Phase Diagram

Phase Diagram

Did you know that it is possible to find the physical state of a substance just with the temperature and pressure values? To do this, it is important to use a physical-mathematical tool called a phase diagram . Keep reading to find out more!

What is the phase diagram?

The phase diagram is a graph that relates pressure and temperature to determine the physical state of a substance.

In this case, with a Cartesian point A(x,y) it can be said whether the compound is solid, liquid or gaseous. Or, analogously, when finding a material in a certain physical state, we find the range of pressure and temperature coordinates to which that body is subject.

water phase diagram
water phase diagram

In the graph above, which represents the water phase diagram, three distinct regions can be seen: 

  • In the upper left part, the temperature and pressure relationships are such that the water is frozen ;
  • In the upper right portion, in turn, the interaction of pressure and temperature allows the substance to be in a liquid state ; It is
  • Finally, in the lower portion, the compound is in the vapor state .

Phase diagram interpretations

The phase diagram curves, in addition to dividing the different regions of the graph, also define dual physical states for the substance.

  • At all points on the yellow line ( solidification curve ), for example, water is both liquid and solid — representing a transition state;
  • On the blue line, called the condensation curve, a similar situation occurs, now with the liquid and gaseous states
  • On the green line, known as the resublimation curve , concomitance occurs with gaseous water and ice. 

Triple point

According to the ideas we have been discussing so far, it is possible to see that the existence of two physical configurations for the same substance is real. In the case of the point where the green, yellow and blue lines intersect, we have the triple point.

Although it does not happen spontaneously in nature easily, the correct temperature and pressure conditions, manipulated in the laboratory, allow water to be solid, liquid and gaseous at the same time, as you can see in the image below:

water phase diagram
water phase diagram

As liquid water boils, bubbles of vapor are released and, at the same time, ice cubes float on the surface of the fluid.

Physical state changes in the phase diagram

The way in which substances change or undergo changes in their temperature and pressure characteristics directly influences their physical state .

In the arrows, we can observe the change in physical state:

  • Arrow A indicates that the substance had an increase in temperature and changed its state from solid to liquid — liquefaction ;
  • Point D demonstrates that the liquid compound has become gaseous — vaporization ; It is
  • Just as arrow E demonstrates a change from ice to gaseous water — sublimation .

On the contrary, processes B, C and F result in the same operation in the opposite way:

  • In the case of B, it goes from liquid to solid, through solidification ;
  • In C, it changes from gaseous to liquid, with condensation ; It is 
  • In F, the gaseous substance becomes solid, through resublimation .

Critical point and gaseous state

Until now, we have observed water in a gaseous state as vapor. In other words, by vaporizing the substance. 

However, we know that there is a point in the phase diagram that allows water to become a uniform gas — so that it is no longer a condensable vapor as we are used to seeing it. 

This point is called the critical point and is located at the moment the condensation curve ends. The critical temperature t c =374ºC and the critical pressure p= 218 atm.

Practical applications of the phase diagram

A good example of an everyday application of the phase diagram is pressure cookers . To achieve a higher cooking temperature, which guarantees greater heat transfer to the food and shorter preparation time.

For a heat source of equal power, the increase in pressure favors a higher boiling temperature for water. You can see this with the graph below, provided in a 1999 Enem exercise:

water phase diagram
water phase diagram

Note that for a pressure of 1 atm, the boiling temperature of water will be 100ºC. However, if we only change the pressure inside the container to 2 atm, for example, it is observed that the boiling temperature reaches approximately 120º C — which catalyzes the food preparation process.

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