Phase Transitions Reading
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.
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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.
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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.
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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:
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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.
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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
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Given this diagram, match each letter (A, B, C, & D) with its appropriate definition.
_______ is the process by which a gas becomes a liquid.
The _______ of a liquid is its resistance to flow.
_______ have definite shape and definite volume.
_______ is the process by which a gas turns directly to a solid.
Answers
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2.
3.
4.
5.
A- Melting Point, B- Boiling point, C- Critical point, D- Triple point
Condensation
Viscosity
Solids
Deposition