Introduction to the Structure and Properties of Matter For our purposes the fundamental unit of matter at the microscopic level is the atom, ion or molecule. Recall that an ion is an atom that has gained or lost electron(s), while a molecule is a strongly bonded (small) group of atoms which we regard as indivisible. Matter, as we know it, at the macroscopic level consists of very large numbers of these fundamental particles “held together” in some way so that we recognise the substance as existing in one of the three phases of matter. The phases of matter may be categorised by the well-known characteristics: Gas Compressible Flows (fluid) Liquid Incompressible Flows (fluid) Solid Incompressible Does not flow Testing these characteristics involves the use of physical measurement: For compressibility we apply a Pressure and measure any Volume change For flow we apply a Stress (force) and measure any velocity gradient (viscosity). As we shall see these characteristics are not wholly true e.g. solids are compressible, though to a far lesser extent than gases. We also know that, for most materials, if we change the temperature we will change the phase of the matter; so we induce freezing and melting or condensation and boiling. It is also true that, under standard conditions, gases have very low densities, liquids have much higher densities and solid densities are generally similar or larger than those of liquids. Thus we observe the close links between these physical quantities and the structure/ properties of matter. This is what Thermodynamics is all about! Note that all of these measurements are made at the macroscopic level and we need know nothing about what the matter is like at the microscopic level in order to derive equations linking these parameters together and thus to explain and predict the behaviour of the matter. However, to get a full and proper picture it is essential to link the microscopic and macroscopic worlds together. The crucial factor at the atomic level must be the existence of forces between the particles and the nature of these forces. The second factor we should consider is the energies of the particles, themselves. We are, in fact, trying to apply principles of mechanics to these particles but will not succeed, as there are so many of them! We find we must invoke the ideas of statistics to properly analyse matter at the microscopic level; this is the subject of Statistical Mechanics. Although we will not go as far as that in this discussion, it is helpful to look at matter in terms of force and energy, in a descriptive manner. Inter-particle forces and energy We can summarise the strength of forces between particle in the different phases of matter and the way energy is held in these phases, through the following table: Phase Forces Energy No inter-particle forces but All the energy is held as Gas phase must be constrained translational kinetic in some volume by energy by the particles external forces (pressure) which will determine the density. Inter-particle forces are Potential energy from the Liquid strong enough to hold weak bonds between the material together in its particles with some kinetic own volume with defined energy (vibration and density. translation) - which facilitates bond breaking and fluidity. Strong inter-particle forces Most of the energy is held Solid hold matter together, as potential energy due to usually in a rigid 3-D strong bonds. Particle arrangement: crystalline motion exists as vibrations solid of definite volume within the crystal which and density. hold kinetic energy. This shows us that the phase of matter is linked to the relative amounts of kinetic and potential energy in the system. If we raise the temperature by putting heat energy into the matter (or lower the temperature by cooling it) we are changing the total internal energy of the system. How is this extra energy held in the system? It can only exist as kinetic energy since the potential energy component is linked wholly to the inter- particle bonds. Hence, changes in the phase of matter result from changes to the way the internal energy is held. We need to discuss two aspects of the microscopic picture in more detail: • The nature of the inter-particle bond; this is best done in the context of solids • The link between the kinetic energy of the particles and the basic thermodynamics of the system; this is covered by the kinetic theory of gases. This seems to leave liquids out in the cold! We must do this to some extent as they are quite hard systems to understand in detail, but hopefully we can discuss some of their properties such as viscosity and surface tension, later.
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