What is Gases: Definition and 501 Discussions

Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or compound molecules made from a variety of atoms (e.g. carbon dioxide). A gas mixture, such as air, contains a variety of pure gases. What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer. The interaction of gas particles in the presence of electric and gravitational fields are considered negligible, as indicated by the constant velocity vectors in the image.
The gaseous state of matter occurs between the liquid and plasma states, the latter of which provides the upper temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases which are gaining increasing attention.
High-density atomic gases super-cooled to very low temperatures are classified by their statistical behavior as either Bose gases or Fermi gases. For a comprehensive listing of these exotic states of matter see list of states of matter.

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  1. Leticia

    I Any articles or resources on how interparticle interactions may affect buoyant force in gases?

    This is for an article I'm writing with other members of my lab. There's a section on the equivalence principle applied to a balloon attached to an accelerated reference frame. I'd like to talk about any possible differences in buoyant behavior for different gas models (ideal, van der Waals...
  2. chocopanda

    Mixing two gases in an isolated system and calculating final p and T

    Hey everyone, I have an attempt at fully solving this problem (my final pressure is ##p_f = 5373,64 hPa##, final temp. is ##T_f = 303,15K = 30C##), but this exercise confuses me very much. First, I have not used the masses in my calculations and I'm pretty sure my prof. accidentally copypasted...
  3. chocopanda

    Mixing two ideal gases with different V, T at constant pressure

    To be honest, thermodynamics is really not my strong suit and I get confused when and how to apply formulas. My thought process is as follows: - there are two ideal gases (ideal gas law applies) - the pressure remains constant (isobaric process), so p1= p2 = p - I imagine there being two...
  4. C

    Ratio of atomic masses of two ideal gases

    For part(b) The solution is, ##1:10##, however, is the wording correct? I don't see how to find the ratio of atomic mass, however, I can solve for the ratio of the molar mass. ##n_A = n_B## from part(a) by setting the internal energy equation for each ideal gas equal ##\frac{M_A}{m_A} =...
  5. jaumzaum

    I How Do You Calculate Equilibrium Conditions After Mixing Two Different Gases?

    Consider that I have 2 adiabatic containers, one with a monoatomic gas at pressure P1, volume V1 and temperature T1 and another with pressure P2, volume V2, temperature T2. If I open a valvule and mix the two gases, how do I calculate the equilibrium pressure and temperature? I know the final...
  6. V

    B Conflicting definition of degree of freedom in Kinetic Theory of Gases

    I am seeing conflicting definitions of degree of freedom in my textbook. If I look at the definition given as per screenshot below then it is the number of independent terms/variables/coordinates used to define the energy of a molecule. But, if I look at the statement of Equipartition of energy...
  7. G

    Understanding the Equipartition Theorem for Ideal Gases

    Hi, I am unfortunately stuck with the following task I started once with the hint that at very low temperatures the diatomic ideal gas behaves like monatomic gas and has only three degrees of freedom of translation ##f=3##. If you then excite the gas by increasing the temperature, you add two...
  8. V

    Why does less dense air rise and more dense air come down?

    My answer given below seems incomplete. Since warm air causes the air to expand in volume, so its density becomes less as compared to the colder air at the top of the room. After this, I generally find all books saying the less dense air rises and more dense air from top comes down and...
  9. C

    Free expansion of Real Gases - Dieterici EoS, Change in Temperature

    I proceeded in the usual manner in which we take ##dU = 0## in the case of free expansion because there is no heat transfer in the box, as well as no work is done. We can write, taking ## U ## as the function of ##V## and ##T##, $$ dU(V,T) = \left( \dfrac{\partial U}{\partial V} \right)_T ~...
  10. R

    Looking for basic theory on mixing of gases

    Hi, In my situation, I am dealing with two ideal gases traveling down a pipe at the same bulk velocities. I am only considering a 1-D treatment at the moment and I would like to find out at which point the two gases should be considered a well mixed, homogeneous mixture. Would anyone know...
  11. Amaterasu21

    What happens to non-respiratory gases in the lungs?

    Hi, a thought just occurred to me. We all learned in school that oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses from the blood into the air to be breathed out. But they never mention nitrogen, argon or any of the other gases in the air! Does something...
  12. M

    I Partition function of mixture of two gases

    I have a question about statistical physics. Suppose we have a closed container with two compartments, each with volume V , in thermal contact with a heat bath at temperature T, and we discuss the problem from the perspective of a canonic ensemble. At a certain moment the separating wall is...
  13. P

    Mathematical equation for the deviation from ideality of real gases

    Hi guys, We have this very common graph where pV deviates from ideality. May I know the equation for such a curve? Secondly, if the x-axis were changed to V, what would the graph look like?
  14. MRG1373

    Diffusion coefficient of gases in liquids

    recently I'm looking for diffusion coefficient of gases in liquids. I have read "THE PROPERTIES OF GASES AND LIQUIDS" from Bruce E. Poling. but in this book it isn't directly mention what is diffusion coefficient of gases in liquids. can we using liquid-liquid models for gas-liquid models?
  15. E

    Cyclic Process for Ideal Gases

    Hello there, is my solution for part d logically correct? Here is my attempt at the solution : Part a : where : P1 = 3P2 Part b : Since P1=3P2, therefore, T1=3T, where T=300K. Thus, T1=900K Part c : Because the final pressure at the end of the cycle is exactly the same as the pressure at...
  16. WMDhamnekar

    MHB Multiple choices question on specific heats of gases

    Two cylinders A and B fitted with pistons contain equal amounts of an ideal diatomic gas at 300 K. The piston of A is free to move, while that of B is held fixed. The same amount of heat is given to the gas in each cylinder. If the rise in temperature of the gas in A is 30 K, then the rise in...
  17. E

    B Gas Interaction in a Box: Force and Pressure Dynamics

    There is a box ##\mathcal{B} \subseteq \mathbb{R}^3## with a partition coinciding with the plane ##z=0##. The gas in the region ##z<0## is initially at ##p_1## and the gas in the region ##z > 0## is initially at ##p_2##, where ##p_1 \neq p_2##. At time ##t=0##, the partition is removed (details...
  18. Uchida

    I Question regarding dh, du, cp, cv for ideal gases

    Hi, Considering the question bellow from a government work selection process:Check the FALSE alternative on the use of thermodynamic properties. In a cylinder-piston type system, the variation of the enthalpy property (Δh) is usually applied to determine the heat (per kilogram) exchanged with...
  19. J

    Is the concentration of greenhouse gases proportional to the temperate

    I can't seem to find an answer to this: If the present level of greenhouse gas concentration were not to change, would the temperature of the planet continue to increase? Or would it level off? Or is a continually increasing concentration of greenhouse gases required to keep raising the planets...
  20. TechieDork

    Need help with calculating a Cv in PHYWE's Heat Capacity of gases

    I've conducted this experiment yesterday. The main goal of this experiment is to find a gas constant R and compare it with its theoretical counterpart but I get stuck in calculating a Cv so I tried to find out what's wrong with my calculations by trying to calculate a Cv from the given data...
  21. aspodkfpo

    Pressure & Force: Gases, Liquids, Solids

    Sorry if I'm asking too many questions, but I'm currently reviewing a textbook and I'm realising quite a few things that I'm conceptually unsure of, so I'm going to be spewing out questions. If a bottle is shaken, dissolved gas comes out of solutions and builds up pressure inside a bottle. As I...
  22. Sabertooth

    A Relativistic Effects on Particles and Gases

    Hello everyone, I was doing some calculations recently regarding particle velocities for different elements at different temperatures and I have a few questions for the experts in here. Usual gas laws in my school book provides information about the velocity of particles in gases, it provides...
  23. Rongeet Banerjee

    Root Mean Square Velocity of Gases

    This question came in NEET Exam 2018.Now my first query is that in the question,the mass of one Oxygen molecule is given wrong.Its exactly half it's true value.I don't think anybody has noticed this before because I couldn't find any change in the printed question on so many different books...
  24. L

    Calculating heat recovery from engine exhaust gases

    Hello, I am doing a study for my final years Uni project, which includes waste heat recovery from gas fuelled engine exhaust. I am using a data sheet for a Jenbacher J620 engine-. In the data sheet it is stated that; exhaust gas mass flow (wet) = 17325 kg hr ( not sure about the wet part?)...
  25. P

    Piston Between Two Ideal Gases

    Attempted Solution: Gas Entropy This system is isothermal: the energy of each gas remains constant. $$dU = 0$$ By the combined statement of the first and second laws, $$dU = TdS - PdV$$ Therefore, $$0 = TdS - PdV$$ $$dS = \frac {PdV}{T}$$ Therefore, $$dS_1 = \frac {P_1 dV_1}{T} = \frac {P_1...
  26. Kaushik

    ##v_{rms}## in the Kinetic Theory Of Gases

    In Kinetic theory of gases, what is the reason behind introducing a new kind of average known as root mean square velocity (##v_{rms}##)? I read the following: The molecules in a container are in constant random motion. So when we add all the velocity vectors to find the average it cancels out...
  27. Kaushik

    Andrew's experiment on liquefaction of gases

    Look at the region from C to B. Why is the pressure constant here while the volume of the gas is decreasing? Is it the phase where gas to liquid transition is going on? What I know In the region from A to B, as the pressure increases the volume of the gas reduces. At C, the state of matter...
  28. electr0dave

    How can I mix gases in a chamber measuring only pressure and temperature

    Hello all! Basically I intend to create an environment inside a chamber with variations of CO2 percentages, using electrovalves. The mixing gas is always compressed air + CO2. For example: create a 50% CO2 concentration with a total pressure of 2 Bar. The chamber is closed, and compressed air...
  29. PCal

    Composition of flue gases by volume on a wet basis and dry basis

    Wet basis 0.75mol C4H10 Requires 4.875 mols O2 Produces 3 mols of CO2 and 3.75 mols of H2O 0.1mol C3H8 Requires 0.5 mols O2 Produces 0.3 mols of CO2 and 0.4 mols of H2O 0.15mol C4H8 Requires 0.9 mols O2 Produces 0.6 mols of CO2 and 0.6 mols of H2O Theoretical oxygen= 6.3mol +10% excess...
  30. S

    I Photon density in photon gases

    Hello everybody, In this Wikipedia article we find an equation for a photon gas which contradicts an equation given by Stefan Weinberg in his book "The first three minutes": https://en.wikipedia.org/wiki/Photon_gas The equation given here has 16 π k^3 ζ(3) T^3 in the numerator and c^3 h^3...
  31. S

    Q&A: How Does Work & Energy Relate to Solids, Liquids & Gases?

    I can understand how this applies to gases as in that scenario work done = pressure * change in volume and if change in volume is zero then W=0 so change in internal energy = Q. But I do not understand how this applies generally to all types of substances, say solid, liquid and gas.
  32. samak55

    Mixture of different gases at different temperatures

    Homework Statement Hello. I have problém with this system. Mixture of two gases. How can i determine temperature T3? Gas 1 (dry air) - I know T1, p1, m1 Gas 2 (water steam) - I know T2, p2, m2 Output (mixture) P3, (m1+m2); p3<p1;p3<p2Homework EquationsThe Attempt at a Solution Is it possible...
  33. A

    Can you change the state of matter by increasing the speed?

    We all know that the state of matter can be changed by increasing temperature or by applying pressure. And, all of these, in some way lead to a change in the particles' speed. The speed of the particle decides the state of matter. So, if we were to throw an object at very high speeds, like 50%...
  34. T

    Energy released upon ionization of different gases

    Hi, I am interested in plasma physics. I understand that once a gas (say Argon) is ionized, light (electromagnetic radiation) is emitted. This is clearly observable from the coloured glow of the gas when it is in a plasma state. Where does the energy for these electromagnetic rays come from? Is...
  35. nineteen

    Why do we study or learn about ideal gases?

    We are learning the lesson about gases/gaseous states at our school and I couldn't help but wonder, why learn about IDEAL GASES... How do ideal gases help us to analyze about real gases?
  36. Bheshaj

    Pressure exerted by a gas (derivation using the kinetic theory of gases)

    In the derivation of finding pressure exerted by a gas using kinetic theory of gases I am not understanding why the time between two collisions is taken as the time for rate of change of momentum when a particle bounces back from the wall. please help me
  37. F

    Kinetic theory of gases and velocity correlations

    I have been reading up on the kinetic theory of gases, and I'm unsure whether I have correctly understood why particle velocities become correlated after colliding. Is it because during the collision they exchange momentum and thus their velocities (and hence trajectories) are altered in a...
  38. M

    Assumptions in the derivation of the kinetic theory of gases

    When deriving the kinetic theory of gases, we take the change in momentum of a particle as it hits one side of a box and divide it by the time over which the collision takes place. The time is derived by taking the total distance the particle traveled in the box (i.e. from one end, off the side...
  39. S

    I Gases: Not Required to Be Blackbodies

    Moderator's note: thread split off from https://www.physicsforums.com/threads/confusion-about-absorption-spectra-cool-gasses-absorb.964474/ Gases are not required to be black bodies. A completely white body can neither absorb nor emit - no matter what its temperature may be. And objects that...
  40. Cool4Kat

    I Confusion about absorption spectra - cool gases absorb?

    So I was taught in school that heated low density gases produce spectral lines and cool low density gases absorb their spectral lines. I mentioned this to my husband and he asked me what the definition of hot and cool were and I had no idea. The more I thought about it, the more confused I...
  41. i_hate_math

    Quantum Gases - Mixing of 3He and 4He

    Homework Statement Consider a solution in which 99% of the atoms are 4He and 1% are 3He. Assuming that the 3He atoms behave as an ideal gas of spin-1/2 particles determine the Fermi energy of the 3He atoms. You may assume that one mole of 4He occupies a volume of 28 cm3.Homework Equations EF =...
  42. Logic hunter

    Why do gases occupy different volumes in eudiometry?

    I know how to solve problems of eudiometery but i don't understand a really basic thing ie how come gases occupy different volumes in an eudiometery tube. (For eg: 15ml of hydrocarbons with 320ml other gases). Gases occupy entire volume of the container, right. So shouldn't all gases occupy the...
  43. S

    I Compression of gases that liquefies

    For example we had a closed system with water at 298K, so it's pressure is 0.031atm. To it we add a gas, A such that we get 1atm of total pressure. That gas undergoes a phase transition at 1.5atm and 298K. So initially, the partial pressure of water is a little smaller than 0.031atm due to the...
  44. M

    Momentum transport in gases in 2d

    I was trying to understand the momentum transport between gas molecules in 2d.In the image below, it is stated that half of the molecules move up(positive velocity in y direction) and half negative.But the author didnt explain why he assumed it.
  45. tanaygupta2000

    B Experimental Virial Equation for real gases

    In the experimental virial equation for real gases, Pv = A(1+BP) (for small values of P) as 'P' approaches zero, then why do we get some finite value of 'Pv' instead of approaching to zero value, since 'Pv' is directly proportional to 'P' ? {P = Pressure of gas, v =...
  46. T

    Calculating Mean Free Path Ratios in a Divided Ideal Gas System

    I have a box with a wall in mid dividing it in 2 sections, and the wall has a hole of diameter d. There is ideal gas in both sections at 150 K in one section and at 300 K in another. How am I supposed to calculate ratio of mean free paths in 2 sections. My attempt: L ~ Volume / Number of...
  47. Spinnor

    I Noble gases with even nuclear spin form superfluids?

    Other than Helium do Noble gases with even nuclear spin form superfluids? Is there a simple quantum mechanical explanation why the difference below of the Melting point and Boiling point of the Noble gases is roughly the same value? A yes or no would suffice. From...
  48. Sophrosyne

    B Why is an electric arc needed for spectroscopy of clear gases?

    To obtain spectroscopy on a gas like hydrogen or helium, an electric arc is passed through a container of the material, and then the emitted light is viewed through a prism which breaks it up into its component wavelengths. There are sharp lines which are formed, caused by excitation of the...
  49. F

    I Line Spectra and low density gases

    Hello, A low density gas, when heated at a temperature T, emits radiation having a line spectrum, i.e. having only discrete spectral lines. Each spectral line corresponds to a very specific energy transition (jump) for the electron in the atom. Some energy transitions are allowed, some are...
  50. Waffle24

    Understanding Pressure and Volume in a U-shaped Tube

    Homework Statement The open legs 1 and 2 of an U-shaped tube have a diameter of 2 cm^2. In leg 2, mercury is poured. When the distance is 16cm from the mercury level up to the valve K1 , then the valve gets closed. The barometer reading is 76cm Hg. a) How big is the pressure of the sealed air...