Chemistry is traditionally divided into a small number of subfields, namely organic, inorganic, analytical and physical chemistry. It's fairly easy to say what the first three are about, but it's much harder to define physical chemistry. The problem is that physical chemistry is all of the following simultaneously:

• A discipline in its own right, with its own set of problems and techniques;

• The source of the basic theory that underlies all of the chemical sciences;

• A provider of experimental methods used across the chemical sciences.

Note that “chemical sciences” includes biochemistry and materials science, among other fields that depend on physical chemistry for at least some of their theory and methods. Physical chemistry's large mandate means that it's difficult to put a finger on what it is exactly. It's a bit like chemistry itself that way: every time you come up with a definition, you immediately think of half a dozen things done under that heading that don't fit.

Rather than trying to give a simple, neat definition of physical chemistry, I'm going to tell you about the big theories that make up physical chemistry. Hopefully, this will give you an idea of what physical chemistry is about, even if we can't wrap it up in a neat package as we can with the other subfields of chemistry.