Sarah S. answered • 06/06/24
High School Science Teacher with 20+ years of Teaching Experience
A P-T diagram refers to a Pressure-Temperature diagram for a substance that is not a mixture. On the horizontal axis you have the temperature and the vertical contains the pressure. As the temperature and pressure increase the phases of matter change.
As you move along the x axis the substance moves from solid to liquid to gas as the temperature increases. As you move along the y axis and the pressure increases the density also increases.
If you move along both axes at once then we see a balance between these two changes and we eventually reach a point called the triple point. It is at this point where the lines for solid-liquid and liquid-gas meet. If the temperature remains low from this point the substance remains a solid, if the temperature rises and the pressure remains low then it becomes a gas, the middle ground is for liquid.
The easiest way to draw a phase diagram is to plot your boiling and freezings point along with their corresponding pressures along with the sublimation point. Once those are plotted then connect the melting point to the sublimation point and the boiling point to the sublimation point. This should give you a graph with three distinct regions and you can label accordingly.
