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

Para um gás ideal, a pressão vezes o volume é uma constante se a temperatura e o número de moles forem constantes. O gráfico de pressão versus volume resulta em uma hipérbole chamada isoterma pV. Para diferentes temperaturas constantes, o conjunto de diagramas de pV é plotado.
Gases não ideais se comportam como um gás ideal em temperaturas mais altas. Se a temperatura de um gás não ideal for reduzida a uma temperatura crítica, a curva pV mostra uma região com inclinações nulas.
Suponha que a temperatura seja reduzida ainda mais, a região com inclinação zero aumenta. Essas regiões representam uma mudança no volume sem qualquer mudança na pressão.
Para a isoterma T2, a região AB na isoterma representa a fase de liquefação. O gás está em estado de vapor até o ponto A e em fase líquida além do ponto B.
Para isotermas com temperaturas inferiores a T2, a liquefação começa em pressões mais baixas e volumes mais altos.
Para isotermas acima das temperaturas críticas, não ocorre liquefação. A temperatura crítica é uma propriedade do gás em estudo para uma determinada pressão crítica.

<ol>
	<li>Young, H.D. e Freedman, R.A. (2012). Física Universitária com Física Moderna. São Francisco, CA: Pearson; seção 18.1; página 596.</li>
	<li>OpenStax. (2019). Física Universitária Vol. 2. [Versão Web]. Obtido de <a href="https://openstax.org/details/books/university-physics-volume-2">https://openstax.org/details/books/university-physics-volume-2</a>; Seção 2.1; página 75 -77.</li>
</ol>

diagramas pv

O diagrama pV, que é um gráfico de pressão versus volume do gás em estudo, é útil para descrever certos aspectos da substância. Quando a substância se comporta como um gás ideal, a equação do gás ideal descreve a relação entre sua pressão e volume. Em um diagrama pV, é comum traçar uma isoterma, que é uma curva que mostra p como função de V com o número de moléculas e a temperatura fixos. Em seguida, para um gás ideal, o produto da pressão do gás e seu volume é constante. Por exemplo, o volume do gás diminui à medida que a pressão aumenta. O gráfico pV resultante é uma hipérbole.

As isotermas pV são hiperbólicas em altas temperaturas e baixas pressões. Conforme resfriamos o gás, o gráfico pV se desvia da hipérbole. Em uma temperatura crítica (Tc), a curva pV mostra um ponto com inclinação zero. À medida que o gás é resfriado ainda mais, a curva pV começa a exibir uma região com inclinação zero. Dentro dessa região, o volume do gás muda, mas sua pressão não muda. Esse comportamento corresponde à ebulição e condensação; quando uma substância está em sua temperatura de ebulição para uma determinada pressão, ela pode aumentar de volume à medida que parte do líquido se transforma em gás (indo de um volume menor para um volume maior) ou diminuir à medida que parte do gás se transforma em líquido (indo de um volume maior para um volume menor), sem qualquer mudança na temperatura ou pressão. Esta é a região onde ocorre a vaporização ou liquefação.

<img alt="Figure1" src="/files/ftp_upload/13669/13669_PT_Figure.svg" style="text-align: center; height: 250px;" />

As partes mais inclinadas das curvas à esquerda da região de transição mostram a fase líquida, que é quase incompressível - uma leve diminuição no volume requer um aumento significativo na pressão. As partes planas mostram a transição líquido-gás, representando combinações de pressão e volume onde líquido e gás podem coexistir. A direita da região de transição mostra a fase gasosa.

As isotermas acima de Tc não passam pela transição líquido-gás. Portanto, o líquido não pode existir acima dessa temperatura, que é a temperatura crítica. Sob pressão suficientemente baixa acima dessa temperatura, o gás possui a densidade de um líquido, mas não irá se condensar; o gás é considerado como estando em um estado supercrítico. Sob pressão mais alta, ele se torna sólido. O dióxido de carbono, por exemplo, não possui fase líquida a uma temperatura acima de 31 graus Celsius. A pressão crítica é a pressão máxima na qual o líquido pode existir. O ponto no diagrama pV que fica na pressão crítica e na temperatura crítica é o ponto crítico.

Diagramas pV

O diagrama pV, que é um gráfico de pressão versus volume do gás em estudo, é útil para descrever certos aspectos da substância. Quando o ...

Education

JoVE Core

Physics

Física

Unit 1

Unidade 1

The Kinetic Theory of Gases

A Teoria Cinética dos Gases

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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

Gas Pressure and Molecular Motion

Gas pressure comes from the motion of molecules in a container. The word gas came from a Flemish word meaning “chaos,” and J. B. van Helmont first used it to describe vapors. In a gas, molecules move in a continuous and random way. When a molecule hits a container wall, its velocity changes. The part of the velocity parallel to the wall stays the same. The part perpendicular to the wall reverses direction, but its size does not change. If the molecule’s velocity changes in the x-direction, its...

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Molecular Speed Curves and Vapor Pressure

Molecular speed curves show how the speeds of gas particles are spread out at a given temperature. For a gas with many molecules, this spread is predictable even though each molecule moves randomly in direction and speed. The pattern is called the Maxwell-Boltzmann distribution. Liquids have a similar speed distribution, but it is not exactly the same as in gases. This difference helps explain boiling and vapor pressure. A molecule must reach at least a certain speed, v a, to escape from the...

maxwell-boltzmann distribution: problem solving

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Maxwell-Boltzmann Gas Speed Ratio

The Maxwell-Boltzmann distribution describes how molecular speeds are spread in a gas. Even though individual molecules move in random directions, the whole gas has a predictable speed pattern. This distribution is written as f(v), where v is molecular speed. The distribution function is used to find the expected number of particles with speeds between v1 and v2. Because N is dimensionless, f(v) has units of seconds per meter. The relationship can also be written in differential form for speed...

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Water Phase Changes on a Pressure Chart

Water phase changes can be mapped on a pressure and temperature chart called a phase diagram. These plots show which phase a substance is in at different conditions. They also reveal where phase transitions happen, because the boundary lines mark the temperatures and pressures where two phases coexist. The water phase diagram shows how pressure affects boiling. As pressure rises, the boiling temperature increases gradually until it reaches 374 °C at 218 atm. A pressure cooker works this way.

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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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Heat Capacity of Gases by Degrees of Freedom

Heat capacity of gases depends on the gas type and the motion of its molecules. It is the amount of heat energy needed to raise the temperature of a unit mass of gas by one degree Celsius. This thermodynamic property is important in science and industry. To solve heat capacity problems, first identify the gas. A gas's molecular structure affects its heat capacity because the molecules have different degrees of freedom. These values can be found from tables or from empirical equations. Next,...

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Gas Mixtures and Their Partial Pressures

Gas mixtures have a total pressure that comes from the partial pressures of each gas. A partial pressure is the pressure one gas would make if it filled the whole container by itself. This idea helps explain how gases behave when they are mixed together. When two or more gases share a container, their molecules move randomly and collide with one another. If the gases are at the same temperature, they reach thermal equilibrium. Their molecules then have the same average kinetic energy. In that...

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Why Lighter Gases Escape Earth First

Earth’s escape velocity is the speed an object needs to leave gravity behind. Near the top of the atmosphere, about 100 km above Earth, that speed is 11.1 km/s. Gas molecules can escape only if their speeds become high enough to reach that limit. The chance of escape depends on gas temperature and molecular speed. According to kinetic theory, the average kinetic energy of gas molecules is proportional to temperature. As temperature rises, the rms speed, or root-mean-square speed, also rises.

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

physics

JoVE Core - Fisica apresenta a teoria e a aplicação de tópicos fundamentais da física, incluindo cinemática, dinâmica, mecânica dos fluidos, estática, ondas e acústica, com vídeos animados concisos e de fácil compreensão. Além disso, os vídeos de cientistas em ação demonstram a aplicação de conceitos relacionados em experimentos de pesquisa clássica e original realizados nos laboratórios de todo o mundo hoje em dia.

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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Precipitation Reactions

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Oxidation-Reduction Reactions

Oxidation&ndash;Reduction Reactions

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Oxidation Numbers

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Acids, Bases and Neutralization Reactions

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Use the reactants in a chemical reaction to predict the products and identify reaction classes (synthesis, decomposition, combustion, single replacement, double replacement). EXPLANATION: Chemical reactions are rearrangements of atoms that follow predicable patterns. There are patterns both at the macroscopic level in the behavior of some of the reaction classes, as well as patterns in the rearrangements of the atoms underlying the reaction. Students should be able to predict the products of the reactions, which also requires the ability to recognize the general patterns for each type of reaction.

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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