(POM) Phase Changes Diagram Lesson

Phase Changes Diagram

The relationships among the solid, liquid, and gas states of a substance can be shown as a function of temperature and pressure in a single diagram. A phase diagram is a graph showing the conditions of temperature and pressure under which a substance exists in the solid, liquid, and gas phases. Examine the general phase diagram shown in the picture below. In each of the three colored regions of the diagram, the substance is in a single state (or phase). The dark lines that act as the boundary between those regions represent the conditions under which the two phases are in equilibrium. In other words, these lines define phase change points.  

Find the X on the pressure axis and presume that the value of X is standard pressure of 1 atm. As one moves left to right across the red line, the temperature of the solid substance is increased while the pressure remains constant. When point A is reached, the substance melts and the temperature B on the horizontal axis represents the normal melting point of the substance. Moving further to the right, the substance boils at point Y and so point C on the horizontal axis represents the normal boiling point of the substance. As the temperature increases at a constant pressure, the substance changes from solid to liquid to gas.

Start right above point B on the temperature axis and follow the red line vertically. At very low pressure, the particles of the substance are far apart from one another and the substance is in the gas state. As the pressure is increased, the particles of the substance are forced closer and closer together. Eventually, the particles are pushed so close together that attractive forces cause the substance to condense into the liquid state. Continually increasing the pressure on the liquid will eventually cause the substance to solidify. For the majority of substances, the solid state is denser than the liquid state and so putting a liquid under great pressure will cause it to turn into a solid. The line segment RS represents the process of sublimation, where the substance changes directly from a solid to a gas. At sufficiently low pressure, the liquid phase does not exist. The point labeled TP is called the triple point. The triple point is the one condition of temperature and pressure where the solid, liquid, and vapor states of a substance can all coexist at equilibrium.  

Answer the questions below to see if you understand how to read a phase diagram.  

Another way to look at phase changes is by the following diagram:

Diagram description Links to an external site.

As we look at the diagram above, let's think about this in terms of heating up an ice cube. In general, as energy (heat) is added, the temperature increases. However, there are two portions of the graph in which energy is being added, but the temperature is not changing. These are seen by the horizontal flat parts on the graph. Why does this occur?

As heat (energy) is added to the ice cube (solid), eventually it will reach a temperature in which the particles start to move faster and spread out. During this change, even though energy is still being added the temperature does not increase. This results in a horizontal line on the graph. All of the energy that is being added is being used to speed up and spread out the particles. Once in a liquid state, as energy is added the temperature will continue to increase again...until it reaches a temperature that causes the particles to speed up and spread out even more. During this change, even though energy is still being added the temperature does not increase. This results in another horizontal line on the graph. All of the energy that is being added is being used to speed up and spread out the particles. Once in a gas state, as energy is added the temperature will continue to increase again.  

In summary, the horizontal flat parts of the graph show when there is a phase change. The first phase change is melting/freezing.  The temperature remains the same while the substance is changing from solid to liquid (liquid to solid). The amount of energy required to change a given mass of a substance from a solid to a liquid is known as heat of fusion.  

The second phase change is vaporization/condensation. The temperature remains the same while the substance is changing from liquid to gas (gas to liquid). The amount of energy required to change a given mass of a substance from liquid to gas is known as heat of vaporization.

[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION