Example 1:
A phase diagram represents the pressure temperature relationships among the different phases of a given substance. There are three regions specified in a phase diagram; solid, liquid and gaseous.
A typical phase diagram is set up with the three phases in predictable fashion. The solid region exists at low to intermediate temperatures and low to high pressure. The liquid region exists at intermediate to high temperatures and intermediate to high pressures. Finally the gaseous region exists at any temperatures and low to intermediate pressures.
There are a few other important features that appear on a phase diagram. There are three equilibrium lines that separate the different phases called phase boundaries. Any point on these lines implies that the two phases that are separated by them are in equilibrium. This is where multiple phases can exist at the same time. The particles at the phase boundaries are taking on the characteristics of both phases. For example, when on the phase boundary between the liquid and solid phases, some of the particles are tightly packed in as a solid while some of the particles are moving more freely like they would in a liquid. Another important feature is the triple point. The significance of this point is that all three phases are in equilibrium at this specific point.
The particles of a substance have different motion, organization, and proximity to each other while in the three different phases. In the solid phase, the particles are packed in very close together. They are still in motion, but since they are so close in proximity, they hardly move. The liquid phase is somewhat similar to the solid phase, but has a few distinct differences. The particles are not as closely packed together as the solid particles. Due to this the different particles stray away from each other more than in the solid phase. In the gaseous phase, the particles move freely with little or no interference. The particles bounce around and stray further apart than the particles in the other two phases.
The phase diagram animation allows you to see what happens when the temperature or pressure increases while the other is held constant. When the temperature increases and the pressure is held constant, the piston rises upward from the cylinder. When the pressure increases and the temperature is held constant, the opposite happens. The piston moves downward toward the cylinder.
This phase diagram animation was for water. It is different than most other phase diagrams due to the negative slope of the liquid-solid equilibrium line. The negative slope indicates that increasing pressure on the surface of ice causes it to melt. This is because ice is less dense than liquid water in the area of the liquid-solid equilibrium. Almost all other solids are denser than their corresponding liquids and they would have positive slopes associated with the liquid-solid equilibrium line.
Example 2:
Phase diagrams exist to show the relationship between temperature and pressure as a pure substance changes phases. Phase regions provide an understanding of the pressure and temperature conditions under which different phases exist. For example, by looking at the phase diagram animation, it can be obtained that solids exist at low temperatures, liquids at moderate temperatures, and gases at high temperatures. These regions are relative to each other at the phase boundaries and the triple point. A phase boundary provides the temperature and pressure at which an equilibrium between two phases exists. The triple point is the point on a phase diagram at which the three phases exist at an equilibrium. Other important features include the critical point, normal freezing or melting point, and normal boiling point.
The phase diagram animation illustrated the particles’ characteristics in different phases. For a substance in the solid phase, the particles are compacted together and move slowly. In the liquid phase, the particles are less compact and move at a more moderate speed. In the gas phase, the particles are very spread out and move quickly about their container. At a phase boundary, as mentioned earlier, the particles are at an equilibrium. For example, at the solid-liquid equilibrium, half the particles are in the solid phase, while the other half is in the liquid phase. The phase boundary that exists at the solid and liquid phases is called the melting curve. As pressure remains constant, temperature increases or decreases and the substance liquefies or solidifies respectively. The sublimination curve is the phase boundary for the solid and gas phases. If temperature remains constant and pressure decreases, the substance sublimes, or changes from solid to gas without going through the liquid phase. The phase diagram animation also used a piston and cylinder to illustrate what happens in the three phases as temperature and pressure increase, decrease, or remain constant. For a substance in either the solid or liquid phase, the piston did not move. This proved that neither a solid nor a liquid can be compressed. For a gas, however, as pressure remains constant and temperature increases, the piston moves up, creating move space in the cylinder for the gas to move around. When temperature is constant and pressure increases, the piston moves down slightly, proving gas is compressible.
The phase diagram for water differs from the phase diagrams for most other substances. The melting curve for most substances has a positive slope; however, the melting curve for water has a negative slope. According to the phase diagram comparison between water and carbon dioxide, this negative slope shows that as pressure increases on ice (the solid form of water), it will change to liquid. This is demonstrated when water forms under the blade of an ice skater’s skate. Compared to carbon dioxide, where its solid form is more dense then its liquid form, the solid form of water is less dense than its liquid form. This fact explains why ice floats in water. The solid form of most other substances would sink in its liquid form because of its density.
Example of an acceptable student essay (7/10):
In the world of chemistry most all compounds and substances have three different phases, solid liquid and gas. A phase diagram represents the three phases and what temperature and pressure it takes to change from each phase. The regions are the different phases a substance can change from.
For example, a substance can change from a solid to a liquid, liquid to a gas, or vise versa. A typical phase diagram is laid out in a graph form with pressure on the Y axis and temperature on the X axis. Then the three different regions are positioned where solid phase is closes to the pressure axis do to low temperatures. Then the liquid is to the right because of higher temperature and degreasing pressure. Then the gas is to the lower right of liquid do to very high temperature and lower pressure at boiling point. Each phase has a different particle motion and organization. Solid particles are very close and tightly pact together, with an orderly definite shape, and slow diffusion. Liquid particles are relatively close, disordered with no shape, and faster diffusion than solids. Gas particles are completely spread out with no shape and diffuse very rapidly.
At a phase boundary the particles will start acting like the particles of the phase it is closer to. For an example, when slower moving liquid particles are close to the liquid and gas boundary, they will start moving faster and spread out. When multiple phases exist, it is called the triple point. This is where solid, liquid, and gas are at a temperature and pressure equilibrium point. As the pressure increases the force of attraction becomes stronger, and as it degreases the force becomes weaker. If temperature changes and the pressure does not, the force remains constant. However, the particles will move more rapidly.
Many substances have a similar phase diagram, but water is a little more different. By increasing just the pressure can change water from a solid to a liquid, where as most substances the temperature has to increase as well. This is because liquid water is denser than the solid state. It is shown in the phase diagram, because of the negative slope on the solid / liquid equilibrium line. This is why ice floats in water.
Example of a poor student essay (3.7/10):
A phase diagram is a diagram showing the various phases of a system, while it also represents pressure and temperature. The diagram has three distinct regions which represent liquid, solid, and gas. The significance of these regions is to show how a substance can exist at all three phases at different temperatures and pressures. A phase diagram usually has temperature on the x axis while pressure is on the y axis. The gas region is in the lower right corner, solid is in the upper left corner and liquid is in the upper right corner. Each of the regions has its own unique make up. Solid has tightly arranged particles, with little movement. Liquid has loser arranged particles, with more movement, and gas has very lose particles, with free movement. At the curves of the phase transitions, where the phases co-exist, the particles can share the qualities of two of the phases at the same time. Along with the phase diagram there is a piston and cylinder diagram. This shows what happens to the piston when you are at the different phases, temperatures, and pressures. In the gas phase, the piston rises as the temperatures raises and the piston lowers as the pressure lowers. On the other hand, in both solid and gas, the piston remains the same as you lower or raise the temperature and the pressure. In a water diagram there are some differences from other phase diagrams. Normally the pressure on a solid would change it to a liquid, but with ice, the solid form of water, it doesn’t. It also has a negative slope on the solid-liquid equilibrium line. As I have learned from this assignment, a phase diagram is a very useful tool to help someone learnt the three phases of a substance and there importance’s.
