7. Investigating matter

Physical chemistry lies at the heart of one of chemistry's principal applications and achievements: the identification of the substances present in a sample and the determination of their abundances and structures. ‘Investigating matter’ considers how the laser and computer have elaborated and refined classical techniques. Various forms of investigative technique are explained including absorption spectroscopy and nuclear magnetic resonance. Mass spectrometry is widely used in organic chemistry to help identify compounds and photoelectron spectroscopy is used both to explore the energies with which electrons are bound inside molecules and to identify species on surfaces. The study of surfaces has been transformed by scanning tunnelling microscopy, which relies on quantum mechanics.

4. States of matter

‘States of matter’ describes the three traditional states — gas, liquid, and solid — and the models used to predict and understand their behaviour. The van der Waals equation of state captures many of the properties of real gases. The classical way of studying the motion of molecules in liquids is to measure its viscosity. Techniques include neutron scattering and nuclear magnetic resonance. X-ray diffraction is used to determine the structures of solids. Intermediate states of matter — where liquid meets gas and liquid meets solid — are also considered. Examples include supercritical fluids, soft matter such as liquid crystals, and graphene, a remarkable and essentially two-dimensional material.

3. Bridging matter

‘Bridging matter’ introduces statistical thermodynamics, which provides the link between the notional insides and outsides of atoms and molecules. It identifies the bulk properties of a sample with the average behaviour of all the molecules that constitute it. A key concept is the Boltzmann distribution, which shows the exponentially decaying function of the energy of molecules as temperature is increased. It captures two aspects of chemistry: stability and reactivity. Statistical thermodynamics is used by physical chemists to understand the composition of chemical reaction mixtures that have reached equilibrium. It also provides an explanation of Le Chatelier's principle, which states that a system at equilibrium responds to a disturbance by tending to oppose its effect.

5. Changing the state of matter

‘Changing the state of matter’ considers the transformations states of matter undergo from one form to another during the processes of freezing, boiling, dissolving, and mixing. All such transformations and the corresponding equilibria that are reached in state transitions can be expressed in terms of the chemical potential and the pushing power it represents. Changes in temperature and pressure can lower or raise boiling and freezing points. Josiah Gibbs' phase rule is concerned with the equilibria between various forms of matter. Phase diagrams help chemists draw conclusions about the compositions of mixtures, minerals, and alloys. Transitions in solutions, including osmosis, and solid-to-solid phase transitions are also discussed.

2. Matter from the outside

‘Matter from the outside’ focuses on the applications of thermodynamics in physical chemistry. Thermodynamics is the science of energy and the transformations that it can undergo. It plays a central role in understanding chemical reactions. There are four laws of thermodynamics: the Zeroth Law establishes the concept of temperature; the First Law concerns the conservation of energy; the Second Law deals with entropy (a measure of the quality of energy); and the Third Law concerns the absolute zero of temperature. The property enthalpy is explained along with Gibbs energy and free energy. Physical chemists can deploy the laws of thermodynamics, laws relating to matter from the outside, to establish relations between properties and to make important connections.

6. Changing the identity of matter

A great deal of chemistry is concerned with changing the identity of matter by the deployment of chemical reactions. Physical chemists are interested in a variety of aspects of chemical reactions, including the rates at which they take place (chemical kinetics) and the details of the steps involved during the transformation (chemical dynamics). Chemical reactions can be achieved simply by mixing and heating, but some are stimulated by light (photochemistry) and others by electricity (electrochemistry). ‘Changing the identity of matter’ explains the key terms of chemical kinetics and chemical dynamics such as spontaneous reaction, reaction quotient, equilibrium constant, rate law, reaction mechanism, rate-determining step, activation energy, and catalysis.

1. Matter from the inside

‘Matter from the inside’ shows that one way to understand how a physical chemist thinks and contributes to chemistry is to start at the atom's interior and then travel out into the world of bulk matter. It begins with the electronic structure of atoms, introduces the role of quantum mechanics in accounting for electron arrangement, and outlines Schrödinger's model of s-, p-, d-, and f-orbitals and the Pauli exclusion principle. Physical chemistry accounts for the general structure of the Periodic Table. The radius, ionization energy, and electron affinity properties of atoms are then considered along with ionic and covalent bonds, and the quantum mechanics of bonds, including valence-bond theory and molecular orbital theory.