# Difference between revisions of "Phase transition"

Entry to be completed by Kevin Tian

Written by Kevin Tian, AP 225, Fall 2011

Generally speaking, a Phase Transition is the the process through which a thermodynamic system changes from one phase to another. The most common and easily observed manifestations of this are the various transitions between the various states of matter (solid, liquid, gas, plasma). However though the most visible, states-of-matter transitions are not representative of all phase transitions. For example one is not necessarily limited to a single phase changing to another (different) single phase. There are transitions where a two component system of single phase transforms to two solid phases, among many others.

## Definition

Figure 1: Taken from [1]. Illustrates the names of the phase transitions between the 4 states of matter: Solid, Liquid, Gas, and Plasma.

The most commonly observed phase transitions are the phase changes of the 3/4 states of matter. If we were to consider a one component system (let's say water), and we alter the external conditions imposed upon the system (such as increase the temperature) then the system will undergo a phase transition (with our example, the water will eventually boil). In essence, as one changes the properties of a system, eventually the system will no longer be in equilibrium. The new phase that forms will be more stable than the phase the system was in previous to the phase transition.

The conditions under which phase transitions occur essentially depend on the system being considered, and what the thermodynamic free energy of the system is (which relates to its temperature, pressure, chemical potential etc.). Most of this would be rather obscure were it not for Phase diagrams.

## Phase Transition 'Order'

It appears to be generally accepted that there are two 'orders' of phase transitions. These are aptly named 'first order' and 'second order'.

• First Order
• These transitions occur in a discontinuous fashion. This is reflected in the order parameter describing the system.
• The system does not entirely transition to the new phase all at once in a smooth fashion.
• There is a discontinuity in the state variables of the system (such as temperature or pressure)
• Examples include melting of a crystal or the crystallization of a liquid
• Second Order
• These transitions occur in a continuous fashion.