In addition, two states of the substance coexist in equilibrium on the lines or curves. A phase transition is the transition from one state of matter to another. There are three states of matter: l iquid, solid, and gas. Phase diagrams illustrate the variations between the states of matter of elements or compounds as they relate to pressure and temperatures. The following is an example of a phase diagram for a generic single-component system:. Phase diagrams plot pressure typically in atmospheres versus temperature typically in degrees Celsius or Kelvin.
The labels on the graph represent the stable states of a system in equilibrium. The lines represent the combinations of pressures and temperatures at which two phases can exist in equilibrium. In other words, these lines define phase change points. The red line divides the solid and gas phases, represents sublimation solid to gas and deposition gas to solid. The green line divides the solid and liquid phases and represents melting solid to liquid and freezing liquid to solid.
The blue divides the liquid and gas phases, represents vaporization liquid to gas and condensation gas to liquid. There are also two important points on the diagram, the triple point and the critical point. The triple point represents the combination of pressure and temperature that facilitates all phases of matter at equilibrium.
With most substances, the temperature and pressure related to the triple point lie below standard temperature and pressure and the pressure for the critical point lies above standard pressure. Therefore at standard pressure as temperature increases, most substances change from solid to liquid to gas, and at standard temperature as pressure increases, most substances change from gas to liquid to solid. However for other substances, notably water, the line slopes to the left as the diagram for water shows.
This indicates that the liquid phase is more dense than the solid phase. This phenomenon is caused by the crystal structure of the solid phase. In the solid forms of water and some other substances, the molecules crystalize in a lattice with greater average space between molecules, thus resulting in a solid with a lower density than the liquid.
Because of this phenomenon, one is able to melt ice simply by applying pressure and not by adding heat. Moving about the phase diagram reveals information about the phases of matter. Moving along a constant temperature line reveals relative densities of the phases.
When moving from the bottom of the diagram to the top, the relative density increases. Moving along a constant pressure line reveals relative energies of the phases. When moving from the left of the diagram to the right, the relative energies increases. Imagine a substance with the following points on the phase diagram: a triple point at.
The solid liquid line is "normal" meaning positive sloping. For this, complete the following:. It ends at the critical point, beyond which the substance exists as a supercritical fluid. The line that connects points A and C is the vapor pressure curve of the solid phase. Along this line, the solid is in equilibrium with the vapor phase through sublimation and deposition. Because no more than three phases can ever coexist, a phase diagram can never have more than three lines intersecting at a single point.
Remember that a phase diagram, such as the one in Figure 7. In practice, however, the conclusions reached about the behavior of a substance in a closed system can usually be extrapolated to an open system without a great deal of error. Far more reproducible than the melting point of ice, which depends on the amount of dissolved air and the atmospheric pressure, the triple point The triple point also represents the lowest pressure at which a liquid phase can exist in equilibrium with the solid or vapor.
At pressures less than 0. The food or beverage is first cooled to subzero temperatures and placed in a container in which the pressure is maintained below 0. Then, as the temperature is increased, the water sublimes, leaving the dehydrated food such as that used by backpackers or astronauts or the powdered beverage as with freeze-dried coffee.
The phase diagram for water illustrated in part b in Figure 7. The melting curve of ice slopes up and slightly to the left rather than up and to the right as in Figure 7. Water behaves this way because it is one of the few known substances for which the crystalline solid is less dense than the liquid others include antimony and bismuth.
The decrease in volume and corresponding increase in density is smaller for a solid or a liquid than for a gas, but it is sufficient to melt some of the ice. The letters refer to points discussed in Example In part b in Figure 7. We have already indicated that the pressure dependence of the melting point of water is of vital importance.
Although at usual temperatures and pressures the phase diagram of water is simple, increasing the pressure above atm crushes the open solid structure of normal ice into increasingly compact structures as shown in Figure 7. This figure is from the Wikipedia. In contrast to the phase diagram of water, the phase diagram of CO 2 Figure 7. Solid CO 2 is generally known as dry ice because it is a cold solid with no liquid phase observed when it is warmed. Also notice the critical point at In addition to the uses discussed in Section 7.
Referring to the phase diagram of water in Figure 7. Given: phase diagram, temperature, and pressure. Asked for: physical form and physical changes. Although phases are conceptually simple, they are difficult to define precisely. Phase Diagram : In this phase diagram, which is typical of most substances, the solid lines represent the phase boundaries.
The green line marks the freezing point or transition from liquid to solid , the blue line marks the boiling point or transition from liquid to gas , and the red line shows the conditions under which a solid can be converted directly to a gas and vice-versa.
The dotted green line is meant to replace the solid green line in the corresponding phase diagram of water. The phase diagram for water is useful for learning how to analyze these diagrams. Along the blue phase boundary, water exists as both a vapor and a liquid. Along the dotted green phase boundary, we see the anomalous behavior of water: it exists as a solid at low-enough temperatures and high-enough pressures.
At the triple point, water in the solid, liquid, and gaseous states coexist. Phase diagrams illustrate the effects selected variables of a system have on the state of matter. Phase diagrams are divided into three single phase regions that cover the pressure-temperature space over which the matter being evaluated exists: liquid, gaseous, and solid states. The lines that separate these single phase regions are known as phase boundaries. Along the phase boundaries, the matter being evaluated exists simultaneously in equilibrium between the two states that border the phase boundary.
By focusing attention on distinct single phase regions, phase diagrams help us to understand the range over which a particular pure sample of matter exists as a particular phase. By examining the phase boundaries and the triple point, researchers can use phase diagrams to understand under which conditions a pure sample of matter exists in two or three state equilibrium.
Phase diagrams can also be used to explain the behavior of a pure sample of matter at the critical point. A Typical Phase Diagram : A typical phase diagram illustrating the major components of a phase diagram as well as the critical point.
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