What is Physical Chemistry ?
Physical Chemistry is the branch of
chemistry dealing with the physical properties
of chemical substances.
That means describing and explaining how specific
chemical substances look and behave in particular
situations, e.g. under certain temperatures and
Physical Chemistry includes study of the physical properties
of many different types of substances and on different
scales (levels of physical detail). That is, it includes
study of the following scales of chemical properties
Macroscopic properties of substances describe how
relatively large quantities of the substance behave
as a group, e.g. melting points and boiling points,
latent heats of fusion and vapourization, thermal
conductivity, specific heat capacity, coefficient
of linear thermal expansion, and many other "physical
Take the chemical substance H20 as an example:
A "drop of water" is liquid. At lower temperatures
it freezes (becoming solid "ice") and at
higher temperatures it evapourates (becoming a gas
called "steam"). The physical state of H20
as a liquid, solid, or gas defines how closely a large
quantity of individual molecules of H20
are attracted to each other. Because the molecules
are very small, there is a large quantity of H20
molecules in a single drop of water. However, these
properties do not describe the chemistry of the H20
molecule: Two hydrogen atoms and one oxygen atom form
each molecule of water, regardless of its temperature,
hence if it solid ice, liquid water, or gaseous steam.
Microscopic properties of substances concern details
of their physical properties observable only using
the magnification provided by microscopes (there are,
of course, different types and powers of microscopes
e.g. light microscopes, electron microscopes, and
more recently scanning probe microscopes). Microscopic
physical properties include, for example, the shapes
and structures of crystals - which can have important
consequences for the behaviour of large sections of
the material of which they are a part e.g. as used
in bridges, aircraft, and so on.
Atomic properties relate to elements. (Recall
that elements consist of many individual atoms,
whereas compounds consist of many molecules - which are, in turn, specific combinations of atoms joined together via chemical bonds.)
Examples of atomic properties of elements include atomic numbers and atomic mass, e.g. the element
Boron whose chemical symbol is B, has atomic number
5 and atomic mass 10.81 (strictly for a free
neutral atom in ground state).
The study of and research into subatomic "particles"
(which are sometimes described in other ways, e.g.
as "energy" or "waves") and their
properties is often classified as part of physics,
rather than chemistry. Advanced physical chemistry
also includes subatomic structures and theories - which
are important for nuclear chemistry.
The study of Physical Chemistry generally involves using
theories, measurements, and techniques either from,
or more usually associated with, physics - to study,
understand and explain chemical substances.
Hence, when asked the question "What is Physical Chemistry", some people talk about the "physics of chemicals", which may be a helpful way to remember or work out a simple definition of physical chemistry.
For example, the following theories and techniques
may be used or described in school/college-level physical
- Static Electricity,
i.e. opposite charges attract but similar charges
This theory is used to explain some types ot chemical
bonds. For example, it may be applied to ions (atoms
that have either "lost" or "gained"
electrons, hence have a positive or negative charge),
which therefore form stable arrays with other types
of (oppositely charged) ions. Static Electricity is
also used to explain van der Waal's forces between
molecules or parts of the same molecule, i.e. the
force between a permanent dipole and a corresponding
- Gas Laws and associated theories
E.g. Boyle's Law, Charles' Law, Ideal Gas Equation,
Dalton's Law of Partial Pressure, Avogadro's Hypothesis,
Gay Lussac's Law, Kinetic Theory of Gases and 'Real
- X-Ray Diffraction
for analysis of crystal structures, incl. the Bragg equation.
- Analysis of Solutions
and Solubility, using:
E.g. Le Chatelier's Principle, Henry's Law, Absorption
Coefficients, Raoult's Law, Mole Fractions, Vapour
Pressures, Partial Pressures, and Molar compositions.
- Laws of Thermodynamics and other assessments of energetics.
e.g. enthaply changes (and measurement of enthalpy
changes), Hess's Law, Born-Haber Cycle, and the concept
"Free Energy G".
- Theory of Radioactivity.
The above list is just a few examples of theories and
techniques used in Physical Chemistry. They may be useful to mention in an essay or extended definition question about "What is Physical Chemistry?". Some of the topics
listed may also overlap with organic
chemistry and/or inorganic
chemistry when applied to specific substances and/or