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  • Interpret a phase diagram.
  • State Dalton’s law.
  • Identify and describe the triple point of a gas from its phase diagram.
  • Describe the state of equilibrium between a liquid and a gas, a liquid and a solid, and a gas and a solid.

Up to now, we have considered the behavior of ideal gases. Real gases are like ideal gases at high temperatures. At lower temperatures, however, the interactions between the molecules and their volumes cannot be ignored. The molecules are very close (condensation occurs) and there is a dramatic decrease in volume, as seen in [link] . The substance changes from a gas to a liquid. When a liquid is cooled to even lower temperatures, it becomes a solid. The volume never reaches zero because of the finite volume of the molecules.

Line graph of volume versus temperature showing the relationship for an ideal gas and a real gas. The line for an ideal gas is linear starting at absolute zero showing a linear increase in volume with temperature. The line for a real gas is linear above a temperature of negative one hundred ninety degrees Celsius and follows that of the ideal gas. But below that temperature, the graph shows an almost vertical drop in volume with temperature as the temperature drops and the gas condenses.
A sketch of volume versus temperature for a real gas at constant pressure. The linear (straight line) part of the graph represents ideal gas behavior—volume and temperature are directly and positively related and the line extrapolates to zero volume at 273 . 15 º C size 12{ +- "273" "." "15"°C} {} , or absolute zero. When the gas becomes a liquid, however, the volume actually decreases precipitously at the liquefaction point. The volume decreases slightly once the substance is solid, but it never becomes zero.

High pressure may also cause a gas to change phase to a liquid. Carbon dioxide, for example, is a gas at room temperature and atmospheric pressure, but becomes a liquid under sufficiently high pressure. If the pressure is reduced, the temperature drops and the liquid carbon dioxide solidifies into a snow-like substance at the temperature 78 º C size 12{ +- "78"°C} {} . Solid CO 2 size 12{"CO" rSub { size 8{2} } } {} is called “dry ice.” Another example of a gas that can be in a liquid phase is liquid nitrogen ( LN 2 ) size 12{ \( "LN" rSub { size 8{2} } \) } {} . LN 2 size 12{"LN" rSub { size 8{2} } } {} is made by liquefaction of atmospheric air (through compression and cooling). It boils at 77 K ( 196 º C ) size 12{ \( –"196"°C \) } {} at atmospheric pressure. LN 2 size 12{"LN" rSub { size 8{2} } } {} is useful as a refrigerant and allows for the preservation of blood, sperm, and other biological materials. It is also used to reduce noise in electronic sensors and equipment, and to help cool down their current-carrying wires. In dermatology, LN 2 size 12{"LN" rSub { size 8{2} } } {} is used to freeze and painlessly remove warts and other growths from the skin.

PV Diagrams

We can examine aspects of the behavior of a substance by plotting a graph of pressure versus volume, called a PV diagram    . When the substance behaves like an ideal gas, the ideal gas law describes the relationship between its pressure and volume. That is,

PV = NkT ( ideal gas ) . size 12{ ital "PV"= ital "NkT"``` \( "ideal gas" \) "." } {}

Now, assuming the number of molecules and the temperature are fixed,

PV = constant ( ideal gas, constant temperature ) . size 12{ size 11{ ital "PV"="constant"``` \( "ideal gas, constant temperature" \) "." }} {}

For example, the volume of the gas will decrease as the pressure increases. If you plot the relationship PV = constant size 12{ size 11{ ital "PV"="constant"}} {} on a PV size 12{ ital "PV"} {} diagram, you find a hyperbola. [link] shows a graph of pressure versus volume. The hyperbolas represent ideal-gas behavior at various fixed temperatures, and are called isotherms . At lower temperatures, the curves begin to look less like hyperbolas—the gas is not behaving ideally and may even contain liquid. There is a critical point    —that is, a critical temperature    —above which liquid cannot exist. At sufficiently high pressure above the critical point, the gas will have the density of a liquid but will not condense. Carbon dioxide, for example, cannot be liquefied at a temperature above 31 . 0 º C size 12{"31" "." 0°C} {} . Critical pressure is the minimum pressure needed for liquid to exist at the critical temperature. [link] lists representative critical temperatures and pressures.

Questions & Answers

Greetings,users of that wonderful app.
Frank Reply
how to solve pressure?
Cruz Reply
how do we calculate weight and eara eg an elefant that weight 2000kg has four fits or legs search of surface eara is 0.1m2(1metre square) incontact with the ground=10m2(g =10m2)
can someone derive the formula a little bit deeper?
what is coplanar force?
what is accuracy and precision
Peace Reply
How does a current follow?
Vineeta Reply
which one dc or ac current.
how does a current following?
AC current
AC current follows due to changing electric field and magnetic field.
you guys are just saying follow is flow not follow please
ok bro thanks
but i wanted to understand him/her in his own language
but I think the statement is written in English not any other language
my mean that in which form he/she written this,will understand better in this form, i write.
ok thanks bro. my mistake
u are welcome
what is a semiconductor
Vineeta Reply
substances having lower forbidden gap between valence band and conduction band
what is a conductor?
replace lower by higher only
convert 56°c to kelvin
How does a current follow?
A semiconductor is any material whose conduction lies between that of a conductor and an insulator.
what is Atom? what is molecules? what is ions?
Abubakar Reply
What is a molecule
Samuel Reply
Is a unit of a compound that has two or more atoms either of the same or different atoms
A molecule is the smallest indivisible unit of a compound, Just like the atom is the smallest indivisible unit of an element.
what is a molecule?
what is a vector
smith Reply
A quantity that has both a magnitude AND a direction. E.g velocity, acceleration, force are all vector quantities. Hope this helps :)
what is the difference between velocity and relative velocity?
Velocity is the rate of change of displacement with time. Relative velocity on the other hand is the velocity observed by an observer with respect to a reference point.
what do u understand by Ultraviolet catastrophe?
A certain freely falling object, released from rest, requires 1.5seconds to travel the last 30metres before it hits the ground. (a) Find the velocity of the object when it is 30metres above the ground.
A vector is a quantity that has both magnitude and direction
the velocity Is 20m/s-2
derivation of electric potential
Rugunda Reply
V = Er = (kq/r^2)×r V = kq/r Where V: electric potential.
what is the difference between simple motion and simple harmonic motion ?
simple harmonic motion is a motion of tro and fro of simple pendulum and the likes while simple motion is a linear motion on a straight line.
a body acceleration uniform from rest a 6m/s -2 for 8sec and decelerate uniformly to rest in the next 5sec,the magnitude of the deceleration is ?
Patricia Reply
The wording not very clear kindly
the magnitude of deceleration =-9.8ms-2. first find the final velocity using the known acceleration and time. next use the calculated velocity to find the size of deceleration.
Firstly, calculate final velocity of the body and then the deceleration. The final ans is,-9.6ms-2
8x6= 48m/-2 use v=u + at 48÷5=9.6
can i define motion like this motion can be define as the continuous change of an object or position
Shuaib Reply
Any object in motion will come to rest after a time duration. Different objects may cover equal distance in different time duration. Therefore, motion is defined as a change in position depending on time.
which country are Mach is a used renewable energy sources in the world
kwot Reply
least the largest renewable energy sources in Ethiopia
what is the largest renewable energy sources in the world?
kwot Reply
our Star
The Sun...in better word
the sun
a flywheel has a mass 200kg and radius of gyration 0.6m.It is given an angular speed of 150rpm in 90 rotations starting from rest.Determine the torque assuming it to be constant that acted on the fly wheel
Swetha Reply

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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