25 Phase Diagram
Learning Objectives
By the end of this section, you should be able to:
Interpret phase diagram for relevant thermodynamic information (e.g. composition, temperature, pressure…)
Phase Transitions
The phase of a substance and separation between molecules are related to the intermolecular forces in a substance. Therefore, a source of energy needs to be supplied when the substance transitions to a higher energy state to separate the molecules in the substance.
Something similar you may recall from a previous chemistry course (say CHEM 154): Bond-breaking requires energy; Bond making releases energy. Similarly, a transition to a lower energy state releases energy.
image obtained from General Chemistry: Principles, Patterns, and Applications/CC BY-NC-SA
Critical Point and Reduced Properties
The phase of a given substance depends on pressure and temperature. Thus, plots of pressure versus temperature showing the phase in each region provide considerable insight into the thermal properties of substances. Such a P-T graph is called a phase diagram. [latex]^{[1]}[/latex]
In the phase diagram, we can observe that the boiling point curve (the line that distinguishes liquid and vapour phases) ends at a point, which is called critical point. Above the critical point, liquid and gas can no longer be distinguished and are classified as a supercritical fluid.
Vapour phase is a gaseous species below its critical temperature. On the other hand, a gaseous phase describes a gaseous species above its critical temperature but below its critical pressure.
Triple point is defined to be the set of pressure and temperature that solid, liquid and vapour are in equilibrium together.
Reduced properties are defined as variables (pressure, temperature) scaled by their values at the critical point. Reduced properties can be useful in coming up with general equations describing different substances.
[latex]T_{c}[/latex] = critical temperature
[latex]P_{c}[/latex] = critical pressure
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[latex]reduced\;temperature:\;\;T_{r}=\frac{T}{T_{c}}[/latex] [latex]reduced\;pressure:\;\;P_{r}=\frac{P}{P_{c}}[/latex] |
Phase Diagram of Water
Critical Pressure = [latex]P_{c} = 22.06 M\!Pa[/latex]
Critical Temperature = [latex]T_{c} = 647 K[/latex]
Triple point [latex]P[/latex] with ice [latex]I_{h}[/latex]= 611 Pa
Triple point [latex]T[/latex] with ice [latex]I_{h}[/latex] = 273.16 K
Ice [latex]I_{h}[/latex] is the most common crystal structure of ice, but ice has a total of 17 known crystal forms. The latest form was discovered in 2014.
Image obtained from Chemistry libretexts / CC BY-NC-SA 3.0
At low temperatures, solid is the stable phase. At moderate temperatures and high pressure, liquid is the stable phase, and at high temperature and low pressure, gas/vapor is the stable phase. Lines separate these phases, representing sets of pressure/temperature for phase changes.
The melting curve or fusion curve of ice/water is very special. It has a negative slope due to the fact th