The line (blue) for the solid-to-liquid transition (fusion reaction) extends from the triple point to arbitrarily high pressures. This line ends in the phase diagram because it reaches the upper limit of the diagram. Similarly, the line (green) for the solid-to-gas transition (sublimation reaction) extends from the triple point to arbitrarily low pressures.
The line for the liquid-to-gas transition (vaporization reaction), however, is different. It extends from the triple point to the critical point, where is ends. The critical point, defined by the critical temperature Tc and critical pressure Pc, marks the point at which it is no longer possible to distinguish between a liquid and gas.
What features of a substance enable us to determine its phase?
A solid is unique in that it is rigid; it has a fixed shape. A substance in the gas or liquid phase does not hold its shape.
What property enables one to distinguish between a gas and a liquid?
In both the liquid and gas phases the molecules of the substance are free to move relative to each other. The only distinction is that in a liquid the molecules are still required to be relatively close to each other, because there is insufficient energy available to completely overcome the intermolecular attractions that draw the molecules together. In a gas the molecules are able to move freely; there is sufficient energy to completely overcome the intermolecular attractions.
If the liquid and gas phases are in equilibrium, it is possible to distinguish the two phases, because the liquid phase has a higher density than the gas phase. Consequently the liquid phase sinks to the bottom of the container and the gas phase occupies the remaining volume above the liquid. (This behavior is illustrated in the closed cylinder animation employed in the Phase Diagram pages.)
What is the effect of increasing the temperature of a liquid?
An increase in temperature produces an increase in the kinetic energy available to the molecules, enabling them overcome the intermolecular forces to a greater extent and move further apart. This behavior decreases the density of the liquid.
What is the effect of increasing the pressure of a gas?
The pressure of the gas in increased by decreasing the volume the gas may occupy (thereby producing more molecule-wall collisions and thus a higher pressure). The molecules are forced to occupy a much smaller space, producing a higher density.
If both the temperature and pressure become sufficiently large, the liquid phase and gas phase have the same density and at this point it becomes impossible to distinguish between the two phases. The thresholds for this behavior are the critical temperature (Tc) and the critical pressure (Pc). If T > Tc and P > Pc, super-critical conditions exist and the sample is said to be a super-critical fluid.
Under super-critical conditions, the molecules in a gas are forced to be in close proximity to each other (just as in a liquid) not because there is insufficient energy to overcome the intermolecular attractions but because there is insufficient room for the molecules to get away from each other (the volume of the container is too small).
In order to better appreciate this effect, carefully examine the phase diagram and the sample in the cylinder shown below. The color of the sample reflects the density of the sample. The red color represents a relatively small density (characteristic of a gas) and the blue color represents a relatively high density (characteristic of a liquid).
1. Set the conditions to Point A (the sample will be in the liquid state). Gradually heat the sample, thereby moving along the isobaric line from Point A to B. This path crosses a phase line, and thus a phase transition should be observed. While crossing the phase line you should see both the liquid (blue) and gas (red) phase in the cylinder at the same time.
2. Reset the conditions to Point A (the sample will once again be in the liquid state).
3. Gradually reduce the volume (increase the pressure) of the sample, thereby moving along the isothermal line from Point A to D. Do you observe a phase transition during this process? What is the effect of moving from Point A to D?
4. Gradually heat the sample, thereby moving along the isobaric line from Point D to C. Do you observe a phase transition during this process? What is the effect of moving from Point D to C?
5. Gradually increase the volume (decrease the pressure) of the sample, thereby moving along the isothermal line from Point C to B. Perform this change very gradually. Do you observe a phase transition during this process? Bear in mind that a phase transition occurs when two different phases coexist at equilibrium (which would be represented by two different color regions in the cylinder). What is the effect of moving from Point C to B?
Your observations should reveal that motion from Point A directly to Point B (via the isobaric line) produces a phase change. One can start at the exact same point (Point A) and travel to the exact same destination (Point B) via the route A to D to C to B without encountering a phase transition! Explain how this is possible.
6. What are the values of Tc and Pc for this substance?
7. At which of the points, A, B, C, or D, is the substance a super-critical fluid?
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