Short Answer

A phase diagram represents different states of a substance—solid, liquid, and gas—by showing the conditions of temperature and pressure under which each state is stable. The diagram is divided into three main regions, each representing one physical state.

The lines separating these regions show the boundaries where two states coexist in equilibrium. Special points like the triple point and critical point also appear on the phase diagram. By locating a point on the graph, we can determine the state of the substance at any temperature and pressure.

Detailed Explanation

Representation of Different States in a Phase Diagram

A phase diagram is a graphical tool used to show how a substance behaves under different conditions of temperature and pressure. It helps us understand when a substance exists as a solid, liquid, or gas. The diagram is divided into regions, each of which corresponds to a particular physical state. The boundaries between these regions show the conditions where two phases can exist together.

A phase diagram is extremely useful in chemistry because it allows us to predict phase changes, determine melting and boiling points, and understand the behaviour of substances at extreme temperatures and pressures. It also provides important information such as the triple point and critical point, which tell us about the unique conditions where phases meet or disappear.

1. Regions Representing Different States

A typical phase diagram has three main regions:

1. Solid Region

This region is usually located at:

• Low temperatures
• High pressures

Here, the particles are closely packed, and the substance is in its solid form.
The boundary between solid and liquid indicates melting or freezing.

1. Liquid Region

This region generally lies between the solid and gas regions.
Liquids exist at:

• Moderate temperatures
• Moderate pressures

Particles are close but can move freely.
The line between liquid and gas shows boiling or condensation.

1. Gas Region

This region occupies:

• High temperatures
• Low pressures

Particles are far apart and move rapidly.
The line between solid and gas shows sublimation or deposition.

Each region corresponds to the stability of that phase under the given conditions.

2. Boundary Lines Between States

A phase diagram has three important lines, also called phase boundaries:

1. Solid–Liquid Line

• Shows melting and freezing conditions
• Slopes differently for different substances
  • For water, it slopes backward due to its unusual structure
• Represents equilibrium between solid and liquid

1. Liquid–Gas Line

• Shows boiling and condensation
• Called the vaporization curve
• Ends at the critical point

1. Solid–Gas Line

• Shows sublimation and deposition
• Represents equilibrium between solid and gas

These lines divide the diagram into regions and show where two states coexist.

3. Triple Point and Its Role in Representing States

The triple point is a crucial part of the phase diagram.
It is the point where all three states—solid, liquid, and gas—exist together in equilibrium.

At this point:

• The substance can melt, freeze, boil, and sublime at the same time
• It shows the unique combination of temperature and pressure where all phases meet

On the phase diagram, it is located where all the boundary lines intersect.

4. Critical Point and Representation of the Supercritical Region

The critical point marks the end of the liquid–gas boundary.
At this point:

• Liquid and gas phases become indistinguishable
• Beyond it, the substance becomes a supercritical fluid

On a phase diagram, it appears at:

• High temperature
• High pressure

The region beyond the critical point is labelled as the supercritical fluid region, where the substance behaves partly like a liquid and partly like a gas.

5. How to Identify the State Using a Phase Diagram

To find the state of a substance using a phase diagram:

1. Locate the temperature on the horizontal axis.
2. Locate the pressure on the vertical axis.
3. Find the point where these two values meet.
4. Check which region the point lies in.
   • Solid region → Substance is solid
   • Liquid region → Substance is liquid
   • Gas region → Substance is gas

If the point lies on a boundary line, the substance exists in two phases at once.

6. Why Phase Diagrams Are Useful

Phase diagrams help in:

• Studying phase changes
• Predicting melting, boiling, and sublimation
• Understanding unusual behaviour (like water’s backward-sloping melting line)
• Designing industrial processes such as gas liquefaction
• Learning how pressure affects boiling and melting points

They provide a complete picture of a substance’s behaviour under different natural and laboratory conditions.

Conclusion

A phase diagram represents different states by dividing temperature–pressure space into solid, liquid, and gas regions. The boundary lines show where two phases coexist, while special points like the triple point and critical point mark unique phase behaviours. By locating a point on the diagram, we can easily identify the physical state of the substance. Phase diagrams are essential tools for understanding phase stability and transitions.