How does specific volume play its role in the phase diagram?

In summary, the conversation discusses the multiphase process of pure substances and the relationship between specific volume, temperature, and pressure. The state postulate states that two out of the three properties are independent. The P-v-T relation is a 3-dimensional concept, with the P-v diagram, T-v diagram, and P-T diagram being 2D projections. In the P-T diagram, specific volume is held constant in any P-T combination, as it goes into or out of the page. The person is seeking clarification on their understanding of this concept.
  • #1
Carbon273
17
4
So, I am casually sifting through a chapter in a thermodynamics textbook talking about the multiphase process that pure substances go through. I understand how the P-v and T-v diagram works and that out of the three properties (specific volume, temperature and pressure), two of them are independent, describing a state postulate. However, I am having a bit of trouble understanding how specific volume plays a role in the P-T diagram. I understand that there is 3-dimensional relation between the three properties called the P-v-T relation and that the P-v diagram, the T-v diagram, and the P-T diagrams are mere 2D projections on the 3D relation. For the P-T diagram, it seems like the specific volume is held constant at any P-T combination seeing that it goes into or out of the page. Can somebody clarify this for me, if I am in the right track?
 
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  • #2
That is certainly a correct interpretation.
 

1. What is specific volume and how is it measured?

Specific volume is a measure of the volume of a substance per unit of mass. It is typically measured in units of cubic meters per kilogram (m^3/kg) or cubic centimeters per gram (cm^3/g). It can be calculated by dividing the volume of a substance by its mass.

2. How does specific volume affect the phase diagram?

Specific volume is one of the variables that can be plotted on a phase diagram. It is typically represented on the y-axis, with temperature on the x-axis. Specific volume plays a critical role in determining the boundaries between different phases of a substance, such as solid, liquid, and gas.

3. How does specific volume change with temperature and pressure?

Specific volume is affected by both temperature and pressure. As temperature increases, the specific volume of a substance typically increases as well. This is because higher temperatures cause molecules to move farther apart, increasing the volume of the substance. Similarly, as pressure increases, the specific volume of a substance typically decreases, as the molecules are forced closer together, reducing the volume.

4. How is specific volume related to density?

Specific volume and density are inversely related. As specific volume increases, density decreases, and vice versa. This is because specific volume is a measure of how much space a substance occupies per unit of mass, while density is a measure of how much mass is contained within a given volume. Therefore, substances with a high specific volume have a low density, and vice versa.

5. Can specific volume be used to predict phase changes?

Yes, specific volume can be used to predict phase changes. On a phase diagram, the specific volume of a substance can help determine the temperature and pressure at which a phase change will occur. For example, the point where the solid, liquid, and gas phases meet on a phase diagram is known as the triple point, and at this point, the specific volume is at its lowest value for that substance.

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