Phase Diagrams

Core Concept

In this tutorial, you will learn about the major features of phase diagrams, also called phase change diagrams, as well as other types of diagrams, and how to interpret and understand their importance.

Topics Covered in Other Articles

Vocabulary

Condensation – when a substance goes from gaseous to liquid state
Vaporization – when a substance goes from liquid to gaseous state
Fusion – when a substance goes from liquid to solid state
Melting – when a substance goes from solid to liquid state
Sublimation – when the substance goes directly from solid to gaseous state
Deposition – when a substance goes directly from gaseous to solid state
Phase Change – when a substance changes from one state of matter to another (e.g. solid to liquid, or liquid to gas)

What are Phase Chase Diagrams?

A phase transition occurs when a substance changes from one state of matter to another state. There are three primary states of matter: liquid, solid, and gas.

A phase diagram is a plot that illustrates the different phases of a substance across multiple variables, most often temperature and pressure. The diagram can help to demonstrate how changing these variables affects the state of matter of a particular substance.

Pressure-Temperature Phase Diagram

The pressure-temperature phase diagram is the most common and basic type.

A phase diagram of the type shown above plots pressure (in atmospheres) versus temperature (in degrees Celsius or Kelvin). The solid lines represent the phase boundaries, also called “coexistence curves,” because there are two phases coexisting in equilibrium with one another. The blue line shows the boiling point, or transition between liquid and gaseous state, the green line shows the freezing point, or transition between liquid and solid state, and the red line shows the conditions where a solid state can be converted directly to gaseous phase and vice versa.

Phase Diagram Descriptions

Here are some common words and phrases you will hear when learning about phase change diagrams.

Fusion (melting or freezing) curve – the curve that shows the transition between liquid and solid states

Vaporization (or condensation) curve – the curve that shows the transition between gaseous and liquid states

Sublimation (or deposition) curve – the curve that shows the transition between gaseous and solid states

Triple point – the point of temperature and pressure at which gas, liquid, and solid coexist in equilibrium

Critical point – the point of temperature and pressure at which the substance is indistinguishable between liquid and gaseous states

Phase boundary – the line that indicates the conditions under which two states of matter coexist in equilibrium

The Special Case of Water

A special case that is commonly discussed is the phase diagram of water. In the diagram from above, the dotted green line represents the solid-liquid coexistence curve in the phase diagram of water.

We can see that for the average substance, raising the pressure of the liquid will cause it to cross the curve and become a solid. However, for water, raising the pressure of the solid (ice) may cause it to cross the curve and become a liquid. This behavior is a consequence of the low density of the crystal structure of ice, which contains a large amount of free space due to hydrogen bonding.

Other Types of Phase Chase Diagrams

The pressure-temperature phase diagram is not the only type. There are many thermodynamic variables that can influence the phase behavior of a substance, including volume, entropy, enthalpy, and chemical potential. You can use any of these variables to generate a phase diagram showing for what values of two variables a sample is solid, liquid, or gaseous.

Since a two-dimensional diagram can only show two variables as axes, sometimes a third variable will be shown by way curves showing where this variable is constant. They have different names depending on the variable they represent. For example, an isobar shows constant pressure, an isochor constant volume, and an isotherm constant temperature.

3D Phase Diagrams

Three-dimensional phase change diagrams plot three thermodynamic variables and show regions of space corresponding to different phases. In this type of diagram, we have a triple line instead of a triple point, and coexistence surfaces instead of coexistence curves. Below is a generic 3D diagram plotting temperature, pressure, and specific volume.

3D phase diagram example — 3D phase diagram

Binary Mixtures

Binary mixtures have phase diagrams plotted against composition, usually represented as mole fraction of one component. This shows how the phase behavior of the mixture depends on the specific makeup of the sample. One of the most important characteristics of this type of plot is the eutectic point, which is the point on the diagram where the melting point is lowest, and usually falls somewhere in the middle. On the diagram below, you can find the eutectic point at about 40 mol % ammonium nitrate.

binary mixture phase change diagram example — Phase diagram for ammonium nitrate and water binary mixture. Above the blue curve is the liquid state, while below is the solid state.

Crystals and Polymorphs

Crystalline solids can have multiple possible structures, dependent on thermodynamic variables. This is why you may hear someone talk about different forms of ice, such as ice II, ice III, or ice IV. You may also see multiple minerals with the same chemical formula, such as calcite and aragonite, which are also different crystalline phases (polymorphs) of the same substance. In cases like this, it is common to see a phase diagram with many phases, all of which are solid.