, there is no temperature at which the liquid phase of CO  2 exists.
  • CO2 forms a supercritical fluid at temperatures less than 31 degrees Celsius.
  • CO2 is a gas under normal conditions of temperature and pressure.
  • Movement across boundary line BO corresponds to a phase change.
  • When the pressure is 4 atm and the temperature is more than -56.7 degrees Celsius, CO  2 exists as a solid.
Watch Solution

Which of the following states of matter is classified as a condensed phase?

a) solid

b) liquid

c) gas

d) Both a and b

e) Both a and b and c

Watch Solution

Draw and label a phase change diagram.

Watch Solution

In the phase diagram of a substance shown  to the right, fill in the blanks to make correct statements.

a) Moving from point B to C results in a transition from _______ phase to ________ phase.

b) At point _______ all three phases of solid, liquid, and gas coexist in equilibrium.

c) At a higher pressure and temperature than point ______ the substance becomes a supercritical fluid.

d) At point F, ______ phase and ______ phase coexist in equilibrium. 

Watch Solution

Choose the answer that correctly assigns the labels on the phase.

A) A = liquid, B = gas, C = solid, D = triple point

B) A = gas, B = liquid, C = solid, D = critical point

C) A = liquid, B = solid, C = gas, D = critical point

D) A = gas, B = solid, C = liquid, D = triple point

E) A = solid, B = gas, C = liquid, D = supercritical fluid

Watch Solution

Superman can squeeze graphite (coal) to such a high pressure that it is transformed to diamond. Let’s consider this claim against the underlying science. The phase diagram for carbon is shown below. 

What pressure does Superman need to exert to change graphite into diamond at room temperature?

Watch Solution

Consider the phase diagram below.

How many distinct solid phases are depicted?

1. 1

2. 3

3. 4

4. 2

Watch Solution

The phase diagram of CO2 is shown here. It's in Russian but you know enough science to be able to translate the words.

Watch Solution

For the choices below, what is the normal boiling point of this substance? 

1. 230°C

2. 100°C

3. 200°C

4. 150°C

What is the critical pressure for this substance?

1. 50 atm

2. 0.08 atm

3. 44 atm

4. 25 atm

5. 1 atm

6. >100 atm

Watch Solution

Sulfur can exist as a gas, a liquid, or as one of two solid states: rhombic and monoclinic. A phase diagram for sulfur is shown below:

a. What is (are) the thermodynamically-stable phase(s) of sulfur at room temperature and pressure (i.e. 1.0 atm at 25°C)?

 

 

b. Which state of sulfur is more dense: rhombic, monoclinic, or liquid?

Watch Solution

What does letter D represent? 

a. fusion curve

b. triple point

c. sublimation point

d. melting point

e. critical point

Watch Solution

A sample is at the temperature and pressure indicated by point (c) on the phase diagram below. What phase is present after the pressure is increased to the critical pressure at constant temperature?

A.  Gas

B.  Liquid

C.  Solid

D.  Supercritical fluid

E.  More than one phase is present

Watch Solution

Consider the phase diagram below.  If the dashed line at 1 atm of pressure is followed from 100 to 500°C, what phase changes will occur (in order of increasing temperature)?

A) condensation, followed by vaporization

B) sublimation, followed by deposition

C) vaporization, followed by deposition

D) fusion, followed by vaporization

E) No phase change will occur under the conditions specified.

Watch Solution

Consider the phase diagram shown.  Choose the statement below that is TRUE.  

A) The triple point of this substance occurs at a temperature of 31°C.

B) At 10 atm of pressure, there is no temperature where the liquid phase of this substance would exist.

C) The solid phase of this substance is higher in density than the liquid phase.

D) The line separating the solid and liquid phases represents the ΔHvap.

E) None of the above are true.

 

Watch Solution

What is the process where molecules go directly from the solid phase to the gas phase?

a. sublimation

b. melting

c. condensation

d. deposition

e. freezing

Watch Solution

Consider the following phase diagram and identify the process occurring as one goes from point C to point D.

a. increasing temperature with a phase change from liquid to vapor

b. increasing temperature with a phase change from solid to liquid

c. increasing temperature with no phase change

d. increasing temperature beyond the critical point

e. increasing temperature with a phase change from solid to vapor

Watch Solution

According to Figure 1, what is the normal boiling point of Xe?

A. < 152 K

B. 161 K

C. 152 K

D. 166 K

E. > 166 K

Watch Solution

You have a sample of Xe at 164 K and 760 torr. You reduce the pressure to 250 torr. According to Figure 1, what happens?

A. It melts.

B. It vaporizes.

C. It condenses.

D. It solidifies.

E. Nothing. No phase change occurs.

Watch Solution

Answer the following questions based on the following phase diagram given below.

a. At what temperature can we no longer tell the difference between the liquid and the gas? _____________

b. Which point represents an equilibrium between the solid, liquid, and gas phase?    _____________

c. Which line segment represents an equilibrium between fusion and freezing? _____________

d. Which line segment represents an equilibrium between sublimation and deposition? _____________

e. Which line segment represents an equilibrium between condensation and vaporization? _____________

f. What is the normal freezing point of this unknown substance? _____________

g. What is the normal boiling point of this unknown substance? _____________

Watch Solution

The graph shows how the vapor pressure of a liquid changes with temperature. Select the choice that best indicates the degree of correctness of this statement: "The normal boiling point of the liquid is 78°C."

(A) The statement is true.

(B) The statement is probably true; additional data would be needed for a final decision.

(C) The statement is probably false; additional data would be needed for a final decision.

(D) The statement is false.

Watch Solution

A gas or vapor may be liquefied only at temperatures

(A) equal to the normal boiling point.

(B) above the normal boiling point.

(C) above the critical temperature.

(D) at or below the critical temperature.

Watch Solution

In the phase diagram, which transition represents the condensation of a gas into a liquid?

(A) B to A

(B) D to B

(C) C to D

(D) A to D

Watch Solution

The phase diagram for a compound is shown below. Describe the phases and transitions as the pressure is increased from 0.5 atm to 1.2 atm at a constant temperature of 75 °C.

 

Watch Solution

The oxides of concern in a coal-burning power plant are sulfur oxides, not nitrogen oxides. The diagram for SO2 is shown below (not drawn to scale).

a. What is the lowest temperature at which liquid SO 2 exists?

b. Which is more dense: solid SO2 or liquid SO2?

c. What is the normal melting point of SO2(s)?

d. Suppose you start with a sample of SO 2 at 0.5 atm and -50°C, and decrease the pressure to 0.001 atm while maintaining the temperature at -50°C. What phase changes, if any, will occur?

e. What acid do you expect to form if SO 2(g) is dissolved in water? ____________________

Explain your reasoning.

Watch Solution

Consider the phase diagram shown below, starting at 0.4 atm and 20°C what phase change will occur if we decrease the temperature to -100°C? 

a) Vaporization

b) Condensation

c) Sublimation

d) Deposition

e) Melting

 

Watch Solution

The phase diagram for a substance is depicted to the below; the substance is a solid at room temperature conditions, 1.0 atm and 20°C. You wish to sublimate the solid, what change(s) must you make in order for this to occur?

a) Increase the temperature.

b) Decrease the pressure.

c) Decrease the temperature, then decrease the pressure.

d) Decrease the pressure, then increase the temperature.

e) Increase the temperature, then increase the pressure.

Watch Solution

What is the name of the process of direct conversion of a solid to a gas?

a) crystallization

b) sublimation

c) vaporization

d) condensation

e) deposition

Watch Solution

A pure substance can exist simultaneously as a solid, liquid, and gas only at

a) no point

b) more information is needed

c) any point

d) the triple point

e) the critical point

 

Watch Solution

Based on the phase diagram shown below, how will the melting point of the substance change if the pressure is increased above 1 atm?

A)      The melting point will decrease.

B)      The melting point will remain the same.

C)      The melting point will increase.

D)      The substance will not melt at pressures of 1 atm and above; instead, the solid sublimes to form the gas phase.

Watch Solution

Give the change in condition to go from a liquid to a gas.

 

A) increase heat or reduce pressure

B) increase heat or increase pressure

C) cool or reduce pressure

D) cool or increase pressure

E) none of the above 

Watch Solution

Which of the following involves a change in temperature during the phase transition?

 

a. deposition of carbon dioxide

b. fusion of ethanol

c. condensation of ammonia

d. all of the above

e. none of the above

Watch Solution