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 pressures.

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 of materials:

  • Macroscopic:
    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 properties.
    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:
    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:
    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).
  • Subatomic:
    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 chemistry:

  • Static Electricity, i.e. opposite charges attract but similar charges repel.
    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 reactions.

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