What is Gas: Definition and 1000 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. Krokodrile

    Engineering Final volume of a gas using the ideal gas equations

    Hey there! for this problem i try to use the combinate gas ecuation. First of all the values its necesary to have it in absolutes: 70 F = 527.67 K 90 F = 549.67 K The ecuation looks like: (200 psig) (1 ft^3)/529.67 K = (0.3 InHg) V2/549.67 K I can eliminate "K" but not psig with InHg for obtain...
  2. Mayan Fung

    Relating the entropy of an ideal gas with partial derivatives

    It looks very easy at first glance. However, the variable S is a variable in the given expression. I have no clue to relate the partial derivatives to entropy and the number of particles.
  3. V

    Construction Gas Strut - ideal mounting position & Force Calculation

    I have a construction made from 4040 aluminum extrusion. The top frame is opening at 90deg with external hinge. the weight of the top is 6Kg The dimensions are 50X75cm externally and 42X67 internally (as the frame width is 4cm) If pictures with dimensions would help please let me know.
  4. T

    Equation of a sound wave with viscous damping in ideal gas

    How can we find a equation of a 1D sound wave in a non-differential form in an ideal gas with viscosity? How does the damping work? How does the wave lose energy at each layer as it propagates? To be clear I am looking for a simple exponential-sinusoidal function for it just in the case of...
  5. L

    Engineering Find the volume of a gas using Boyle's law

    Is this right go confused with the 37 degrees Q) pV = Constant pressure of a gas is ‘10’ (kPa) and the constant ‘37 degrees celcus’ = ‘0.6’, calculate the volume that the gas. pV = C C/p = V 0.6/10 = C V = 0.06
  6. M

    Engineering Gas turbine (mean line) analysis question

    Hi, I was attempting another turbomachinery question and am struggling with a few concepts. The question is: "A single stage gas turbine for power generation is designed for the following overall duty: - Power output is 4.2 MW, and mass flow rate is 70 kg/s - Inlet stagnation pressure = 3 bar...
  7. sliwinskit

    Cabinet with doors lifted by a gas spring

    Hello everybody. I'm trying to figure out how to calculate gas springs for specific doors not with hinge, but lifted up, like on the picture. Do anybody experience with such type of mechanism? Also I'm looking for helpful software, for gas spring calculation (different types). I found screens...
  8. L

    Engineering System Constants for Perfect Gas

    Q) I have to explain the relationships between system constants for a perfect gas. Hi can somebody explain this to me. I am kind of confused what this means and where to start and what to do. Also not 100% sure what a perfect gas is.
  9. L

    Pressure profile of a gas flow through an orifice

    Hello, The question will probably be related to mechanical engineering / chemical engineering / aeronautics. I come from the field of optics and have no background at all on fluid dynamics. I'm trying to calculate the dependence of pressure on distance, i.e. P(x) in a gas flow problem: I...
  10. mcas

    Find the change in entropy for an ideal gas undergoing a reversible process

    We know that $$dU=\delta Q + \delta W$$ $$dU = TdS - pdV$$ So from this: $$dS = \frac{1}{T}dU + \frac{1}{T}pdV \ (*)$$ For an ideal gas: $$dU = \frac{3}{2}nkdT$$ Plugging that into (*) and also from ##p=\frac{nRT}{V}## we get: $$S = \frac{3}{2}nk \int^{T_2}_{T_1} \frac{1}{T}dT +...
  11. Z

    Gaussian Distribution of Ideal Gas

    My attempt : $$P(n) = \frac{1}{\mathcal{Z}} Exp[(n\mu -E)/\tau]$$, use $$\lambda = e^{\mu/\tau}$$, then the distribution can be written as $$P(n) = \frac{1}{\mathcal{Z}} \lambda^nExp[-E/\tau]$$ Note that the average number of particle can be written as $$<N>= \lambda \partial \lambda ( log...
  12. P

    Rate of heat flux from from hot gas in a pipe to water outside

    I have solved the first 2 parts. For the 3rd part, I have obtained the equation: T(x) - T0 = (T1 - T0)e^(-Φx/fc), where f = fm in the question. How do I obtain that expression for H? Thank you!
  13. J

    Finding pressure of gas in u shaped tube with liquid

    The answer is suppose to be 0.9432. Initially I thought the pressure inside the glass ball would just be the same as the atmospheric pressure because these equal pressures would cancel each other out, but obviously that's not true. I can calculate the density of the gas using the equation...
  14. B

    How Do You Determine The Number Of Moles In A Volume Of Gas?

    Sadly, I don't even know what I don't know. Could somebody generate a ballpark figure of how many moles of hydrogen you might have in a sphere 2 meters across, 75F and roughly 16 psi? Even with all that information I'm clueless. (I'll now resume twig-fishing for termites...)
  15. Z

    Entropy change for spontaneous/ irreversible gas expansion

    When trying to describe why the entropy goes up for a irreversible process, such as gas expanding into a vacuum, it seems fairly easy at a high level. the valve between the two chambers opens, the free expansion occurs, the pressure drops proportional to the volume change and the temp remains...
  16. D

    Chemistry Volume of a Gas from a thermal decomposition

    Ca(HCO3)2 -> CaCO3 + H2O + CO2 First I evaluate the moles of calcium carbonate (don't mind the units, just to save time) ##\frac {80.0}{40,00+12.01+3*16,00}= 0,799 mol## From the equation, correct me if I am wrong , one mole of CaCO3 is proportional to one mole of CO2, so from this I can...
  17. D

    Chemistry Ideal gas law problem with two cylinders

    my answer will be ##P_1=2 P_2## but I have some doubts, if that is correct or not
  18. PuzzledMonkey

    Mystery of Fizzing Gas Duster - Videos Included

    While using an ordinary gas duster (difluoroethane) to clean some electronics, I noticed something weird (see linked videos): After spraying the gas duster and placing it on a hard surface e.g. countertop, the can emits a fizzing noise. Picking up the can causes the fizzing to stop. The process...
  19. K

    Thermodynamics: Ideal gas model

    Do particles have air in between them in the ideal gas model? I think the answer is 'no, but I am not quite sure about the explanation. Is it because in an ideal gas model, the volume of the particles is negligible? Thank you.
  20. naviakam

    Black body radiation vs electric discharge in a gas

    Black body radiation formula contains power and exponential terms. Electric discharge in a gas results in the ion acceleration; the ion distribution comprises power and exponential terms too. Any connection between these two phenomena (i.e. black body and potential) could be established?
  21. M

    I Trying to get a physical understanding of a Fermi gas

    I would like to get a more physical interpretation of conduction electrons (fermi gas) in a metal. I imagine ionized valence electrons close to the ions, with the fermi level (highest energy electrons) of the gas participating in conduction. A point of confusion for me...the first ionization...
  22. L

    Thermodynamics: gas expansion formula or approximation error?

    FIRST TYPE: REVERSIBLE PROCESS At the temperature of 127 ° C, 1 L of CO2 is reversibly compressed from the pressure of 380 mmHg to that of 1 atm. Calculate the heat and labor exchanged assuming the gas is ideal. Q = L = - 34.95 J CONDUCT 380 mmHg = 0.5 atm L = P1 * V1 * ln (P1 / P2) = 0.5 * 1...
  23. S

    Position of piston related to ideal gas

    a. The piston will be at rest when all its kinetic energy converted into work to push the gas, so: $$\frac{1}{2}m_0 c^2=P_0. \Delta V$$ $$\frac{1}{2}m_0 \frac{29}{4} \frac{P_0.V_0}{m_0}=P_0.\Delta V$$ $$\frac{29}{8} V_0=\Delta V$$ $$\frac{29}{8} L_0 = L_0 - L$$ $$L=-\frac{21}{8} L_0$$ My...
  24. sandmanvgc

    (Dry) Volume Ideal Gas Law Calculation

    If the question was asking for (dry) volume, how would you do that?
  25. P

    Gas effusing through hole, working out time dependence

    Consider instead a thermally insulated container of volume V with a small hole of area A, containing a gas with molecular mass m. At time t = 0, the density is ##n_0## and temperature is ##T_0##. As gas effuses out through a small hole, both density and temperature inside the container will...
  26. I

    Gas Laws - pumped tires exploding at the top of a mountain

    When you pump a bicycle tire before riding up a mountain, the tires may explode. I think it is because of Boyle's law, where since the external pressure decreases, the volume of the tire is supposed to increase and it explodes. The solution says that as the air in the tire expands on heating...
  27. AndreasC

    Non-interacting gas in homogeneous gravitational field

    It even gives a hint, it says "consider two horizontal surfaces z1 and z2 and think about what thermodynamic equilibrium means for particles traveling from one surface to the other". This really trips me up because I am not sure what to do with this. Obviously in equilibrium the number of...
  28. P

    Gas in mercury manometer, finding its normal volume

    Some notation: - the difference between the heights of mercury, which is effectively the height of the mercury in the open end of the tube is ##h_{diff}## - the volume of gas inside the sealed off end is ##V_{inside}## - the volume of gas when let outside, "normal volume", is ##V_{outside}## -...
  29. P

    Kinetic Theory - A gas mixture effuses through a hole, find the pressure change

    So I know Dalton's law as stated above which I think is applicable in this question. Then I know the effusion rate is ##\frac{1}{4} n \bar{v}##, and from this we can make a differential for the time evolution of the number density of the gas in the container which is: ##\frac{dn}{dt} =...
  30. L

    Chemistry Partial pressure of He in this gas mixture

    My progress: P(HE) = 0.877*0.75/(0.75+0.5) = 0.5262 atm is this right?
  31. J

    Ideal gas law problem -- Pneumatic piston movement with air temperature changes

    I have come up with the change in height as 170 cm. My professor does not want to solve for the problem for a reason I do not understand. 170 cm is not part of the answer key. The answer according to the answer key is 65 cm. My attempt is: Initial temperature: p=F/A; (50 *9.8) / (pi * 0.05^2)...
  32. H

    I Transparency of a gas compared to a plasma

    I imagine an empty region of space, in between stars, or even galaxies. This void is filled only with a uniform distribution of hydrogen particles. I won't specify how dense this field is, but I would like to know which is more transparent to photons, a gas of hydrogen atoms, hydrogen...
  33. Kathhhriine

    How is the Ideal Gas Law Derived from Temperature and Pressure Relationships?

    I figured that T' is a common factor for both relationships and from there deduceted that T'=p2xt1/p1=v1xt2/v2. However, I don't understand how that can be further manipulated to PV=KT.
  34. T

    1420 MHz--- the emission frequency of cold hydrogen gas

    I recently finished reading Paul Davies book The Eerie Silence, which is a book about the SETI (Search for ExtraTerrestrial Intelligence) project. In The Eerie Silence, Davies says that scientists using radio telescopes to search for radio messages from space aliens set their radio telescopes...
  35. amilton

    I Why does a hydrogen gas tube produce a hydrogen atomic spectrum?

    To measure the atomic hydrogen spectrum people often uses hydrogen gas tubes as light source. Since the gas in the tube is the molecule ##H_2## , why we obtain the spectrum of atomic hydrogen? My guess is that because the voltage is so high, so that the molecules are totally dissociated. If...
  36. AndreasC

    I Help with an ideal gas canonical ensemble partition function integral

    Where does the volume even come from? Any help would be appreciated!
  37. SQUB

    Photoemission from a non-interacting electron gas

    I had another excercise of the long list of the same topic (solid state physic) where I need a bit of help. All other excercise where about interband transition, dispersion relation, refracting and absorption coefficient, x-rays and so on, and I managed to solve them or I think I will be able...
  38. Will26040

    How do you approximate between the Cp of a gas and a solid?

    I have the Cp of Benzoyl peroxide (BPO) in gas form (454.39 J/molK). What approximation could I make to find the Cp of solid BPO as I cannot find this info online? thanks
  39. Mikkel

    I At what wavelength do we observe ionized gas?

    Hello! My question is this: At which wavelength do we observe ionized gas? My understanding is this: An ionized gas is basically a soup of particles flying around, because of the extreme temperatures (plasma?). So if we have particles moving around with such high velocities, I would expect to...
  40. Mayan Fung

    I Fermi gas in relativistic limit

    In a statistical mechanics book, I learned about the degenerate pressure of a Fermi gas under the non-relativistic regime. By studying the low-temperature limit (T=0), we got degenerate pressure is ##\propto n^{5/3}## (n is the density). And then I was told that in astrophysical objects, the...
  41. LCSphysicist

    Thermodynamics: work done on the atmosphere by an expanding gas

    V = V(n,P) PV = nc dV = (-cn/P²)dP + (c/P)dn dW = (cn/P)dP - (c)dn This was how i tried to attack the problem, but it will end with log and probably it is wrong, any tips to fix it?
  42. A

    Motion of a rocket with propellant exhaust gas speed = constant

    Now this is how I've tried to solve this $$ v_e = u0 \cdot ln \frac {M} {M- μ \cdot t} $$ After putting in the values I get this; $$ v_e = 200 * ln 0,36 $$ $$ v_e = 73,54 \frac m s $$ Now I'd say that this is the correct way to do it, but this part is confusing me "What is the speed of the...
  43. Abdalrahman Azab

    Question about two-stage light gas guns

    Hello ... I had a question about the capabilities of the two-stage light gas guns which are used to study the effect of small meteors on spacecraft shields ... is it theoretically possible to achieve velocity above 7 km / s for a projectile of about 16 grams weight and 15 mm diameter using a...
  44. D

    Automotive Diesel engine parts for post-treatment of diesel engine gas exhaust

    Hi folks, Any idea about what is it? I am required to find parts like these items which are made of stainless steel 304 and used on the post-treatment of diesel engine gas.
  45. L

    Engineering Differences between an Ideal gas and a perfect gas?

    Is this right for difference between idea gas and perfect gas. trying to get it into head but can't find simple explation.Idea gas it is a fictious matter that follows the PV=nRuT or PV = mRuT equation, which has predermined conditions of ideal conditions of the the gas. As temperature for...
  46. P

    Problem related to the Ideal Gas equation -- Nitrogen under pressure

    Solution from the textbook: My work: I constantly get 1.55kg. I also tried dividing the first and the second equation (pxV=m/M x R x T with different values). How did they come up with the equation in the solution? Also, I am sorry if I posted it in the wrong place and didn't follow the rules...
  47. A

    Heat Capacity of a Van Der Waals' gas in an Open system

    In our class, we're using Wassermann's Thermal physics as textbook. I always try to solve all question which included in Text book. But sometime when I meet a problem that look like easy but actually hard, I'm so embarrassed. This problem do also. First, in the textbook grand potential for van...
  48. Z

    What is the Saha equation and how does it apply to hydrogen gas?

    I have no clue. I am stuck. I would appreciate If someone could help me out. I just know I need to use the Saha equation.
  49. T

    Why does the pressure of a gas bubble change as it rises?

    The correct answer is 'C'. I'm having trouble understanding the solution shown on this link (http://www.engineeringadmissionsassessment.com/2019-solutions.html) - scroll down to Section 2, Question 8. From what I've gathered, [final pressure = initial pressure x 'R^3/r^3'], as PV must be...
  50. K

    Proof question related to the Ideal Gas Law

    A cylinder contains an initial volume V1 = 1m^^3 of a perfect gas at initial pressure p1 = 1 bar, confined by a piston that is held in place by a spring. The gas is heated until its volume is doubled and the final pressure is 5 bar. Assuming that the mass of the piston is negligible and that the...
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