Name: pressure units and tools for gas measurement
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 2 min 57 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.3

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Definition and Measurement of Pressure: Atmospheric Pressure, Barometer, and Manometer

Recall that a gas always assumes the shape and volume of the vessel in which it is contained. Microscopically, a gas consists of moving particles that collide with each other and the walls of their container.

Each collision with the wall exerts a small force on the area of contact. The forces resulting from a large number of such collisions add up to its pressure.

Fewer gas particles lead to fewer collisions, a lower force per unit area, and hence, a lower pressure. Pressure, therefore, is directly proportional to the number of particles in a given volume, or the density of the gas.

Air &#8212; a mixture of different gaseous atoms and molecules &#8212; exerts pressure on everything on the surface of the earth. This is atmospheric pressure, and it is measured using a barometer. A traditional barometer is a mercury-filled glass tube that is inverted into a dish containing mercury.

In response to pressure exerted by the atmosphere, the height of the mercury column rises or falls. Measuring the height provides a measure of the atmospheric pressure.

At sea level, the atmospheric pressure is 1 atm, which raises the column of mercury to a height of 760 millimeters of mercury. At higher altitudes, where the air is less dense, the height of the mercury column falls, indicating a lower atmospheric pressure.

The pressure of a gas trapped in a container is measured using a manometer. A closed-end manometer consists of a mercury-filled U tube. One end is sealed in a vacuum, and the other end is connected to the container that is filled with the gas sample.

The gas particles push down on the mercury in one end of the tube, creating a height difference that corresponds to the pressure exerted by the gas.

In an open-end manometer, one end is left open to the atmosphere. This measures the pressure of the gas sample relative to atmospheric pressure.

When the gas exerts a pressure higher than the atmospheric pressure, the height difference is added to the atmospheric pressure. This yields the pressure of the gas.

Conversely, when the atmosphere exerts a pressure greater than the gas sample, the height difference is subtracted from the atmospheric pressure to get the pressure of the gas.

definition and measurement of pressure: atmospheric pressure, barometer, and manometer

Gas pressure is caused by force exerted by gas molecules colliding with the surfaces of objects. Although the force of each collision is very small, any surface of an appreciable area experiences a large number of collisions in a short time, which can result in high pressure.

In general, pressure is defined as the force exerted on a given area: 

<img alt="Pressure formula \(P = \frac{F}{A}\) equation; key concept in physics, static equilibrium." src="/files/core/CDNSource/lessons/1035/1035_Equations-01.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

Pressure is directly proportional to force and inversely proportional to area. Thus, pressure can be increased either by increasing the amount of force or by decreasing the area over which it is applied; pressure can be decreased by decreasing the force or increasing the area.

The SI unit of pressure is the pascal (Pa), with 1 Pa = 1 N/m2, where N is the newton, a unit of force defined as 1 kg&#183;m/s2. One pascal is a small pressure; in many cases, it is more convenient to use units of kilopascal (1 kPa = 1000 Pa) or bar (1 bar = 100,000 Pa). Pressure can also be measured using the unit atmosphere (atm). 

<h4>Measuring Pressure</h4>

Atmospheric pressure, the force exerted by the atmosphere on the earth&#8217;s surface, is measured with a barometer. A barometer is a glass tube that is closed at one end, filled with a nonvolatile liquid, such as mercury, and then inverted and immersed in a container of that liquid. The atmosphere exerts pressure on the liquid outside the tube, the column of liquid exerts pressure inside the tube, and the pressure at the liquid surface is the same inside and outside the tube. The height of the liquid in the tube is, therefore, proportional to the pressure exerted by the atmosphere.

In the barometer, mercury is the preferred choice over water, since it is 13.5 times denser than water. The atmospheric pressure can support a column of mercury that is only about 0.760 m tall, whereas a column of water would need to be 10.3 m tall. This makes a column of mercury a convenient way to measure pressure.

Standard atmospheric pressure of 1 atm at sea level (101,325 Pa) corresponds to a column of mercury that is about 760 mm (29.92 in.) high. The pressure exerted by a fluid due to gravity is known as hydrostatic pressure, p:

<img alt="Pressure formula \(P = \rho gh\), static fluid concept equation, educational diagram." src="/files/core/CDNSource/lessons/1035/1035_Equations-02.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq2" />

where h is the height of the fluid, &#961; is the density of the fluid, and g is the acceleration due to gravity.

A manometer is a device used to measure the pressure of a gas trapped in a container. A closed-end manometer is a U-shaped tube with one closed arm, one arm that connects to the gas to be measured, and mercury in between. The distance between the liquid levels in the two arms of the tube, h, is proportional to the pressure of the gas in the container. In an open-end manometer, one arm of the tube is open to the atmosphere. In this case, the distance between the liquid levels corresponds to the difference in pressure between the gas in the container and the atmosphere. 

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/9-1-gas-pressure">Openstax, Chemistry 2e, Section 9.1: Gas Pressure</a>.

Gas pressure is caused by force exerted by gas molecules colliding with the surfaces of objects. Although the force of each collision is very small, any ...

Education

JoVE Core

Gases

KEY TERMS AND CONCEPTS

pressure units and tools for gas measurement

Pressure is the force applied to a given area, and gas pressure comes from gas molecules colliding with surfaces. Each collision is tiny, but many collisions over a small area can create high pressure. Pressure increases when force increases or when the area gets smaller. It decreases when force drops or when the area gets larger.

Pressure is measured in pascals, or Pa, in the SI system. One pascal equals 1 newton per square meter, and a newton is a unit of force equal to 1 kg·m/s2. A pascal is a small unit, so kilopascal (kPa) and bar are often more useful. Pressure can also be reported in atmospheres, or atm.

Atmospheric pressure is the force the atmosphere exerts on Earth’s surface. It is measured with a barometer. A barometer uses a glass tube that is closed at one end, filled with a nonvolatile liquid such as mercury, and then turned upside down in a container of the same liquid. The atmosphere presses on the liquid outside the tube, and the liquid column inside the tube rises to a height that matches that pressure.

Mercury is preferred in a barometer because it is 13.5 times denser than water. That means atmospheric pressure supports only about 0.760 m of mercury, while a water column would need to be 10.3 m tall. Standard atmospheric pressure at sea level is 1 atm, or 101,325 Pa, and it corresponds to a mercury column about 760 mm, or 29.92 in., high.

Fluid pressure from gravity is called hydrostatic pressure. It depends on the height of the fluid, its density, and the acceleration due to gravity. A manometer uses this idea to measure gas pressure in a container. A closed-end manometer has one sealed arm, one arm connected to the gas, and mercury between them. The height difference between the liquid levels gives the gas pressure.

An open-end manometer has one arm open to the atmosphere. In that setup, the height difference shows the pressure difference between the trapped gas and the air around it. Together, barometers and manometers show how fluid height can be used to measure atmospheric pressure and gas pressure.

Pressure Units and Tools for Gas Measurement

pressure-units-and-tools-for-gas-measurement

Pressure is the force applied to a given area, and gas pressure comes from gas molecules colliding with surfaces. Each collision is tiny, but many collisions over a small area can create high pressure. Pressure increases when force increases or when the area gets smaller. It decreases when force drops or when the area gets larger. Pressure is measured in pascals, or Pa, in the SI system. One pascal equals 1 newton per square meter, and a newton is a unit of force equal to 1 kg·m/s2. A pascal...

gas laws: boyle's, gay-lussac, charles', avogadro's, and ideal gas law

gas-laws-boyle-s-gay-lussac-charles-avogadro-s-and-ideal-gas-law

How Gas Pressure, Volume, and Temperature Change

Gas pressure, volume, temperature, and amount are linked by several gas laws. Scientists found these relationships by studying how a confined gas changes when one variable is held constant and another changes. These patterns apply to an ideal gas, which is a useful model for gases under low pressure and high temperature. Gay-Lussac’s law, also called Amontons’s law, connects pressure and temperature. If gas is trapped in a rigid container, the volume and number of moles stay constant. When the...

applications of the ideal gas law: molar mass, density, and volume

applications-of-the-ideal-gas-law-molar-mass-density-and-volume

Ideal Gas Law at STP and Gas Density

The ideal gas law links pressure, volume, number of moles, and temperature. It helps explain how gas volume changes when pressure or temperature changes. At standard temperature and pressure, or STP, the values are 273.15 K and 1 atm. At STP, one mole of an ideal gas occupies about 22.4 L. This is called the standard molar volume. The gas can be hydrogen, oxygen, argon, or carbon dioxide, and the volume is still about 22.4 liters. Half a mole occupies 11.2 L, and 2 moles occupy 44.8 L. The...

mixtures of gases: dalton's law of partial pressures and mole fractions

mixtures-of-gases-dalton-s-law-of-partial-pressures-and-mole-fractions

Gas Pressure in Mixtures: Partial Pressures

Gas pressure in mixtures is described by partial pressure and mole fraction. When gases do not chemically react, each gas in the mixture acts as if it were alone in the container. The pressure from one gas is called its partial pressure. For a mixture of three gases, A, B, and C, the total pressure equals the sum of their partial pressures: PA, PB, and PC. This is Dalton's law of partial pressures. It states that the total pressure of a mixture of ideal gases is the sum of the pressures from...

chemical stoichiometry and gases: using ideal gas law to determine moles

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Stoichiometry of Gases with the Ideal Gas Law

Chemical stoichiometry links the amounts of reactants and products in a chemical reaction. For gases, those amounts can also be measured by volume. If a gas’s volume, pressure, and temperature are known, the ideal gas equation can be used to find the number of moles present. The same idea works in reverse. If the moles of a gas are known, its volume can be found at any temperature and pressure. The transcript gives an example using 8.88 g of gallium reacting with excess hydrochloric acid.

basic postulates of kinetic molecular theory: particle size, energy, and collision

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Kinetic Molecular Theory and Gas Behavior

Kinetic molecular theory explains gas behavior at the particle level. It gives a microscopic model for what happens when pressure or temperature changes. Gas laws describe how macroscopic properties change, but they do not explain why those changes happen. The theory was put into a complete form by Rudolf Clausius in 1857. It was built from hundreds of experimental observations about gases. Those observations helped explain the different gas laws through a few basic postulates. One key idea...

kinetic molecular theory and gas laws explain properties of gas molecules

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Gas Pressure Patterns in Kinetic Molecular Theory

Kinetic molecular theory explains gas pressure by linking it to the motion of gas molecules. Gas pressure comes from rapidly moving molecules hitting the walls of a container. It depends on how many molecules strike a unit area in a given time. The theory also helps explain the main gas laws. In Gay-Lussac’s law, also called Amontons’s law, raising temperature increases the average speed and kinetic energy of gas molecules. If volume stays constant, the molecules collide with the walls more...

kinetic molecular theory: molecular velocities, temperature, and kinetic energy

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Gas Particle Speed Distributions and Temperature

Gas particle speed distributions and temperature are key ideas in the kinetic molecular theory. This theory explains the gas laws by describing how gas molecules move, collide, and share energy. Gas molecules do not all move at the same speed. A gas sample has an average speed and an average kinetic energy, but individual molecules move at different speeds. Their collisions are elastic, which means momentum is conserved. Because so many molecules collide, the overall speed distribution stays...

behavior of gas molecules: molecular diffusion, mean free path, and effusion

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Gas Particle Motion, Diffusion, and Effusion

Gas particle motion, diffusion, and effusion explain how gases spread and escape in everyday settings. Gas molecules move at very high speeds, but they collide with other gas molecules and change direction many times before reaching a target. At room temperature, a gas molecule can experience billions of collisions each second. The mean free path is the average distance a molecule travels between collisions. This distance becomes larger when pressure decreases. In general, the mean free path...

real gases: effects of intermolecular forces and molecular volume deriving van der waals equation

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Real Gases: When the Ideal Gas Law Breaks Down

Real gases are gases that do not always follow the ideal gas law, PV = nRT, especially when pressure is high or temperature is low. The ideal gas model assumes gas particles have no attractive or repulsive forces and take up negligible space compared with the container. At room temperature and 1 atm or less, many gases behave close to ideally. When conditions change, the measured relationships between pressure, volume, and temperature can drift away from the ideal prediction. One way to see...

key terms and concepts

Learn how pressure is measured with pascals, atmospheres, barometers, and manometers, plus why mercury columns show atmospheric and gas pressure.

Pressure Units and Tools for Gas Measurement | JoVE K12

chemistry

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Gas laws

Virginia  Standards of Learning

HIgh School

CH.6

CH.6.b

Earth Science

ES.11

ES.11.a

Arizona Science Standards

High School

Physical Science

HS+C.P1

HS+C.P1U1.3

Texas Essential Knowledge and Skills

CHEM.10

CHEM.10.B

Alabama Course of Study Science

Matter and Its Interactions

9-12.C.1.4.e

9-12.C.4.7.b

9-12.PS.3.5.c

Earth and Space Science

Earth's Systems

9-12.ESS.2.7.c

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New York State Science Learning Standards

HS. Structure and Properties of Matter

HS-PS1-9.

Classifying Matter by State

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Intermolecular Forces

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The internal energy of a substance&#8212;the total kinetic energy of all its molecules and the potential energy of their associated forces&#8212;depends ...

Phase Transitions

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Phase Transitions: Vaporization and Condensation

The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize ...

Vapor Pressure

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Clausius-Clapeyron Equation

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Phase Diagrams

A phase diagram combines plots of pressure versus temperature for the liquid-gas, solid-liquid, and solid-gas phase-transition equilibria of a substance. ...

Use models to explain how changes in energy affect the arrangement and movement of the particles in solids, liquids, and gases, as well as the relative strengths of their intermolecular forces.

PS.Chm.7.1

Energy Basics

Chemical reactions, such as those that occur when you light a match, involve changes in energy as well as matter.

Chemical changes and their accompanying ...

First Law of Thermodynamics

Energy Conservation

Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some form ...

Internal Energy

The total of all possible kinds of energy present in a substance is called the internal energy (U), sometimes symbolized as E. Suppose a system with ...

Quantifying Heat

Thermal Energy&#160;

Microscopically, thermal energy is the kinetic energy associated with the random motion of atoms and molecules. Temperature is a ...

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Enthalpy

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Constant Pressure Calorimetry

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Using Differential Scanning Calorimetry to Measure Changes in Enthalpy

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Hess's Law

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Standard Enthalpy of Formation

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Enthalpies of Reaction

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PS.Chm.7.2

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Basic Postulates of Kinetic Molecular Theory: Particle Size, Energy, and Collision

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Kinetic Molecular Theory and Gas Laws Explain Properties of Gas Molecules

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Kinetic Molecular Theory: Molecular Velocities, Temperature, and Kinetic Energy

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Real Gases: Effects of Intermolecular Forces and Molecular Volume Deriving Van der Waals Equation

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Ideal Gas Law

Source: Laboratory of Dr. Andreas Z&#252;ttel - Swiss Federal Laboratories for Materials Science and Technology
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	Determining the Mass of the Unknown LiquidExpand
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Use mathematics and computational thinking to explain the relationships among pressure, temperature, volume, and quantity of gas, both qualitatively and quantitatively.

Understand the relationship among pressure, temperature, volume, and phase.

Pressure Units and Tools for Gas Measurement

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