Name: vapor pressure and boiling behavior
Uploaded: 2025-02-13T15:43:08.000Z
Duration: 1 min 28 s
Channel: JoVE

La pression de vapeur, qui influence l’ébullition et la cavitation, est la pression exercée par une vapeur en équilibre avec son liquide à une température donnée et indique la tendance des molécules à s’échapper de la surface du liquide.
Dans un récipient fermé et évacué, les molécules liquides s’échappent dans l’espace, augmentant la pression jusqu’à ce que l’équilibre soit atteint, où le taux de molécules qui s’échappent est égal au taux de retour. Cette pression d’équilibre est la pression de vapeur.
La pression de vapeur dépend de la température. À mesure que la température augmente, les molécules gagnent de l’énergie cinétique, surmontant les forces intermoléculaires et s’échappant plus facilement dans la phase vapeur, ce qui augmente la pression de vapeur.
Par exemple, l’eau à 100 °C a une pression de vapeur de 101,3 kPa, ce qui correspond à la pression atmosphérique au niveau de la mer, ce qui la fait bouillir. À des altitudes plus élevées, avec une pression atmosphérique plus basse, l’eau bout à des températures plus basses car sa pression de vapeur correspond plus rapidement à la pression atmosphérique réduite.
La surveillance de la pression de vapeur est cruciale dans la conception et le fonctionnement des pompes et des turbines afin d’éviter la cavitation.
La cavitation se produit lorsque la pression locale du fluide descend en dessous de la pression de vapeur, formant des bulles de vapeur qui s’effondrent violemment, endommageant l’équipement.

pression de vapeur d'un fluide

La pression de vapeur d'un fluide est un concept crucial en mécanique des fluides, influençant des phénomènes tels que l'ébullition et la cavitation. La pression de vapeur désigne la pression exercée par une vapeur en état d'équilibre thermodynamique avec sa phase liquide correspondante à une température spécifique. Elle représente la tendance des molécules à s'échapper de la surface du fluide vers la phase vapeur.

Lorsqu'un liquide est placé dans un récipient fermé avec un petit espace d'air et que l'espace est évacué, les molécules de vapeur s'échappent du liquide vers l'espace. Au fur et à mesure que davantage de molécules s'échappent, la pression dans l'espace augmente jusqu'à atteindre un équilibre, où le taux de molécules s'échappant du liquide est égal au taux de molécules y retournant. Cette pression d'équilibre est la pression de vapeur du liquide.

La pression de vapeur dépend de la température. À mesure que la température augmente, l'énergie cinétique des molécules augmente, ce qui fait que davantage de molécules surmontent les forces intermoléculaires et s'échappent vers la phase vapeur. Il s'ensuit que la pression de vapeur augmente avec la température. Par exemple, l'eau à 100 °C a une pression de vapeur de 101,3 kPa (1 atm), ce qui lui permet d'ébullition à la pression atmosphérique normale. Cependant, à des altitudes plus élevées, où la pression atmosphérique est plus basse, l'eau bout à des températures plus basses car sa pression de vapeur est égale à la pression atmosphérique réduite à l'altitude la plus élevée.

Pression de vapeur d'un fluide

La pression de vapeur, qui influence l’ébullition et la cavitation, est la pression exercée par une vapeur en équilibre avec son liquide à une température donnée et ...

Education

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Civil Engineering

Génie civil

Unit 1

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Fluid Properties

Propriétés des fluides

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

Vapor Pressure of Fluid

vapor pressure and boiling behavior

Vapor pressure and boiling behavior are closely linked in fluids. Vapor pressure is the pressure a vapor exerts when it is in thermodynamic equilibrium with its liquid at a given temperature. It shows how strongly molecules tend to leave the liquid surface and move into the vapor phase.

A closed container helps show how vapor pressure forms. If a liquid sits in a sealed space with a small air gap, and that space is evacuated, vapor molecules escape from the liquid into the empty space. The pressure rises as more molecules collect there. Soon, the rate of molecules leaving the liquid equals the rate returning to it. That balance creates the vapor pressure of the liquid.

Temperature changes vapor pressure. When temperature increases, the molecules gain kinetic energy. More molecules can overcome intermolecular forces and escape into the vapor phase. For that reason, vapor pressure increases as temperature increases.

Water gives a clear example. At 100°C, water has a vapor pressure of 101.3 kPa, or 1 atm. That is why it can boil at standard atmospheric pressure. At higher elevations, atmospheric pressure is lower. Water then boils at a lower temperature because its vapor pressure reaches the reduced outside pressure sooner.

Vapor Pressure and Boiling Behavior

characteristics of fluids

Fluid Behavior in Liquids and Gases

Fluids include liquids and gases, and they behave differently from solids because of how their molecules are arranged. In solids, molecules are tightly packed and held by strong intermolecular forces. That structure helps solids keep their shape and resist deformation. Fluids have molecules that are farther apart and held by weaker forces. Because of this, they can flow and change shape easily. A fluid is any substance that deforms continuously when shearing stress is applied. Shearing stress...

density, specific weight, specific gravity and compressibility of fluid

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Fluid Properties: Density, Weight and Compression

Density, specific weight, specific gravity, and compressibility are key fluid properties. Density is the mass per unit volume of a fluid. It helps describe how much mass a fluid system contains and affects buoyancy, pressure, flow behavior, viscosity, thermal conductivity, and sound propagation. In pipeline design, knowing density helps engineers make sure the pipe can support the fluid’s mass. Specific weight is the weight per unit volume. It is found by multiplying density by gravitational...

viscosity of fluid

How Temperature Changes Fluid Viscosity

Viscosity describes how much a fluid resists flow and deformation. It comes from internal friction between fluid layers that move past each other. Dynamic viscosity, shown by the Greek letter mu (μ), measures the force needed to move one layer over another. Newtonian fluids, such as water and air, have a linear relationship between shearing stress and the rate of shearing strain. Their viscosity stays constant, even when the applied stress changes. Non-Newtonian fluids, like ketchup,...

types of fluids

Newtonian vs Non-Newtonian Fluid Behavior

Fluids can be grouped by how they respond to shear stress. Shear stress is a force that acts across a material, and shear strain rate describes how quickly it changes shape or flows. In this lesson, fluids are divided into Newtonian and non-Newtonian types. Newtonian fluids follow Newton’s law of viscosity. Their shear stress changes in a linear way as the shear rate changes. That means their viscosity stays constant, so their flow is predictable and easier to analyze. Water, air, oil, and...

newtonian fluid: problem solving

Newtonian Fluid Viscosity and Shear Stress

Newtonian fluids have a constant viscosity, so their shear stress and shear strain rate stay directly proportional. This creates a linear, predictable link between force and flow. Water, air, and light oils are common examples of Newtonian fluids. A simple way to study this behavior is to place the fluid between two parallel plates. One plate stays still, and the other moves at a constant velocity. The fluid forms a velocity gradient, which is the change in speed across the distance between...

vapor pressure of fluid

Vapor pressure and boiling behavior are closely linked in fluids. Vapor pressure is the pressure a vapor exerts when it is in thermodynamic equilibrium with its liquid at a given temperature. It shows how strongly molecules tend to leave the liquid surface and move into the vapor phase. A closed container helps show how vapor pressure forms. If a liquid sits in a sealed space with a small air gap, and that space is evacuated, vapor molecules escape from the liquid into the empty space. The...

surface tension of fluid

Why Water Drops and Needles Float

Surface tension helps explain why water forms drops, supports small objects, and moves in thin tubes. It is a property of fluids at the boundary between a liquid and a gas, or between two immiscible liquids. At the surface, molecules are pulled inward by neighboring molecules. That inward pull makes the surface act a little like a stretched elastic membrane. Inside a fluid, molecules are attracted in all directions. Surface molecules do not have the same balance of forces. They feel a net...

capillarity in fluid

Capillary Action in Narrow Spaces

Capillary action is the movement of liquid through narrow spaces without any outside force. It depends on surface tension and adhesion between the liquid and nearby solid surfaces. This effect is common in narrow tubes, porous materials, and fine particles. Surface tension comes from cohesive forces between liquid molecules at the liquid-air boundary. These forces create a thin skin that resists outside pressure. When a capillary tube is placed in a liquid, the liquid level changes because of...

design example: deciding thickness of lubricating fluid in a shaft

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Lubricant Film Thickness for Shaft Bearings

Lubricant film thickness for shaft bearings is calculated to keep a rotating shaft from touching its bearing housing. A controlled layer of lubricant reduces friction, wear, and energy loss. It also helps the machine run smoothly by preventing metal-to-metal contact. The calculation starts with the shaft’s tangential velocity. First, the rotational speed is converted to angular velocity. Then that value is multiplied by the shaft radius. The result is the shaft’s linear speed at its surface...

key terms and concepts

Learn how vapor pressure forms, why it rises with temperature, and how it explains boiling in water at sea level and higher elevations.

Vapor Pressure and Boiling Behavior | JoVE K12

JoVE Core Civil Engineering provides a foundational exploration of essential construction materials in civil engineering, focusing on bricks, stones, steel, wood, and concrete through easy-to-understand animated videos. This collection covers the properties, manufacturing processes, and practical applications of these materials. Additionally, the accompanied science-in-action videos demonstrate the research experiments performed in laboratories.

civil engineering

Vapor Pressure and Boiling Behavior

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