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The figure has two plots of Pressure, p, on the vertical axis as a function of volume, V, on the horizontal axis, at several different temperatures. Figure a shows six isotherms labeled, from the bottom to top, T 1, T 2, T C, T 3, T 4 and T 5. A note on the graph tells us that these temperatures are also in increasing order. The graphs show that pressure generally decreases with increasing volume for all temperatures, except at low temperatures when pressure is constant as a function of volume during a phase change. The phase change occupies a region in the plot shaded in blue and labeled Liquid-vapor equilibrium region. Figure b is the same plot, zoomed in to show the p V diagram in and around the shaded liquid vapor region. Above the shaded region, the curves decrease monotonically. The curve that is still outside but just touches the peak of the liquid vapor region is labeled as the critical isotherm, T c. The point at which this curve meets the shaded region is labeled the critical point. The region to the left of the shaded region and at pressures lower than the pressure of the critical point is the liquid region. The region to the right of the shaded region is the vapor region. The right edge of the shaded region is the saturation curve. The region above the critical isotherm is labeled as true but not ideal gas.
pV diagrams. (a) Each curve (isotherm) represents the relationship between p and V at a fixed temperature; the upper curves are at higher temperatures. The lower curves are not hyperbolas because the gas is no longer an ideal gas. (b) An expanded portion of the pV diagram for low temperatures, where the phase can change from a gas to a liquid. The term “vapor” refers to the gas phase when it exists at a temperature below the boiling temperature.

The isotherms above T c do not go through the liquid-gas transition. Therefore, liquid cannot exist above that temperature, which is the critical temperature (described in the chapter on temperature and heat). At sufficiently low pressure above that temperature, the gas has the density of a liquid but will not condense; the gas is said to be supercritical    . At higher pressure, it is solid. Carbon dioxide, for example, has no liquid phase at a temperature above 31.0 ºC . The critical pressure is the maximum pressure at which the liquid can exist. The point on the pV diagram at the critical pressure and temperature is the critical point (which you learned about in the chapter on temperature and heat). [link] lists representative critical temperatures and pressures.

Critical temperatures and pressures for various substances
Substance Critical temperature Critical pressure
K °C Pa atm
Water 647.4 374.3 22.12 × 10 6 219.0
Sulfur dioxide 430.7 157.6 7.88 × 10 6 78.0
Ammonia 405.5 132.4 11.28 × 10 6 111.7
Carbon dioxide 304.2 31.1 7.39 × 10 6 73.2
Oxygen 154.8 –118.4 5.08 × 10 6 50.3
Nitrogen 126.2 –146.9 3.39 × 10 6 33.6
Hydrogen 33.3 –239.9 1.30 × 10 6 12.9
Helium 5.3 –267.9 0.229 × 10 6 2.27


  • The ideal gas law relates the pressure and volume of a gas to the number of gas molecules and the temperature of the gas.
  • A mole of any substance has a number of molecules equal to the number of atoms in a 12-g sample of carbon-12. The number of molecules in a mole is called Avogadro’s number N A ,
    N A = 6.02 × 10 23 mol −1 .
  • A mole of any substance has a mass in grams numerically equal to its molecular mass in unified mass units, which can be determined from the periodic table of elements. The ideal gas law can also be written and solved in terms of the number of moles of gas:
    p V = n R T ,

    where n is the number of moles and R is the universal gas constant,
    R = 8.31 J/mol · K .
  • The ideal gas law is generally valid at temperatures well above the boiling temperature.
  • The van der Waals equation of state for gases is valid closer to the boiling point than the ideal gas law.
  • Above the critical temperature and pressure for a given substance, the liquid phase does not exist, and the sample is “supercritical.”

Conceptual questions

Two H 2 molecules can react with one O 2 molecule to produce two H 2 O molecules. How many moles of hydrogen molecules are needed to react with one mole of oxygen molecules?

2 moles, as that will contain twice as many molecules as the 1 mole of oxygen

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Under what circumstances would you expect a gas to behave significantly differently than predicted by the ideal gas law?

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A constant-volume gas thermometer contains a fixed amount of gas. What property of the gas is measured to indicate its temperature?


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Questions & Answers

What is differential form of Gauss's law?
Rohit Reply
help me out on this question the permittivity of diamond is 1.46*10^-10.( a)what is the dielectric of diamond (b) what its susceptibility
a body is projected vertically upward of 30kmp/h how long will it take to reach a point 0.5km bellow e point of projection
Abu Reply
i have to say. who cares. lol. why know that t all
is this just a chat app about the openstax book?
Lord Reply
kya ye b.sc ka hai agar haa to konsa part
MPL Reply
what is charge quantization
Mayowa Reply
it means that the total charge of a body will always be the integral multiples of basic unit charge ( e ) q = ne n : no of electrons or protons e : basic unit charge 1e = 1.602×10^-19
is the time quantized ? how ?
What do you meanby the statement,"Is the time quantized"
Can you give an explanation.
there are some comment on the time -quantized..
time is integer of the planck time, discrete..
planck time is travel in planck lenght of light..
it's says that charges does not occur in continuous form rather they are integral multiple of the elementary charge of an electron.
it is just like bohr's theory. Which was angular momentum of electron is intral multiple of h/2π
determine absolute zero
The properties of a system during a reversible constant pressure non-flow process at P= 1.6bar, changes from constant volume of 0.3m³/kg at 20°C to a volume of 0.55m³/kg at 260°C. its constant pressure process is 3.205KJ/kg°C Determine: 1. Heat added, Work done, Change in Internal Energy and Change in Enthalpy
Opeyemi Reply
U can easily calculate work done by 2.303log(v2/v1)
Amount of heat added through q=ncv^delta t
Change in internal energy through q=Q-w
please how do dey get 5/9 in the conversion of Celsius and Fahrenheit
Gwam Reply
what is copper loss
timileyin Reply
this is the energy dissipated(usually in the form of heat energy) in conductors such as wires and coils due to the flow of current against the resistance of the material used in winding the coil.
it is the work done in moving a charge to a point from infinity against electric field
Ashok Reply
what is the weight of the earth in space
peterpaul Reply
As w=mg where m is mass and g is gravitational force... Now if we consider the earth is in gravitational pull of sun we have to use the value of "g" of sun, so we can find the weight of eaeth in sun with reference to sun...
g is not gravitacional forcé, is acceleration of gravity of earth and is assumed constante. the "sun g" can not be constant and you should use Newton gravity forcé. by the way its not the "weight" the physical quantity that matters, is the mass
Yeah got it... Earth and moon have specific value of g... But in case of sun ☀ it is just a huge sphere of gas...
Thats why it can't have a constant value of g ....
not true. you must know Newton gravity Law . even a cloud of gas it has mass thats al matters. and the distsnce from the center of mass of the cloud and the center of the mass of the earth
please why is the first law of thermodynamics greater than the second
Ifeoma Reply
every law is important, but first law is conservation of energy, this state is the basic in physics, in this case first law is more important than other laws..
First Law describes o energy is changed from one form to another but not destroyed, but that second Law talk about entropy of a system increasing gradually
first law describes not destroyer energy to changed the form, but second law describes the fluid drection that is entropy. in this case first law is more basic accorging to me...
define electric image.obtain expression for electric intensity at any point on earthed conducting infinite plane due to a point charge Q placed at a distance D from it.
Mateshwar Reply
explain the lack of symmetry in the field of the parallel capacitor
Phoebe Reply
pls. explain the lack of symmetry in the field of the parallel capacitor

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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