What is Vapor: Definition and 272 Discussions

In physics, a vapor (American English) or vapour (British English and Canadian English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature, which means that the vapor can be condensed to a liquid by increasing the pressure on it without reducing the temperature. A vapor is different from an aerosol. An aerosol is a suspension of tiny particles of liquid, solid, or both within a gas.For example, water has a critical temperature of 647 K (374 °C; 705 °F), which is the highest temperature at which liquid water can exist. In the atmosphere at ordinary temperatures, therefore, gaseous water (known as water vapor) will condense into a liquid if its partial pressure is increased sufficiently.
A vapor may co-exist with a liquid (or a solid). When this is true, the two phases will be in equilibrium, and the gas-partial pressure will be equal to the equilibrium vapor pressure of the liquid (or solid).

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  1. maryelin

    How to get the density equation using the Reidel Equation for vapor liquids?

    I need to know how to get the density equation using the Reidel Equation for vapor liquids... how is the calculations to get
  2. P

    Vapor pressure -- How does water still boil at 100°C in an open pot?

    Vapor pressure is the pressure of the vapor when it is in equilibrium with its liquid. This only happens when the container where the liquid is present is closed. Indeed, when the container is open, this liquid-vapour equilibrium is never reached, because the partial pressure of the vapor (at a...
  3. M

    I Water bath and water vapor + air mixture in a closed system

    Hello, At my work I came up with the following question: Say we have a vacuum chamber. Inside the vacuum chamber we have at the bottom a water bath. The rest of the chamber is filled with a air and water vapor mixture. The whole system is placed in a room, is at room temperature and is in...
  4. M

    Internal Combustion Engines: Liquid Gasoline or Vapor?

    In an Internal Combustion Engine, does liquid gasoline itself burn/combust, or is it the gasoline VAPOR emitted from the liquid gasoline by evaporation that burns/ combusts? (assuming that Oxygen is present in the Air and Fuel mixture)
  5. B

    Vapour Absorption Cycle Questions

    Hi all, I'm doing some research on vapour absorption cycle machines, mainly water ammonia refrigerators. I've got a few questions. 1. What is the role of the water absorber? ie why do we need the water? 2. In the absorber we have a water ammonia solution. Is the ammonia a gas in the solution...
  6. Jonathon

    How to Factor Vapor Retarders into Relative Humidity & thus Mold Risk

    Relative humidity can be calculated if you know dewpoint and temperature - by formulae such as the August-Roche-Magnus approximation. All methods - all formulae - ignore vapor resistance (perms). Surely scientists/engineers have a way to then, as a subsequent step, bring vapor retarders into...
  7. Dario56

    Vapor - Liquid Equilibrium Equation (VLE)

    Most fundamental equation for VLE is $$ \mu_i^L = \mu_i^V $$ It states that for every component chemical potential must be equal in both liquid and vapor phase at equilibrium. However, in my thermo textbook, this equation is derived for isolated systems while usually when dealing with VLE...
  8. I

    Chemistry Vapor pressure of ethanoic acid vs. ethanol

    I think the answer is D because both molecules can hydrogen bond with their OH groups, but the C=O bond in ethanoic acid is polar and contributes to dipole-dipole interactions. The solution says B is the correct answer, because ethanoic acid molecules strongly hydrogen bond so that most...
  9. Limebat

    Newbie Physics Student Asks: Does Vapor to Ice Change Acceleration/Velocity?

    *My bad if this question is a tad ree ree. I've just completed my first year of college and am still inexperienced. I just study physics for fun.* My intuition says the momentum of the water vapor is still conserved during the phase shift, as this question most probably relates to the...
  10. I

    Chemistry Equilibrium of Methanol Vapor Decomposition

    The solution says that when the effusion rate ratio is multiplied by the equilibrium mole ratio of H2 to CH3OH, the effused mixture will have 33.0 times as much H2 as CH3OH. I don't understand why. I just set the equilibrium mole ratio of H2 to CH3OH as equal to 33.0 times, Why is this...
  11. S

    Mass of vapor per hour for heating water

    I thought about using Black principle to solve this question but I am confused about the final temperature of system Q released by vapor = Q absorbed by water mvapor . cvapor . ΔTvapor + mvapor.Lv + mvapor . cwater . ΔT2 = mwater . cwater . ΔTwater But what is the final temperature of the...
  12. D

    Phase Equilibrium of Liquid and Vapor Under External Pressure

    Suppose you have a container of water at a given temperature T (say normal room temperature) with a vacuum above it. Presumably water will evaporate until there is sufficient vapor that the pressure of it above the water is the SVP for that temperature. Now suppose that there is air above the...
  13. jaumzaum

    Why can shock waves condense water vapor?

    Why can a shock wave condensate water droplets in the air and produce the visible vapor cone that we see when objects move faster than the speed of sound. Also, does this condensation happens only when the object is moving with a velocity greater than sound velocity? I don't understand the...
  14. E

    Does pressure affect equilibrium vapor pressure (or RH)?

    I'm wondering if I'm on the right track and if anyone is willing to steer me on if not: Equilibrium vapor pressure (EVP—also referred to as saturation vapor pressure) is dependent only on temperature. Outside pressure has no bearing. Now, of course, with lower external pressure (atmospheric)...
  15. S

    Mercury vapor from blood pressure devices

    I came across some articles about the health hazard from broken mercury-based sphygmomanometers. This led me to think about vapor leakage from good devices during normal operation. The top of the glass tube is at ambient atmospheric pressure. There is supposed to be a membrane at the top that...
  16. M

    Optical Laser to refract/reflect water vapor and smoke.... angles?

    Building visibiity sensor... What is best angle to detect water vapor reflection from laser, and refraction? I plan to have two open cylindrical containers painted flat black and put inside each other so the overlap is about 0.5-1" (adj to limit ambient light vs airflow). Laser is cheap red...
  17. fsonnichsen

    Sodium Vapor Lamps (low pressure) for spectral calibration and diffraction work

    Just curious if anyone out there is using the old low pressure sodium vapor lamps for spectral calibration and diffraction work. I was surprised that I do not see any bench ready laboratory lamps from the usual vendors. I understand that the last manufacturer of the low pressure bulbs, Philips...
  18. T

    Is cavitation worse at depth? (Fluid pressure and Vapor pressure boiling)

    I am wondering about the impact of the hydro static pressure of a fluid on its boiling point. The simplest real world example scenario I can think of is the rate/onset of cavitation at a large depth vs a shallow depth. As we increase the submarine propellor speed to a speed where the adjacent...
  19. D

    Calculate the amount of water vapor

    Hello 4m3/min is 240m3 /hr. Temperature is 293K, at 25C the amount of water is 22.8g/m3 and at 20C it is 17.1g/m3. Relative humidity is 70% and absolute pressure 9bar. Solution: @ 25C relative humidity lowers the water content from 22.8 to 15,96g/m3 so in total 3830.4g/hr 240m3 or air is...
  20. T

    What element produces a vapor in comet tails?

    I would like to find out what element produces a vapor or a stream in a heavy charged ionic vacuum would lithium produce such a thing? Comet tails.. just wondering
  21. C

    Could somebody please explain the vapor compression cycle?

    Heat is taken from food in the refrigerated space to the evaporator which has evaporated (I am assuming saturated vapor) refrigerant flowing through it. My first stumble would be, if this is true, why doesn't the temperature increase for the fluid coming out of the evaporator (unless it is...
  22. M

    Water vapor absorption in THz range

    Hi everyone! Terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique that is using the radiation in the THz range. https://en.wikipedia.org/wiki/Terahertz_time-domain_spectroscopy A THz-TDS setup consists of an emitter and detector. The emitter creates a short broadband...
  23. HotFurnace

    B Effects of Cooling and Heating in a Cloud Chamber

    Good day everybody! As we have already known the structure and operation of the cloud chamber, can someone explain to me why the cooling is done at the bottom of the chamber and the heating at the top, but not vise versa?? How this affect the function of the cloud chamber?
  24. R

    Chemical Vapor Deposition - Diamond

    Are there any instructional video's on how to make your own CVD Diamond optics. I was looking online and CVD Diamond wafers are EXTREMELY expensive. Is it possible to make a plasma chamber for CVD if so does anyone have any good text recommendations What is the catch
  25. P

    I Chemical potential on a solid and its vapor pressure

    I have been reading the book "Nanostructures and Nanomaterials" by G. Cao and Y. Yang, and was intrigued by the following passage in page 33: "Assuming the vapor of solid phase obeys the ideal gas law, for the flat surface one can easily arrive at: μv − μ∞ = −kTlnP∞, where μv is the chemical...
  26. Jed K

    How does water vapor in the atmosphere form under normal conditions?

    So I understand generally how humidity in atmospherics work but there's this one thing I can't reconcile with the fundamentals. First, my understanding of the basics. Water boils at 212 F @ 14.7 psia (1 ATM). At 213F @ 14.7 psia that water is now 1F superheated vapor. So we're all on the same...
  27. dromeascr

    What is the pressure surrounding a liquid at its boiling point?

    Hello everyone, before I start I just want to mention that I am not an expert in physics whatsoever, so please be as specific as you can get if you wish to provide an answer. (The question itself might be considered stupid to be honest) I read the definition of the boiling point recently and...
  28. P

    Where can I find water vapor images?

    I am searching for images of the daytime sky taken within the water vapor absorption bands at 940nm, 1600 nm, 1860 nm, 2700 nm or 2900 nm. Specifically I am trying to see if multispectral images of in-band and adjacent out-of-band photos could be subtracted to show areas of slight water vapor...
  29. B

    A Why Do LS and jj Coupling Differ in Spectral Line Transitions?

    My question is divided into two: a. One of Hg spectral lines is a strong one, 253.7 nm, emitted by transition from 3P1 (triplet) to 1S0 (singlet). I'm afraid I don't read the 1st state right, since it says S=1 (due to 2S+1=3) & J=1, hence (?) L=0. However, P means L=1, isn't it ? What am I...
  30. N

    Can water vapor go directly into a solid

    If water vapor is pulled inwards and cooled at a fast enough rate could if be frozen back into a solid form? i understand that they would have to be froze together as soon as contact is made but if this is possible what would the temperature have to be? And could this be the only thing that can...
  31. EastWindBreaks

    Heated piston & cylinder device with saturated water and vapor states

    [Mentor note: Thread title changed to describe actual problem being presented] 1. Homework Statement Homework Equations The Attempt at a Solution I understand you have to interpolate temperature and pressure of the saturated vapor from the table, since there is no matched final specific...
  32. Z

    B Absorption bands of liquid and vapor water

    Water vapor exists at a higher energy state than liquid water...but does that make a difference in the absorption bands? In other words, are the absorption bands of vapor and liquid water identical? Thanks!
  33. I

    Deriving the vapor pressure latent heat dependence

    Homework Statement There is a picture attached showing the entire problem. Equation 2.78 is the Clausius Clapeyron equation. Homework Equations Clausius Clayperon equation. L = L'T (since there is a linear dependence on temperature) The Attempt at a Solution $$ \frac{de_s}{dT} =...
  34. K

    How to calculate : Mass flow of water vapor in the air (kg/h)

    Hello Homework Statement Lets say i have the given parameters: air volume flow : V = 36 m3/h Relative humidity: RH=33% air mixing ratio : X = 5.5 (1g H2O) / 1kg (DRY AIR) (calculated from RH) air temperature : T= 20 C So i know how much mass of water is the air compared to the air's "dry air"...
  35. M

    Osmotic Pressure vs Vapor Pressure

    If I were asked to calculate the vapor pressure, and I calculated the osmotic pressure and converted it to mmHg, Would I get the right answer?
  36. D

    Would HE light applied to mercury vapor produce electricity?

    I'm likely going to have a ton of questions in the following weeks about a wide range of particle physics etc. If (electricity X mercury vapor) = light, then multiplication being transitive, (light X mercury vapor) = electricity. Can someone inform? I've found only mercury vapor as a metallic...
  37. U

    Filter Material which Blocks Volatile Liquid, Passes Vapor

    Hi, All. Given a scenario of a closed volume of volatile liquid and it's associated vapor above boiling point (ex. butane in a can at room temp.) which is dispensed through some tube or line: Is there some filtering medium capable of ensuring that only vapor may pass through the line, producing...
  38. Const@ntine

    Thermodynamics: Steam is added to an ice cube

    Homework Statement Vapors of 100 C is added to an ice cube of 0 C. How much of the ice cube has melted and what's the final temperature if the masses of the steam and the ice cube are 10.0 g & 50.0 g respectively? Homework Equations Lw = 3.33 x 105 J/kg cw = 4186 J/kg*C Q = m*c*ΔΤ Q =...
  39. oobgular

    How Does Altitude Affect Humidity Sensor Readings?

    Homework Statement So I sent a humidity sensor up in a weather balloon, which gave a reading for relative humidity. I eventually want an absolute humidity, but I am unsure whether I need to correct the output based on the changing density and atmospheric pressure with altitude. Homework...
  40. A

    Could you create thrust with water vapor?

    Whilst I was thinking of hydrogen combustion jet engines. I thought of a little idea... What if you could create thrust with water vapor the same way you could with the exhaust gas of a jet engine. (NOTE: I have no experience in engineering and I only have a mild interest in it). If this is...
  41. wasup23

    I How does relative humidity drop below 100%

    How is it that once the dew point is reached, moisture continues to condense. What is causing the imbalance of partial water vapor pressure? Does the rise in atmospheric temperature simply pass up the evaporation rate too quickly?
  42. mango99

    Vapor pressure of water between two heated plates on a press

    Hi, I'm trying to do some figuring on vapor pressures. If one were to take two plates heated to 220F and press water between them in a press that exerts 100psi, what would the resulting vapor pressure of the water be? Now, what temperature would you heat the plates to if you were using a press...
  43. B

    Maximum vapor velocity from a heat exchanger into a turbine

    Hi Everyone, I am looking at designing a small impulse steam turbine that runs off of saturated refrigerant vapor delivered from a shell and tube heat exchanger. With the design of the turbine, I am looking to maximize the velocity that the steam can exit from a diverging nozzle in order to be...
  44. DelfinDelfin

    Heating Water with vapor at certain pressure

    Homework Statement We wish to heat water from 10 °C to 50 °C using vapor at 180 °C and 1.5 bar of pressure. How much cold water do we need to mix with every Kg of vapor to obtain the result? Homework Equations I am not sure. I think the problem is related with exergy and energy balance The...
  45. S

    Can propane/air mixtures be safely ignited at extremely high pressures?

    Hello, I am working on a project that involves burning a mixture of propane and air under pressure to propell a projectile. This has been done by many people including myself in the past at pre ignition pressures of up to around 10 ATM. However I want to take it a step further and attempt a...
  46. R

    Why does external pressure on a liquid increase vapor pressure

    I was reading an online chemistry textbook that said "when a liquid is subjected to hydrostatic pressure (for example, by an inert, non-dissolving gas that occupies the vapor space above the liquid surface), the vapor pressure of the liquid is slightly raised." (link...
  47. N

    Thermodynamics: vapor pressure water, given the gibbs energy

    Homework Statement Calculate the vapor pressure of water at 25°C, based on the Gibbs free energy when vaporising from liquid water to vapor (so at 1 atm and 25°C ). Homework EquationsThe Attempt at a Solution After integrating d g/d p= RT/p. I get my formula p = p0*exp (-delta gm(p0, T)/RT). I...
  48. Z

    Gas Law applying to air vs water vapor

    Say we have a tire filled with air (equal to ambient air surrounding it in every way). pressure on the gauge is 0. Then, we pressurize it to 1 ATM (15psi). If we heat the tire , the air will be heated and expand based on gas law, or is there a factor that changes it based on the air's...
  49. C

    I Parts per million of vapor above a liquor in a sealed jar

    How would you calculate the concentration of a vapor above a liquid in a sealed jar in parts per million? Supposing that the liquid is a 10% solution of X with known vapor pressure. We've gotten to this point: 1ppm = 1mgX/ g solution 1ppm = 1mgX/1mL solution (assuming it's water and water has...
  50. grandpa2390

    Why does Water Vapor have 12 degrees of freedom?

    Homework Statement How many degrees of freedom does water vapor have Homework Equations Translational up to 3 rotational up to 3 Vibration up to 6 The Attempt at a Solution Well I said water vapor had 3 translational. It can move along the x, y, or z axis I said it had 2 rotational (the...