# 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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...

26. ### ##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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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...