Phase Diagrams - Online Tutor, Practice Problems & Exam Prep

A phase diagram is a graphical representation of the states of matter (solid, liquid, gas) of a pure substance under varying temperature and pressure. The triple point marks the unique conditions where all three states are in equilibrium. The critical point is the end of the liquid-gas boundary, beyond which a supercritical fluid exists. Phase changes, such as melting (fusion), sublimation, and vaporization, are depicted by curves on the diagram. At normal pressure (1 atmosphere or 760 mmHg), substances have specific normal melting and boiling points, which can be identified on the diagram. Understanding phase diagrams is crucial for grasping how changes in temperature and pressure affect a substance's state.

States of Matter of Phase Diagrams

Hey everyone. So a phase diagram represents a map of the physical state of a pure substance as a function of pressure, which is on our Y axis, and temperature, which is on our X axis. If we take a look here at a typical phase diagram, here we have our pressure and it's on our Y, and then down here is our temperature, which is X. We're going to say here that we have different portions of the face diagram that you need to become familiar with in order to fully understand what happens to any pure substance as we play around with its temperature and pressure.

Now here we're going to say a triple point. This is a unique set of conditions where three states of matter are stable and in equilibrium with one another. For us to identify the triple point, we first need to realize that three states of matter are represented on a typical phase diagram. So we take a look here. Let's say we're starting off in this area of our phase diagram. In this area, we would say that our pressure is higher and our temperature is lower. These conditions are ideal for the solid form of a pure substance. So here we have our solid.

If we start to increase our temperature, we can have our solid basically melt into a liquid. So here we have our liquid phase within this region. And then finally, if we keep pushing the envelope in terms of temperature, if we want or we decrease the pressure greatly, we go into the gaseous state. So here this is our gaseous state here. Now our solid, liquid, and gas, they all meet here at point 1.1. Here represents our triple point.

Next, we have our critical point. This is just our final set of pressure and temperature conditions where a liquid and gas are distinguishable. So if we take a look here, point 2 represents my critical point and again this is our triple point. So at this critical point, basically our pressure is pretty high up there and our temperature is pretty high. If we keep moving towards this direction though, we're going to go to a place where it's no longer a liquid, it's no longer a gas, it becomes something called a supercritical fluid.

Now if you want to think about what's a good example of a supercritical fluid. Now if you've watched television and movies, you'll see every time we have space explorations towards the sun. You can see the sun being depicted as like lava or something close to that. In reality, the sun is made up of hydrogen gas. If that's such a high temperature and pressure, then it kind of looks a little bit like lava. But it's not exactly liquid, it's not exactly a gas, it's supercritical fluid.

So just remember when we're looking at a typical phase diagram, these are the portions you need to remember. Where we have our pressure on the Y, our temperature on the X axis, we have our solid, liquid, and gaseous phases within their different sectors. And then we have our triple point which connects our three phases together and then finally our critical point. Beyond that point, we're no longer a liquid or gas, but instead a supercritical fluid.

Phase Diagrams Example 1

Here are the example question, States at a temperature of 60�C and 1.20 atmospheres. Determine the physical state of the substance from the phase diagram shown above. All right, so we're dealing with 60�C, which we know is somewhere between 50�C here and 80.

Next they tell me the temp on the pressure, 1.20 atmospheres. Well, if we take a look here on our Y axis we have 760 tour. Remember 760 tour represents one atmosphere, but here we have a pressure of 1.20 which means our pressure is above this line here. So our pressure is higher than 760 Tor and the temperature is around 60.

So we can approximate this looks around 60 or so and it'll be somewhere above this line, so it'll fall within this sector of my face diagram. Now if we look that means it resides within the liquid phase. So here my answer would be option A. The unknown substance would be a liquid when the temperature is 60�C and 1.20 atmosphere based on the phase diagram shown before us.

The critical point of this substance occurs at what temperature?

• A. 0 °C
• B. –78.5 °C
• C. –56.7 °C
• D. 31 °C
• E. 100 °C

Phase Diagrams Concept 2

We're now going to take a look at the phase changes associated with a typical phase diagram. Now recall that a phase change is a reversible change that involves a transition between the three states of matter, solids, liquids and gases. And we're going to say associated with this phase change is what we call a phase change curve. This is just our line segment within phase diagrams that separates 3 states of matter. Well, two states of matter, two states of matter from each other.

So if we take a look here at this typical phase diagram, we have our solids, our liquids and our gases. We're going to say here that in this blue line, we're separating the solid phase from the liquid phase. Now if you think about it, what do we call going from a solid to a liquid? We call that melting. And in chemistry, a fancier term for it is fusion. So we're going to say here that this is our melting or what we call our fusion curve.

For the green one, we're separating the solid phase from the gaseous phase. If we think about it, what do we call going from a solid to a gas South? We've talked about this several chapters ago. Going from a solid to a gas is called sublimation. So this green line would be our sublimation curve. And then finally we have our liquid and our gas phases being separated by this red line. Thinking about going from a liquid to a gas, that's vaporization. So this is our vaporization curve.

Now with this whole idea of phase changes, we have involved our normal pressure. So normal pressure is 760 Tor or 760mm of mercury or just one atmosphere. We're going to say here that normal pressure is the name given to the standard pressure of one atmosphere or 760mm of mercury or Tor. And associated with noble pressure we have normal melting point and normal boiling point. Normal melting point is the phase transition between a solid, a liquid at normal pressure and normal boiling point would just be the phase transition between a liquid to a gas at normal pressure.

Now how do we able to see that on this typical phase diagram? Well here is our normal pressure and what I'm going to do here is I'm going to trace to this blue line. Remember at this blue line is where we have the transition between solid to liquid. So this line is where melting can occur. If we trace down from there, we hit 50� Celsius. 50�C represents our normal melting point temperature. Now boiling has to do with us going from liquid to gas. So again continuing tracing, we hit this red line here where vaporization can occur and then you just trace down here we get 80 degrees. 80� here would represent the normal boiling point of this substance.

Alright, so just remember the different types of phase change curves that are associated with any typical phase diagram. And remember, if we know the normal pressure, we can apply that to find our normal melting point and our normal boiling point for any given substance.

Phase Diagrams Example 2

Here we're told that a substance has a triple point of -45�C and 500mm of mercury. What is the most likely phase change to occur for a solid sample of this substance as it's heated from -60�C to 10�C at a pressure of 490mm of mercury? All right. So first of all, they tell us our triple point. And on this phase diagram, our triple point would be here, the point in which all three phases meet together.

Now if we take a look, we're told that it happens at 500 meters, millimeters of mercury and at -45�. Now they're telling us the the whole new pressure is a little bit lower than that. It's at 490. So it's a little different, Not much different, but it's a little different. And we're starting at -60, so let's say -60 is here, and then let's say that 10�C is over here. It doesn't really matter in terms of spacing. We know that -60 is going to be to the left of -45 and we know 10�C will be to the right.

We know that we're starting out as a solid. So we know we're somewhere within this phase here and we're just traveling from our solid phase and getting to 10�C which is over here. Remember, if we were to basically suction off the different phases, phases of our phase diagram, we'd have our solid, our liquid and our gas. We just went from the solid phase, we skipped the liquid phase and ended up in the gaseous phase.

Remember, going from a solid straight to a gas, we call that sublimation. That would mean that option E is our correct answer.

Arrow I corresponds to:

• A. Equilibrium
• B. Sublimation
• C. Condensation
• D. Vaporization
• E. Deposition

What is the normal freezing point of this unknown substance?

• A. 0 °C
• B. –200 °C
• C. 200 °C
• D. 400 °C
• E. 100 °C

At what temperature can we no longer tell the difference between the liquid and gas phases?

• A. 100 °C
• B. 200 °C
• C. 400 °C
• D. 800 °C
• E. 820 °C

Here’s what students ask on this topic:

What are phase diagrams?

Phase diagrams are graphical representations that show the stability of different phases of a material at various temperatures and pressures. Essentially, they map out the conditions under which a substance will exist in a solid, liquid, or gaseous state, or in a combination of phases. Each region in a phase diagram represents a different phase, and the lines that separate these regions are known as phase boundaries.

At a phase boundary, two phases can coexist in equilibrium. Important points on a phase diagram include the triple point, where all three phases can coexist, and the critical point, beyond which the distinction between liquid and gas phases disappears. By studying a phase diagram, you can predict how a substance will behave when subjected to different environmental conditions, which is crucial in fields like chemistry, materials science, and engineering.

What is the eutectic point in phase diagrams?

The eutectic point in phase diagrams represents a specific combination of temperature and composition where multiple phases (solid phases in particular) coexist in equilibrium. At this point, a mixture of substances melts or solidifies at a single, fixed temperature, which is the lowest possible for the given system. This temperature is known as the eutectic temperature. When a eutectic mixture is cooled down to the eutectic temperature, it solidifies into a mixture of the different solid phases simultaneously, rather than one solid phase crystallizing before the other. Similarly, when a eutectic mixture is heated to the eutectic temperature, it melts as a whole into a liquid solution. The resulting solid mixture typically has a fine-grained microstructure, which can be beneficial in various applications, such as in soldering and casting, where a lower melting point is desired. The eutectic composition defines the proportions of the components in the mixture that will exhibit this precise melting and solidifying behavior.

How do you read phase diagrams?

Reading a phase diagram involves understanding the layout and the information it represents about a substance's states of matter under different conditions of temperature and pressure. Here's a brief guide:

1. Axes: The horizontal axis typically represents temperature, while the vertical axis represents pressure.
2. Phases: The diagram is divided into regions that represent the different phases of the substance—solid, liquid, and gas.
3. Lines or Curves: The lines, or phase boundaries, indicate the conditions at which two phases can coexist in equilibrium. These include:
   • The solid-liquid line (melting/freezing line)
   • The liquid-gas line (boiling/condensation line)
   • The solid-gas line (sublimation/deposition line)
4. Triple Point: This is a unique point on the diagram where all three phases can coexist in equilibrium. It's the intersection of the three phase boundaries.
5. Critical Point: This point marks the end of the liquid-gas boundary. Beyond this point, the liquid and gas phases are indistinguishable, and the substance is in a supercritical fluid state.

To read the diagram, find the temperature and pressure of interest and determine which region or line it falls on to know the phase of the substance or the phase transition occurring at those conditions.