Phase Transitions

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 13:37

We have now looked at the physical properties which chemists use to define the solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions., liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container, and gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. phases. In a solid, atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume., ions or molecules, are locked into an organized, long range latticeThe points in space that define the ordered, repeating arrangement of atoms, ions, or molecules in a crystal. structure, unable to move beyond an average position due to intermolecular forces. In a liquid, this structure breaks down, molecules can slip past each other, but they are still held together by attractive forces. In a gas, these attractive forces are overcome, and the substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. expands to fill space, each particle having gained mobility to break free of the others.

Substances can be transformed from one phase into another. Solids melt into liquids and liquids boil to form vapors at temperatures which depend on their molecular properties, so chemists are interested in these transitions between phases.

We are all familiar with the changes in macroscopic properties that accompany these transitions. YouTube has time lapse movies of ice meltingThe process of a liquid forming from a solid. on a small scale, or of the more environmentally critical arctic ice melt from 1979 to 2007.

If your workstation is subscribed to JCE Software, you can view the following video of solid sulfur melting.

This is a familiar process. As the solid melts, the resulting liquid is able to flow and conforms to the shape of the container. HeatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. from a flame is needed to bring about this transition.

On a microscopic level melting involves breaking the intermolecular interactions between molecules. This requires an increase in the potential energy of the molecules, and the necessary energy is supplied by the Bunsen burner. Melting (or freezingThe process of forming a solid from a liquid.) can, in some cases, be caused by changing just the pressureForce per unit area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container..

Boiling is equally familiar. Under specific temperature and pressure conditions, liquids start to bubble, and are converted to a gaseous form. A YouTube video "Boiling water with ice" shows that water boils at low temperatures if the pressure is reduced.

If your workstation is subscribed to JCE Software, you can view the following video of hexane boiling.

Heat energy is absorbed when a liquid boils because molecules which are held together by mutual attraction in the liquid are jostled free of each other as the gas is formed. Such a separation requires energy. In general the energy needed differs from one liquid to another depending on the magnitude of the intermolecular forces. We can thus expect liquids with strong intermolecular forces to boil at higher temperatures. It should be noted as well, that because there is a distribution in the kinetic energies of molecules, a equilibrium between gas and liquid phase is established at temperatures other than the boiling pointThe temperature at which the vapor pressure of a liquid equals the pressure of the gas in contact with the liquid; usually this is atmospheric pressure., and this behavior is another aspect of phase transitions that chemists study.

For phase transitions from solid to liquid, liquid to gas, or solid to gas, energy is required because they involve separation of particles which attract one another. Further, we can predict under which conditions of temperature and pressure such transitions will occur.