Name: liquid-gas changes on pv curves
Uploaded: 2025-05-22T09:05:49.000Z
Duration: 1 min 18 s
Channel: JoVE

K12 Standards

Tennessee Academic Standards

High School

Chemistry

Matter and Its Interactions

CHEM1.PS1.6

pV-Diagrams

Per un gas ideale, la pressione moltiplicata per il volume è una costante se la temperatura e il numero di moli sono costanti. Il grafico della pressione rispetto al volume si traduce in un'iperbole chiamata isoterma pV. Per diverse temperature costanti, viene tracciato l'insieme dei diagrammi pV.
I gas non ideali si comportano come un gas ideale a temperature più elevate. Se la temperatura di un gas non ideale si riduce a una temperatura critica, la curva pV mostra una regione con pendenze nulle.
Supponiamo che la temperatura si abbassi ulteriormente, che la regione con pendenza zero aumenti. Queste regioni rappresentano una variazione di volume senza alcuna variazione di pressione.
Per l'isoterma T2, la regione A-B sull'isoterma rappresenta la fase di liquefazione. Il gas è in uno stato di vapore fino al punto A e in una fase liquida oltre il punto B.
Per le isoterme con temperature inferiori a T2, la liquefazione inizia a pressioni più basse e volumi più elevati.
Per isoterme superiori alle temperature critiche, non avviene alcuna liquefazione. La temperatura critica è una proprietà del gas in studio per una data pressione critica.

<ol>
	<li>Young, H.D e Freedman, R.A. (2012). Fisica universitaria con fisica moderna. San Francisco, CA: Pearson; sezione 18.1; pagina 596.</li>
	<li>OpenStax. (2019). Fisica universitaria Vol. 2. [Versione web]. Estratto da <a href="https://openstax.org/details/books/university-physics-volume-2">https://openstax.org/details/books/university-physics-volume-2</a>; sezione 2.1; pagine 75 -77.</li>
</ol>

diagramma pv

Il diagramma pV, che è un grafico di pressione vs volume del gas studiato, è utile nel descrivere certi aspetti della sostanza. Quando la sostanza si comporta come un gas ideale, l'equazione dei gas ideali descrive la relazione tra la sua pressione e il volume. In un diagramma pV, è comune tracciare un'isoterma, che è una curva che mostra p come una funzione di V con il numero di molecole e la temperatura fisse. Quindi, per un gas ideale, il prodotto della pressione del gas e il suo volume è costante. Per esempio, il volume del gas diminuisce con l'aumento della pressione. Il risultante grafico pV è un'iperbole.

Le isoterme pV sono iperbole ad alte temperature e basse pressioni. Quando raffreddiamo il gas, il grafico pV devia dall'iperbole. Ad una temperatura critica (Tc), la curva pV mostra un punto con zero pendenza. Come il gas si raffredda ulteriormente, la curva pV inizia a mostrare una regione con zero pendenza. All'interno di questa regione, il volume del gas cambia, ma non la sua pressione. Questo comportamento corrisponde all'ebollizione e alla condensazione; quando una sostanza è alla sua temperatura di ebollizione per una particolare pressione, essa può aumentare il suo volume mentre una parte del liquido si trasforma in gas (passando da un volume basso ad uno alto) o diminuirlo mentre una parte del gas diventa liquido (passando da un volume alto ad uno basso), senza nessun cambiamento di temperatura o pressione. Questa è la regione in cui avvengono la vaporizzazione o la liquefazione.

<img alt="Figure1" src="/files/ftp_upload/13669/13669_PT_Figure.svg" style="text-align: center; height: 250px;" />

Le parti ripide delle curve a sinistra della regione di transizione mostrano la fase liquida, che è quasi incomprimibile – una leggera diminuzione nel volume richiede un aumento significativo di pressione. Le parti piatte mostrano la transizione da liquido a gas, rappresentando combinazioni di pressione e volume dove liquidi e gas possono coesistere. La parte destra della regione di transizione mostra la fase gassosa.

Le isoterme sopra Tc non attraversano la transizione da liquido a gas. Di conseguenza, il liquido non può esistere sopra questa temperatura, la temperatura critica. Ad una pressione sufficientemente bassa al di sopra di tale temperatura, il gas ha la densità di un liquido ma non condensa; il gas è detto essere supercritico. Ad una pressione più alta, è solido. Il diossido di carbonio, per esempio, non ha una fase liquida ad una temperatura maggiore di 31 gradi Celsius. Il punto sul diagramma pV alla pressione e temperatura critica è il punto critico.

Diagramma pV

Il diagramma pV, che è un grafico della pressione rispetto al volume del gas oggetto di studio, è utile per descrivere alcuni aspetti della sostanza. Quando il ...

Education

Didattica

JoVE Core

Physics

Fisica

Unit 1

Unità 1

The Kinetic Theory of Gases

La teoria cinetica dei gas

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

liquid-gas changes on pv curves

pV curves show how gas pressure changes with volume. A pV diagram is a graph of pressure versus volume for the gas being studied. It helps describe how the substance behaves. When the gas acts like an ideal gas, the ideal gas equation gives the link between pressure and volume.

A common pV curve is an isotherm, which is a line drawn at fixed temperature and fixed number of molecules. For an ideal gas, pressure times volume stays constant. That means volume goes down as pressure goes up. The curve is a hyperbola, especially at high temperatures and low pressures.

As the gas is cooled, the curve no longer stays perfectly hyperbolic. At the critical temperature, Tc, the pV curve reaches a point with zero slope. Cooling farther creates a flat region where the volume can change without changing the pressure. This is the part of the curve where boiling and condensation occur.

In this transition region, a substance can expand as liquid changes into gas or contract as gas changes into liquid. The temperature and pressure stay the same during this change. To the left of the transition region, the curve is steep. This shows the liquid phase, which is almost incompressible, so a small decrease in volume needs a large increase in pressure.

The flat part of the curve represents liquid and gas coexisting. To the right of this region, the curve shows the gas phase. Isotherms above Tc do not pass through the liquid-gas transition. That is why liquid cannot exist above the critical temperature. At low enough pressure above Tc, the gas can have the density of a liquid but still not condense. This is called a supercritical gas. At higher pressure, the substance is solid. Carbon dioxide is one example, because it has no liquid phase above 31 degrees Celsius. The critical pressure is the highest pressure at which the liquid can exist, and the point at the critical pressure and critical temperature is the critical point.

Liquid-Gas Changes on pV Curves

equation of state

Gas Behavior in the Equation of State

The equation of state describes how a gas’s pressure, volume, temperature, and number of moles are related. These macroscopic properties help define the condition, or state, of the matter in a thermodynamics system. The relationship between these quantities can be written as a simple mathematical expression. In some cases, it is too complex to express in a formula. Even then, the link between the quantities still exists. When an equation of state cannot be written mathematically, scientists...

ideal gas equation

Gas Laws Behind the Ideal Equation

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van der waals equation

How Real Gases Deviate from Ideal Behavior

Real gases often deviate from the ideal gas law, especially at low temperatures and high pressures. The van der Waals equation improves the ideal gas model by adding two corrections that account for these real-world effects. One correction accounts for attractive forces between molecules. These forces become stronger at higher densities and lower the measured pressure, so the equation adds a pressure term based on the square of the molar density and a positive constant a. The second...

pv-diagrams

pV curves show how gas pressure changes with volume. A pV diagram is a graph of pressure versus volume for the gas being studied. It helps describe how the substance behaves. When the gas acts like an ideal gas, the ideal gas equation gives the link between pressure and volume. A common pV curve is an isotherm, which is a line drawn at fixed temperature and fixed number of molecules. For an ideal gas, pressure times volume stays constant. That means volume goes down as pressure goes up. The...

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Avogadro's Number and Gas Particle Motion

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molecular kinetic energy

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Molecular Speed Curves and Vapor Pressure

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maxwell-boltzmann distribution: problem solving

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Maxwell-Boltzmann Gas Speed Ratio

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Water Phase Changes on a Pressure Chart

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Gas collisions: path length and time

Gas collisions in a cylinder help explain the mean free path and mean free time. Gas molecules move in random motion. They collide with each other and change speed and direction. The mean free path is the average path length between collisions. The mean free path depends on how many molecules are packed into a volume. It varies inversely with density. When more molecules are present, collisions happen more often, so the mean free path becomes shorter. It also depends on molecule size. Larger...

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key terms and concepts

Explore how pV curves show ideal gas behavior, boiling, condensation, the critical point, and why carbon dioxide has no liquid phase above 31°C.

Liquid-Gas Changes on pV Curves | JoVE K12

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Florida Department of Education

Physical Science

SC.912.P.12

SC.912.P.12.10

Georgia Standards of Excellence

9-12

SC5.

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Gas Laws: Pressure, Volume, Temperature, and Moles

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CHEM1.PS1.1

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CHEM1.PS1.2

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CHEM1.PS1.3

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CHEM1.PS1.4

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Conduct investigations to explore and characterize the behavior of gasses (pressure, volume, and temperature), develop models to represent this behavior, and construct arguments to explain this behavior. Evaluate the relationship (qualitative and quantitatively) at STP between pressure and volume (Boyle's Law), temperature and volume (Charles' Law), temperature and pressure (Gay-Lussac Law) and moles and volume (Avogadro's Law), and evaluate and explain these relationships with respect to kinetic molecular theory. Be able to understand, establish, and predict the relationships between volume, temperature, and pressure using combined gas law both qualitatively and quantitatively. EXPLANATION: When exploring the behavior of gasses, it is important to consider experimental design. Experiments used to show the relationships between these sets of variables should include one independent and one dependent variable. Other variables should be held constant. Pressure should serve as the dependent variable because it cannot be manipulated directly. Avogadro's Law can be used to relate the amount of gas in two different containers in lieu of knowing actual number of gas particles in an individual container. Individual demonstrations can be performed to explore each of the different gas laws.

CHEM1.PS1.5

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CHEM1.PS1.7

Liquid-Gas Changes on pV Curves

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