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

W przypadku gazu doskonałego ciśnienie razy objętość jest stała, jeśli temperatura i liczba moli są stałe. Wykres ciśnienia w funkcji objętości daje w wyniku hiperbolę zwaną izotermą pV. Dla różnych stałych temperatur wykreślany jest zestaw wykresów pV.
Gazy niedoskonałe zachowują się jak gaz doskonały w wyższych temperaturach. Jeśli temperatura gazu niedoskonałego spadnie do temperatury krytycznej, krzywa pV pokazuje obszar o zerowym nachyleniu.
Załóżmy, że temperatura jest jeszcze bardziej obniżona, zwiększa się obszar o zerowym nachyleniu. Obszary te reprezentują zmianę objętości bez żadnej zmiany ciśnienia.
W przypadku izotermy T2 region A-B na izotermie reprezentuje fazę upłynniania. Gaz znajduje się w stanie parowym do punktu A i w fazie ciekłej poza punktem B.
W przypadku izoterm o temperaturach niższych niż T2 upłynnianie rozpoczyna się przy niższych ciśnieniach i większych objętościach.
W przypadku izoterm powyżej temperatur krytycznych nie zachodzi upłynnienie. Temperatura krytyczna jest właściwością badanego gazu dla danego ciśnienia krytycznego.

<ol>
	<li>Young, H.D i Freedman, R.A. (2012). Fizyka uniwersytecka z fizyką współczesną. San Francisco, Kalifornia: Pearson; sekcja 18.1; Strona 596.</li>
	<li>Protokół OpenStax. (2019). Fizyka uniwersytecka Vol. 2. [Wersja internetowa]. Źródło: <a href="https://openstax.org/details/books/university-physics-volume-2">https://openstax.org/details/books/university-physics-volume-2</a>; sekcja 2.1; strona 75 -77.</li>
</ol>

wykresy pv

Wykres pV , będący wykresem ciśnienia w funkcji objętości badanego gazu, jest pomocny w opisywaniu niektórych aspektów substancji. Kiedy substancja zachowuje się jak gaz doskonały, równanie gazu doskonałego opisuje zależność między jej ciśnieniem i objętością. Na wykresie pV często wykreśla się izotermę, czyli krzywą przedstawiającą p jako funkcję V przy liczbie cząsteczek i ustalonej temperaturze. Wtedy dla gazu doskonałego iloczyn ciśnienia gazu i jego objętości jest stały. Na przykład objętość gazu zmniejsza się wraz ze wzrostem ciśnienia. Powstały wykres pV jest hiperbolą.

Izotermy

pV są hiperboliczne w wysokich temperaturach i niskich ciśnieniach. W miarę ochładzania gazu wykres pV odbiega od hiperboli. W temperaturze krytycznej (Tc) krzywa pV pokazuje punkt o zerowym nachyleniu. W miarę dalszego schładzania gazu krzywa pV zaczyna przedstawiać obszar o zerowym nachyleniu. W tym obszarze zmienia się objętość gazu, ale nie zmienia to jego ciśnienia. To zachowanie odpowiada wrzeniu i kondensacji; gdy substancja ma temperaturę wrzenia pod określonym ciśnieniem, może zwiększyć swoją objętość, gdy część cieczy zamieni się w gaz (przechodząc z mniejszej objętości do większej objętości) lub zmniejszyć, gdy część gazu zamieni się w ciecz (przechodząc z większej objętości do mniejszej objętości), bez jakiejkolwiek zmiany temperatury lub ciśnienia. Jest to region, w którym zachodzi odparowanie lub upłynnienie.

<img alt="Figure1" src="/files/ftp_upload/13669/13669_PT_Figure.svg" style="text-align: center; height: 250px;" />

Strome części krzywych na lewo od obszaru przejściowego pokazują fazę ciekłą, która jest prawie nieściśliwa – niewielki spadek objętości wymaga znacznego zwiększenia ciśnienia. Płaskie części pokazują przejście ciecz-gaz, reprezentujące kombinacje ciśnienia i objętości, w których ciecz i gaz mogą współistnieć. Prawa strona obszaru przejściowego pokazuje fazę gazową.

Izotermy powyżej Tc nie przechodzą przez przejście ciecz-gaz. Dlatego ciecz nie może istnieć powyżej tej temperatury, temperatury krytycznej. Przy dostatecznie niskim ciśnieniu powyżej tej temperatury gaz ma gęstość cieczy, ale nie ulega kondensacji; mówi się, że gaz jest nadkrytyczny. Przy wyższym ciśnieniu jest stały. Na przykład dwutlenek węgla nie ma fazy ciekłej w temperaturze powyżej 31 stopni Celsjusza. Ciśnienie krytyczne to maksymalne ciśnienie, pod jakim może istnieć ciecz. Punkt na wykresie pV przy ciśnieniu krytycznym i temperaturze jest punktem krytycznym.

Wykresy pV

Wykres pV, który jest wykresem ciśnienia w funkcji objętości badanego gazu, jest pomocny w opisie niektórych aspektów substancji. Kiedy ...

Education

JoVE Core

Physics

Fizyka

Unit 1

Jednostka 1

The Kinetic Theory of Gases

Kinetyczna teoria gazów

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

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

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van der waals equation

How Real Gases Deviate from Ideal Behavior

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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...

kinetic theory of an ideal gas

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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mean free path and mean free time

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...

heat capacity: problem-solving

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,...

dalton's law of partial pressure

Gas Mixtures and Their Partial Pressures

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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 Physics introduces the theory and application of fundamental physics topics, including kinematics, dynamics, fluid mechanics, statics, waves, and acoustics, with concise and easy-to-understand animated videos. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

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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Understand and be prepared to use values specific to chemical processes: the mole, molar mass, molarity, and percent composition. EXPLANATION: During the early and mid-1800s, a tremendous amount of early research poured into building an understanding of the way that atoms combine. This work required analysis of the percent composition of different substances. Joseph Proust's work with tin oxides led him to publish his law of definite proportions noting that a given pure substance will have a constant ratio for the combination of elements making up the substance. Building on Proust's work, John Dalton published his law of multiple proportions and atomic theory noting that multiple patterns exist for the way that atoms combine, yet the combining atoms are invariable. Meanwhile Amedeo Avogadro hypothesizes that given equal pressures and temperatures, two equal volumes of gas will contain equal numbers of atoms. Acknowledging that the exact number of atoms was unknown in these gaseous volumes, the Latin word "mole" was used. "Mole" translates to "lump" recognizing the ambiguous quantity of gas. A number of attempts have been undertaken historically to quantify the mole. Robert Millikan's oil drop experiment revealed the change of a single electron, which could then be divided into Faraday's Constant for the charge of a mole of electrons. Dividing the charge of a mole of electrons by the charge of a single electron yields the number of electrons in a mole, $6.022\times10^{\wedge}23$. Students should be familiar with how to translate between the given quantities. Student's first use atomism/Dalton's atomic theory in third grade noting that matter is made up of particles too small to be seen. This the first time where students will undertake the process of counting these invisible particles. Electrolysis of water provides a tangible experience to demonstrate the constituent parts of water, though the definite proportions of water may not be easily seen due to the solubility of the gasses produced. The concept of relative mass can be explored by comparing small groups of like items. For example, relative masses of beans can be compared with pinto, white, and black beans. If bags are created with equal numbers of like beans, students can develop a concept of why the smallest bean will have a relative mass of 1, analogous to hydrogen.

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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Analyze solutions to identify solutes and solvents, quantitatively analyze concentrations (molarity, percent composition, and ppm), and perform separation methods such as evaporation, distillation, and/or chromatography and show conceptual understanding of distillation. Construct argument to justify the use of certain separation methods under different conditions. EXPLANATION: In seventh grade, students developed an understanding that chemical and physical properties of a sample of matter could be used to identify a substance. Building on this understanding, students now explore the use of these physical and chemical properties to separate mixtures. Analysis of the substances is performed to aid in further discussion and facilitate further quantitative analysis.

CHEM1.PS1.7

Liquid-Gas Changes on pV Curves

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