# Phase Transitions

All of the matter that we encounter in our everyday lives is found in one of the three phases: solid, liquid, or gas. They are important in multiple ways. From simply understanding why an ice cube melts, to understanding how our refrigerator works, phase transitions are crucial to understand in chemistry. In this section it is vital to gain better knowledge of what phase transitions actually are and why these everyday events occur.

### Introduction

A phase transition is the transition from one state of matter to another. There are three states of matter: liquid, solid, and gas.

• Liquid: A state of matter that consists of loose, free moving particles which form the shape set by the boundaries of the container in which the liquid is in. This happens because the motion of the individual particles within a liquid is much less restricted than in a solid. One may notice that some liquids flow readily whereas some liquids flow slowly. A liquid's relative resistance to flow is viscosity.
• Solid: A state of matter with tightly packed particles which do not change the shape or volume of the container that it is in. However, this does not mean that the volume of a solid is a constant. Solids can expand and contract when temperatures change. This is why when you look up the density of a solid, it will indicate the temperature at which the value for density is listed. Solids have strong intermolecular forces that keep particles in close proximity to one another. Another interesting thing to think about is that all true solids have crystalline structures. This means that their particles are arranged in a three-dimensional, orderly pattern.  Solids will undergo phase changes when they come across energy changes.
• Gas: A state of matter where particles are spread out with no definite shape or volume. The particles of a gas will take the shape and fill the volume of the container that it is placed in. In a gas, there are no intermolecular forces holding the particles of a gas together since each particle travels at its own speed in its own direction. The particles of a gas are often separated by great distances.

### Key Terms

Now that we have got an understanding of the key concepts involving phase transitions, there are also key terms in how we classify the movement from one state of matter, to another.

• Fusion (commonly referred to as melting)- The transition by which a solid becomes a liquid.
• Vaporization (commonly referred to as boiling)– The transition by which a liquid becomes a gas. This process takes place when the vapor pressure of the liquid is equal to the pressure of the atmosphere on its surface.
• Sublimation- An unusual process by which a solid goes directly to the gas phase without turning into a liquid first. Sublimation is seen in substances such as carbon dioxide, which has a relatively high vapor pressure and relatively low intermolecular force.
• Solidification (commonly referred to as freezing)- The transition by which a liquid becomes a solid.
• Condensation- The transition by which a gas becomes a liquid. We see condensation form on the outside of a cold glass when water vapor in the air turns into a liquid.
• Deposition– The transition by which a gas vapor goes directly into the solid phase without becoming a liquid first. This process is the opposite of sublimation.
• Evaporation- The transition by which a liquid on the surface of a sample changes to the gas phase.

### Phase Diagram

The relationships between the key terms mentioned above can be expressed in a standard phase diagram.

The principle goal of this diagram is to show how pressure and temperature affect each of the phases. The major points that you should know on this diagram are the following:

• Critical point (also called critical state)- The critical point is the point between the liquid phase and the gas phase with no phase boundary. It is also the point at which temperature and pressure are equal.
• Triple point- This is when temperature and pressure coexist in all three phases.
• Melting point- This is the temperature when a phase change exists between a solid and a liquid.
• Boiling point- This is the temperature when a phase change exists between a liquid and a gas.

### Useful Information

In understanding how a phase diagram works, you must know the different forces that can exist, which hold molecules together. This concept will come up in your latter reading but it is good to know how these forces affect the behavior of the phase transitions. You should know how liquids, solids, and gases are made up of particles and how these particles are able to be held together. It is important to realize that as particles undergo a phase change, the pressure and temperature affect the bonds between the molecules. These forces are called intermolecular forces or Van Der Waals Forces. Van Der Waals Forces can be further broken down into three sub-forces.

• Dispersion Forces (also known as London forces)- These forces are caused by the movement of electrons in the molecules. The stronger the molecular mass, the stronger the forces are going to be. The force also depends on the shape of the molecule. This type of force is present in most molecules.
• Dipole-Dipole Forces- These forces rely on the movement of electrons in bonds, rather than in the molecule. To determine molecules with higher dipole-dipole forces, you can look at their electronegativity. The higher the electronegativity, the stronger the force.
• Hydrogen Bond Forces- These forces occur when hydrogen atoms bond to highly electronegative atoms such as oxygen, nitrogen, and fluorine. This bond greatly affects the boiling point and freezing point.

This concept is important because it helps to understand that if you have one molecule with with all three of these forces, the molecules in this substance are going to be held so tightly together. This means that it will require more energy to break them apart, creating the boiling point to be higher than that of another molecule, like water. Understanding why examples like this one just described will benefit you in understanding phase transitions in general.

### Problems

1. Given this diagram, match each letter (A, B, C, & D) with its appropriate definition.

2. _______ is the process by which a gas becomes a liquid.

3. The _______ of a liquid is its resistance to flow.

4. _______ have definite shape and definite volume.

5. _______ is the process by which a gas turns directly to a solid.

1. A- Melting Point, B- Boiling point, C- Critical point, D- Triple point
2. Condensation
3. Viscosity
4. Solids
5. Deposition

### References

1. Petrucci, Ralph H., et al. General Chemistry: Principles and Modern Applications. Upper Saddle River, NJ: Prentice Hall, 2007
2. Curan, Greg. Chemistry. The Career Press, Inc. Franklin Lakes, NJ: Homework Helpers, 2004

### Contributors

• Shyall Bhela (UCD), Candace Wong-Sing (UCD)

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