10.000solldCritical pointsupercritical1,000fuldliquidcriticalpoin· Critical point is the end point of a10-gasiplepoiphase equilibrium curve.STT300200250350400temperatureT (K). The most prominent example is the liquid-vaporcritical point, the end point of the pressure-temperature curve that designates conditionsunder which a liquid and its vapor can coexist· At the critical point, defined by a criticaltemperature Tc and a critical pressure pc, phaseboundariesvanish.6
Critical point • Critical point is the end point of a phase equilibrium curve. • The most prominent example is the liquid-vapor critical point, the end point of the pressuretemperature curve that designates conditions under which a liquid and its vapor can coexist. • At the critical point, defined by a critical temperature 𝑇𝑐 and a critical pressure 𝑝𝑐 , phase boundaries vanish. 6
· At the critical point, only one phaseexists. The heat of vaporization iszero. There is an inflection point in theconstant-temperature line (criticalisotherm) on a PV diagram. Thismeans that at the critical point:(%),= (%)=0· Above the critical point one has astate of matter that is continuouslyconnected with (can be transformedwithout phase transition into) boththe liquid and the gaseous state. It iscalled supercritical fluid
• At the critical point, only one phase exists. The heat of vaporization is zero. • There is an inflection point in the constant-temperature line (critical isotherm) on a PV diagram. This means that at the critical point: • Above the critical point one has a state of matter that is continuously connected with (can be transformed without phase transition into) both the liquid and the gaseous state. It is called supercritical fluid. 7
Tricritical point·It is possible that threephases, such as solid, liguid andvapour, can exist togetherin equilibrium (P = 3).If thereisonly one component, there are no degrees of freedom (F =O)whenthere arethreephases.. Therefore, in a single-component system,this three-phasemixturecan onlyexistat a singletemperatureand pressure,which is known as a triple point. Here there are twoequationsμsol(T,p) = μlig(T,p) = μvap(T,p)which are sufficientto determinethe two variables T and p· It is also possible for other sets of phases to form a triplepoint, for example in the water system there is a triple pointwhere ice I, ice Ill and liquid can coexist.8
Tricritical point • It is possible that three phases, such as solid, liquid and vapour, can exist together in equilibrium (𝑃 = 3). If there is only one component, there are no degrees of freedom (𝐹 = 0) when there are three phases. • Therefore, in a single-component system, this three-phase mixture can only exist at a single temperature and pressure, which is known as a triple point. Here there are two equations 𝜇𝑠𝑜𝑙(𝑇, 𝑝) = 𝜇𝑙𝑖𝑞(𝑇, 𝑝) = 𝜇𝑣𝑎𝑝(𝑇, 𝑝) which are sufficient to determine the two variables T and p. • It is also possible for other sets of phases to form a triple point, for example in the water system there is a triple point where ice I, ice III and liquid can coexist. 8