A phase diagram is a representation of different phases of a system consists of a substance or many substances at two different thermodynamic conditions such as temperature and pressure. Phase diagram can also be drawn between other thermodynamic conditions such as between temperature and volume or temperature and solubility etc. Phase diagram which is drawn between temperature and pressure shows the state of a substance or mixture of substances at any given time at particular temperature and pressure. It is a very useful tool in order to understand the nature of a single component substance or mixture of substance. You can vary one thermodynamic condition such as temperature by keeping the other constant such as pressure.
Phases of a substance mean solid, liquid and gas phase of a matter. A phase has a complete distinct physical and chemical properties, whereas two phases are separated by a phase boundary. A phase diagram is a graphical representation of the substance phases, consists of the curved lines and the space between the two lines represent a specific phase of the matter at given pressure and temperature, whereas any point at the curve lines shows the equilibrium between two phases.
Phase diagram explanation
In this section, we describe how one can use the phase diagram to study the relationship between two phases and effects of varying thermodynamic conditions like pressure and temperature on properties of the substance. Before varying values of temperature by keeping pressure constant or vice versa we will first define certain point and features of the phase diagram. Three spaces in phase diagram correspond to solid, liquid and vapours states of a substance. These three spaces and three curve lines. The first space indicates the solid phase, the second space represents the liquid while the third space shows the vapour state of a substance at a given temperature and pressure have distinct physical and chemical properties. The line of curve separates the solid and liquid phases/liquid and vapour phases/solid and vapour phases, also showing the equilibrium between two phases which means at any point on the line, two phases are interconvertible at given temperature or pressure.
In a typical phase diagram, one can see two points which are very important to define in order of explaining the graph. One of these points is called a critical point and the other one is called triple point.
Critical point (C) is explained as a point at which liquid and vapour phases of a substance are not distinguishable and the values of temperature and pressure at this point is called critical temperature and critical pressure (Fig. 1).
Triple point, as mentioned above that phase diagram has three curve lines which are also called phase boundaries. The first curve line (A, T) separated the solid and vapour phase, second curve line (T, B) separates the solid and liquid phase whereas third curve (T, C) separates the liquid and vapour phases of a substance (Fig. 1). Triple point (T) in phase diagram represents the point, where all these three curves meet and all three phases are in equilibrium due to a unique combination of temperature and pressure values. At this point, all three phases are interconvertible under controlled temperature and pressure values.
Fig. 1. Typical phase diagram
The phase diagram of a substance can be used to identify the physical and chemical properties of that substance. Here, we will study a general phase diagram by considering different values of one variable while keeping the other variable value constant. In a phase diagram temperature values are drawn on x-axis, whereas pressure values on y-axis.
If we will keep the pressure value constant on the y-axis and increase the temperature, a substance will be converted into a liquid phase from its solid phase. Whereas, if keep the constant value at high pressure will increase the melting point temperature at which solid converts into a liquid phase. So, the same substance will now melt into a liquid at a higher temperature. This is due to the fact that phase boundary slope which separates the liquid and solid phases bend towards right-hand side along the x-axis as the slope move upwards along the y-axis. So, the solid and liquid phase boundary at high constant pressure can be crossed by increasing temperature values gradually (Fig. 2). At the phase boundary where solid and liquid phases are in equilibrium if we increase the pressure, it can result in the reversible reaction and the liquid phase will be again changed into the solid phase. One can easily conclude that higher pressure results in an increase in melting temperature. By maintaining a constant temperature and decreasing the pressure, the solid phase will also be converted into the liquid phase.
In another case, an increase in temperature at very low constant pressure will result in the conversion of the solid phase directly into the vapors without converting into the liquid phase. The bottom curve line A, T represents that phase boundary at which solid and vapor phases are in equilibrium (Fig. 3). The pressure should be very low for the conversion of solid directly into vapors and at this phase solid is called subliming. So, one can cross this boundary by increasing temperature at constant low pressure or at constant temperature by lowering pressure. Sublimation is defined as a process, where solid can be converted into vapors.
The third curve is boundary phase between liquid and vapors (T, C) which can be crossed by simply increasing the temperature or lowering the pressure while keeping the other factor constant (Fig. 4). Liquid can be converted into vapors by increasing the temperature at constant pressure. But if the constant pressure value is high then it will increase the boiling temperature of the liquid. Decreasing the pressure level at constant temperature will result in crossing the phase boundary and liquid will eventually converted into the vapors. So, as in the case of melting temperature, increase in pressure also result in increasing the boiling temperature.
In order to further explain the phase diagram, we will study the phase diagram of water and carbon dioxide as both compounds have distinct phase diagram and it will give us a better chance to understand different properties of phases of matter.
Water phase diagram
The phase diagram of water is very unique in a sense that boundary which separates the solid phase (ice) and liquid phase bend towards the left side as compared to other substance, where it bends towards right side. The reason of left bend of solid-liquid phase boundary is because ice has low density as compared to liquid water. At low temperature and low pressure the liquid phase of water can be converted into the solid phase.
At low constant pressure and increase in temperature lead to the melting of the ice but if we increase the temperature at constant high pressure the melting temperature will decrease as compared to melting temperature at low constant pressure. By increasing pressure ice phase of water can be converted into liquid phase easily, so in the case of water increasing in pressure results in decreasing the melting temperature (Fig. 5). On the phase boundary of solid and liquid, the equilibrium can be shifted with increasing pressure but system will work to decrease the pressure in order to maintain the equilibrium at the boundary phase.
Another very interesting condition can be observed from the phase diagram of water i.e., if we decrease the pressure of liquid phase of water while keeping the temperature constant at a certain value, the line of lower pressure crosses the liquid phase space then solid-phase space and finally ended in vapor phase space (Fig. 6). In normal conditions on can observe that lowering the pressure at constant temperature result in the conversion of the liquid phase to the vapor phase. But in case of water, liquid phase transforms into a solid phase (ice) and solid phase then converts into the vapor phase.
Phase diagram of water is showing that it has a triple point (T) at very low pressure and its critical point temperature is 374 ᵒC, it meant that above this temperature it is almost impossible to convert gas phase of water into liquid phase through increasing pressure. Normal melting and boiling points of water can be found by drawing a line at 1 atmospheric pressure (atm) by increasing the temperature. The point where line crosses the solid-liquid phase boundary showing water melting temperature and the point where it crosses the liquid-vapor phase boundary showing it boiling temperature (Fig 7).
CO2 phase diagram
The phase diagram of CO is more normal than phase diagram of water, as it’s solid and liquid phase boundary tend to bend right side which means that increase in temperature and pressure leads to the conversion of solid phase into liquid phase. But an important thing one can observe easily in phase diagram of CO2 is that, it has triple point (T) at much higher pressure which is calculated as 5.11 atm. So in order to have CO2 in liquid phase, the system should have 5.11 atm pressure which is difficult to attain (Fig 8).
Under normal conditions (1 atm) CO2 presents in gaseous phase and lowering temperature of the gaseous phase below -78 ᵒC results in its conversion into solid phase of CO2 which is also called dry ice. Similarly at 1 atm, the solid phase of CO2 sublime into gaseous phase and never converted into liquid phase (Fig. 9). The critical temperature and pressure of CO2 lies in normal zone and critical point found at 73 atm and 31.1 ᵒC.
As under normal conditions, CO2 never occurs in the liquid phase making it a good candidate for several applications. It is used as a chilling agent because it does not add liquid to the system, where it is used for cooling. In many cases, CO2 used as a solvent because it has a low viscosity so it can penetrate surfaces and smalls spaces efficiently. It is widely used in decaffeinating the coffee, as a refrigerant and dry-cleaning agent.
CK12 Intermediate – https://courses.lumenlearning.com/cheminter/chapter/phase-diagram-for-water/
Libretexts – https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/States_of_Matter/Phase_Transitions/Phase_Diagrams