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Some people make physical chemistry sound more confusing than it really is. One of their best tricks is to define it inaccurately, saying it is ‘the physics of chemicals’. This definition is sometimes quite good since it suggests we look at a chemical system and ascertain how it follows the laws of nature. This is true, but it suggests that chemistry is merely a sub-branch of physics, and the notoriously mathematical nature of physics impels us to avoid this otherwise useful way of looking at physical chemistry. Physical Chemistry by Paul Monk
An alternative and more user-friendly definition tells us that physical chemistry supplies ‘the laws of chemistry, and is an addition to the making of chemicals. This is a superior lens through which to view our topic because we avoid the bitter aftertaste of pure physics, and start to look more closely at physical chemistry as applied science: we do not look at the topic merely for the sake of looking, but because there are real-life situations requiring a scientific explanation. Nevertheless, most practitioners adopting this approach are still overly mathematical in their treatments and can make it sound as though the science is fascinating in its own right, but will sometimes condescend to suggest an application of the theory they so clearly relish.
But the definition we will employ here is altogether simpler, and also broader: we merely ask ‘why does it happen?’ as we focus on the behavior of each chemical system. Every example we encounter in our everyday walk can be whittled down into small segments of thought, each so simple that a small child can understand. As a famous mystic of the 14th century once said, ‘I saw a small hazelnut and I marveled that everything that exists could be contained within it’. And in a sense she was right: a hazelnut looks brown because of the way light interacts with its outer shell – the topic of spectroscopy (Chapter 9); the hazelnut is hard and solid – the topic of bonding theory (Chapter 2) and phase equilibria