Faza (termodinamika) – razlika između verzija
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U [[fizika|fizičkim naukama]], '''faza''' je [[skup]] stanja makroskopskih fizikalnih sistema koji imaju relativno unifoman hemijski sastav i fizičke osobine (npr. [[gustoća|gustoću]], [[kristalna struktura|kristalnu strukturu]], [[indeks prelamanja]], itd.).<ref>{{cite book |last=Modell|first=Michael|author2=Robert C. Reid|title=Thermodynamics and Its Applications |publisher=Prentice-Hall|location=Englewood Cliffs, NJ |date=1974 |isbn=0-13-914861-2}}</ref><ref name="Fermi2012">{{cite book|author=Enrico Fermi|title=Thermodynamics|date=25 April 2012|publisher=Courier Corporation|isbn=978-0-486-13485-7}}</ref>{{rp|86}}<ref name="Adkins1983">{{cite book|author=Clement John Adkins|title=Equilibrium Thermodynamics|date=14 July 1983|publisher=Cambridge University Press|isbn=978-0-521-27456-2}}</ref> U sistemu koji se sastoji od leda i vode u staklenoj tegli, kocke leda su jedna faza, voda je druga, a vlažni vazduh iznad vode je treća faza. Staklo tegle je još jedna zasebna faza.
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At room temperature and pressure, the water jar reaches equilibrium when the air over the water has a humidity of about 3%. This percentage increases as the temperature goes up. At 100 °C and atmospheric pressure, equilibrium is not reached until the air is 100% water. If the liquid is heated a little over 100 °C, the transition from liquid to gas will occur not only at the surface, but throughout the liquid volume: the water boils.
== Broj faza ==
{{see also|Multifazna tečnost}}
[[Image:phase-diag2.svg|thumb|right|280px|A typical phase diagram for a single-component material, exhibiting solid, liquid and gaseous phases. The solid green line shows the usual shape of the liquid–solid phase line. The dotted green line shows the anomalous behavior of water when the pressure increases. The [[triple point]] and the [[Critical point (thermodynamics)|critical point]] are shown as red dots.]]
For a given composition, only certain phases are possible at a given [[temperature]] and pressure. The number and type of phases that will form is hard to predict and is usually determined by experiment. The results of such experiments can be plotted in [[phase diagram]]s.
The phase diagram shown here is for a single component system. In this simple system, which phases that are possible depends only on [[pressure]] and [[temperature]]. The markings show points where two or more phases can co-exist in equilibrium. At temperatures and pressures away from the markings, there will be only one phase at equilibrium.
In the diagram, the blue line marking the boundary between liquid and gas does not continue indefinitely, but terminates at a point called the [[critical point (thermodynamics)|critical point]]. As the temperature and pressure approach the critical point, the properties of the liquid and gas become progressively more similar. At the critical point, the liquid and gas become indistinguishable. Above the critical point, there are no longer separate liquid and gas phases: there is only a generic fluid phase referred to as a [[supercritical fluid]]. In water, the critical point occurs at around 647 [[Kelvin|K]] (374 °C or 705 °F) and 22.064 [[Pascal (pressure)|MPa]].
An unusual feature of the water phase diagram is that the solid–liquid phase line (illustrated by the dotted green line) has a negative slope. For most substances, the slope is positive as exemplified by the dark green line. This unusual feature of water is related to ice having a lower density than liquid water. Increasing the pressure drives the water into the higher density phase, which causes melting.
Another interesting though not unusual feature of the phase diagram is the point where the solid–liquid phase line meets the liquid–gas phase line. The intersection is referred to as the [[triple point]]. At the triple point, all three phases can coexist.
Experimentally, the phase lines are relatively easy to map due to the interdependence of temperature and pressure that develops when multiple phases forms. See [[Gibbs' phase rule]]. Consider a test apparatus consisting of a closed and well insulated cylinder equipped with a piston. By charging the right amount of water and applying heat, the system can be brought to any point in the gas region of the phase diagram. If the piston is slowly lowered, the system will trace a curve of increasing temperature and pressure within the gas region of the phase diagram. At the point where gas begins to condense to liquid, the direction of the temperature and pressure curve will abruptly change to trace along the phase line until all of the water has condensed.
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