Phase Diagrams: Videos & Practice Problems

A phase diagram is a graphical representation of the states of matter (solid, liquid, gas) of a pure substance under varying temperature and pressure. The triple point marks the unique conditions where all three states are in equilibrium. The critical point is the end of the liquid-gas boundary, beyond which a supercritical fluid exists. Phase changes, such as melting (fusion), sublimation, and vaporization, are depicted by curves on the diagram. At normal pressure (1 atmosphere or 760 mmHg), substances have specific normal melting and boiling points, which can be identified on the diagram. Understanding phase diagrams is crucial for grasping how changes in temperature and pressure affect a substance's state.

States of Matter of Phase Diagrams

A phase diagram is a graphical representation that illustrates the physical state of a pure substance based on its pressure and temperature. In this diagram, pressure is plotted on the vertical (y) axis, while temperature is plotted on the horizontal (x) axis. Understanding the different regions of a phase diagram is crucial for comprehending how a pure substance behaves under varying temperature and pressure conditions.

Within a typical phase diagram, there are three primary states of matter: solid, liquid, and gas. The area where the pressure is high and the temperature is low corresponds to the solid state of the substance. As the temperature increases, the solid can melt into a liquid, which occupies a different region of the diagram. Further increasing the temperature or significantly decreasing the pressure leads to the gaseous state.

One of the key features of a phase diagram is the triple point, denoted as point 0.1. This unique condition represents the specific temperature and pressure at which all three states of matter (solid, liquid, and gas) coexist in equilibrium. Understanding the triple point is essential for recognizing how substances transition between different states.

Another important feature is the critical point, marked as point 0.2. This point indicates the conditions at which the distinction between liquid and gas phases disappears. Beyond the critical point, the substance enters a state known as a supercritical fluid, where it exhibits properties of both liquids and gases. An example of a supercritical fluid can be found in the extreme conditions present in the Sun, where hydrogen gas exists at high temperatures and pressures, resembling a state that is neither purely liquid nor gas.

In summary, a phase diagram provides valuable insights into the behavior of pure substances under varying temperature and pressure. Key concepts include the identification of solid, liquid, and gas phases, the significance of the triple point where all three states coexist, and the critical point beyond which substances become supercritical fluids. Understanding these elements is fundamental for studying thermodynamics and material properties.

Phase Diagram Example

To determine the physical state of a substance at a specific temperature and pressure using a phase diagram, it is essential to analyze the given values in relation to the diagram's axes. In this case, we are examining a temperature of 60 degrees Celsius and a pressure of 1.20 atmospheres.

First, convert the pressure from atmospheres to torr for clarity. Since 1 atmosphere is equivalent to 760 torr, a pressure of 1.20 atmospheres translates to:

1.20 \, \text{atm} \times 760 \, \text{torr/atm} = 912 \, \text{torr}

Next, locate the temperature on the phase diagram. The temperature of 60 degrees Celsius falls between the lines representing 50 degrees Celsius and 80 degrees Celsius. On the y-axis, the pressure of 912 torr is above the 760 torr line, indicating that the pressure is higher than 1 atmosphere.

By plotting these values on the phase diagram, we find that the point corresponding to 60 degrees Celsius and 912 torr lies within the liquid phase region. Therefore, based on the analysis of the phase diagram, the substance is in a liquid state at the specified conditions.

In conclusion, at a temperature of 60 degrees Celsius and a pressure of 1.20 atmospheres, the unknown substance is identified as a liquid.

The critical point of this substance occurs at what temperature?

• A. 0 °C
• B. –78.5 °C
• C. –56.7 °C
• D. 31 °C
• E. 100 °C

Answer: (D)

Phase Changes in Diagrams

Phase changes are reversible transitions between the three states of matter: solids, liquids, and gases. These transitions are represented in a phase diagram, which includes phase change curves that separate the different states. In a typical phase diagram, the blue line indicates the boundary between the solid and liquid phases, known as the melting curve or fusion curve. This curve represents the process of melting, where a solid transforms into a liquid.

The green line in the diagram separates the solid phase from the gaseous phase, representing sublimation, the process where a solid changes directly into a gas. Lastly, the red line indicates the boundary between the liquid and gas phases, known as the vaporization curve, which corresponds to the process of vaporization, where a liquid turns into a gas.

Understanding these phase changes also involves the concept of normal pressure, defined as 1 atmosphere (760 torr or 760 millimeters of mercury). At this standard pressure, we can identify the normal melting point and normal boiling point of a substance. The normal melting point is the temperature at which a solid transitions to a liquid under normal pressure, while the normal boiling point is the temperature at which a liquid transitions to a gas under the same conditions.

In the phase diagram, tracing the blue line down to the temperature axis reveals the normal melting point, which, for example, might be at 50 degrees Celsius. Similarly, tracing the red line down indicates the normal boiling point, which could be at 80 degrees Celsius. By understanding these curves and points, one can determine the phase transitions of any given substance at normal pressure.

Sublimation Phase Change Example

In this scenario, we are examining the phase changes of a substance with a known triple point at -45 degrees Celsius and 500 millimeters of mercury. The triple point is the unique set of conditions where all three phases—solid, liquid, and gas—coexist in equilibrium. As we analyze the situation, we note that the pressure is slightly lower at 490 millimeters of mercury, and we are heating a solid sample from -60 degrees Celsius to 10 degrees Celsius.

Starting at -60 degrees Celsius, which is below the triple point temperature, the substance exists in the solid phase. As the temperature increases towards 10 degrees Celsius, we observe that this temperature is above the triple point. However, since the pressure is below the triple point pressure, the phase diagram indicates that the solid will transition directly to the gas phase without passing through the liquid phase. This process is known as sublimation, where a solid transforms directly into a gas.

Thus, as the solid sample is heated from -60 degrees Celsius to 10 degrees Celsius at a pressure of 490 millimeters of mercury, the most likely phase change occurring is sublimation. This means that the correct answer to the question is that the solid will skip the liquid phase and transition directly into the gaseous phase.

Arrow I corresponds to:

• A. Equilibrium
• B. Sublimation
• C. Condensation
• D. Vaporization
• E. Deposition

Answer: (C)

What is the normal freezing point of this unknown substance?

• A. 0 °C
• B. –200 °C
• C. 200 °C
• D. 400 °C
• E. 100 °C

Answer: (E)

At what temperature can we no longer tell the difference between the liquid and gas phases?

• A. 100 °C
• B. 200 °C
• C. 400 °C
• D. 800 °C
• E. 820 °C

Answer: (E)

Here’s what students ask on this topic:

What are phase diagrams?

Phase diagrams are graphical representations that show the stability of different phases of a material at various temperatures and pressures. Essentially, they map out the conditions under which a substance will exist in a solid, liquid, or gaseous state, or in a combination of phases. Each region in a phase diagram represents a different phase, and the lines that separate these regions are known as phase boundaries.

At a phase boundary, two phases can coexist in equilibrium. Important points on a phase diagram include the triple point, where all three phases can coexist, and the critical point, beyond which the distinction between liquid and gas phases disappears. By studying a phase diagram, you can predict how a substance will behave when subjected to different environmental conditions, which is crucial in fields like chemistry, materials science, and engineering.

What is the eutectic point in phase diagrams?

The eutectic point in phase diagrams represents a specific combination of temperature and composition where multiple phases (solid phases in particular) coexist in equilibrium. At this point, a mixture of substances melts or solidifies at a single, fixed temperature, which is the lowest possible for the given system. This temperature is known as the eutectic temperature. When a eutectic mixture is cooled down to the eutectic temperature, it solidifies into a mixture of the different solid phases simultaneously, rather than one solid phase crystallizing before the other. Similarly, when a eutectic mixture is heated to the eutectic temperature, it melts as a whole into a liquid solution. The resulting solid mixture typically has a fine-grained microstructure, which can be beneficial in various applications, such as in soldering and casting, where a lower melting point is desired. The eutectic composition defines the proportions of the components in the mixture that will exhibit this precise melting and solidifying behavior.

How do you read phase diagrams?

Reading a phase diagram involves understanding the layout and the information it represents about a substance's states of matter under different conditions of temperature and pressure. Here's a brief guide:

1. Axes: The horizontal axis typically represents temperature, while the vertical axis represents pressure.
2. Phases: The diagram is divided into regions that represent the different phases of the substance—solid, liquid, and gas.
3. Lines or Curves: The lines, or phase boundaries, indicate the conditions at which two phases can coexist in equilibrium. These include:
   • The solid-liquid line (melting/freezing line)
   • The liquid-gas line (boiling/condensation line)
   • The solid-gas line (sublimation/deposition line)
4. Triple Point: This is a unique point on the diagram where all three phases can coexist in equilibrium. It's the intersection of the three phase boundaries.
5. Critical Point: This point marks the end of the liquid-gas boundary. Beyond this point, the liquid and gas phases are indistinguishable, and the substance is in a supercritical fluid state.

To read the diagram, find the temperature and pressure of interest and determine which region or line it falls on to know the phase of the substance or the phase transition occurring at those conditions.