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. Z

    Why isn't the temperature of this vapor T_sat?

    A rigid tank with a volume of 2.00m3 contains 5.77 kg of saturated liquid-vapor mixture of water at 75 deg C. Now the water is slowly heated. Determine the temperature at which the liquid in the tank is completely vaporized. Why isn't the temperature just 75C? If it is already a mixture in...
  2. A

    Vapor Pressure In Closed Cylinder

    Hi There, I am trying to understand if I have a vacuum in the below scenario or simply just really low pressure. A pump supplies a pipe with a constant supply of water at one end (Point A) the water discharges several kilometers away at some lower elevation (Point B). At "Point A" a valve...
  3. The Jason 1970

    Calculating Power Output of a Vapor Core Reactor

    Hi I am someone researching vapor core reactors from a historical perspective. I have a technical question. In a congressional document I found it is stated that a vapor core reactor would have 35kW of power per square inch. Am I right in calculating that a cubic inch of a vapor core reactor...
  4. M

    Stoichiometric air/fuel ratio of gasoline vapor

    Hello, Can someone tell me the optimum stoichiometric air/fuel ratio of gasoline VAPOR? I know that with liquid gasoline the optimum stoichiometric air/fuel ratio is 14.7 parts ambient air to 1 part gasoline. Thank you in advance for your help.
  5. M

    Question about vapor pressure....

    A QUESTION. Suppose I enclose water within a large noncombustible substance, say a block of concrete. The water is completely enclosed and sealed = no way for it to escape. Now, I heat the block. I get it very hot. At any temperature I can raise it to, will it still be just water at the...
  6. E

    Does Pressure in Phase Diagram refer to vapor pressure?

    I am somewhat confused by what pressure refers to in a phase diagram? In a closed box it makes sense to me that the vapor pressure would eventually equilibriate at a pressure determined by the temperature. However, say you have an open box. It makes sense that the liquid would boil when the...
  7. S

    Mixture of Water & Kerosene: Vapor Pressure?

    When we mix water and kerosene, what is the vapor pressure of the mixture? Is it just that of kerosene, or an average of vapor pressure of both liquids? (Kerosene floats on top of water which is why I felt like asking this question)
  8. Ravi Singh choudhary

    Difference between Partial pressure and vapor pressure

    Partial pressure must be less than or equal to the vapor pressure if there is no liquid present. However, when both vapor and liquid are present and the system is in the phase equilibrium, the partial pressure of the vapor must be equal to the vapor pressure and system is said to be saturated...
  9. G

    Why are there still bubbles after the pump?

    Hi everyone, Got a question I'm trying to solve here. It is for an experiment I'm running at achool. I got a pipe system with a centrifugal pump. There is cavitation in front of the pump due to the pressure falling below the water Vapor pressure. When the water flow enters to the...
  10. L

    Why is the pressure of fluid important for vapor pressure and cavitation?

    I straggle with getting full grasp of concept for Cavitation. I was finding statement that : If The pressure of fluid is lower or equal to the Vapor Pressure then it will start to boiling. And I can't understood the reason behind this. Why actually it happens? I understood where we came to the...
  11. Biker

    How Does Vapor Pressure Cause Water to Boil?

    I have searched about this topic all over the internet and non of them seem to explain how vapor pressure is equal to atmospheric pressure. All I need is some forces diagrams and some explanations. How when we increase the vapor pressure it makes the liquid boil faster? Isnt the vapor pressure...
  12. B

    Ice sublimating to water vapor

    Suppose an ice cube is in a vessel and it is at a temperature slightly below 0 degrees Cesius. Suppose further that the inside of the vessel is a vacuum. If the temperature of the vessel is increased to slightly above zero Celsius and the vacuum is maintained would the state change in the ice...
  13. D

    Permeation of water vapor through seals: vacuum vs dry gas

    From reading about the permeation of gasses through polymer seals, I am led to believe that given two identical sealed chambers, one containing a vacuum and the other pressurized perfectly dry nitrogen, water vapor would permeate through the seals and into the chambers at the same rate. This...
  14. L

    Nylon deterioration from hydrogen released water vapor?

    Hello, I am by far not very firmiliar with physics nor engineering. I had an idea a couple years ago involving the replacement of the propane in hot air balloons with hydrogen gas as the fuel to burned to provide the heat to lift the balloon. One thing I didn't realize back then was that...
  15. K

    Does Condensation of Water Vapor Constitute Heat Transfer?

    Scenario A: An icicle on the eaves of Uncle Ivan’s dacha in far off Verkhoyansk vaporizes a number of water vapor molecules at an ambient temperature of -80°C. Over the succeeding months, one of these molecules makes its random way to the surface of a cloud droplet over Ouagadougou. Along the...
  16. S

    How to calculate amount of vapor given q(t) and p(t)?

    Hi everybody, I am trying to build a small model which basically should be able output "mass flow of water vapor as a function of time" given following inputs: - initial mass liquid water m_l_0 [kg] - initial temperature of liquid water T_l_0 [°C] - initial pressure p_0 [Pa] - heat added as...
  17. M

    I Question about converting propane vapor volume to liquid gallons

    Hello, I work for a propane company where i unload propane rail cars. I am trying to figure out how much propane is being shipped back to the refinery after I off load the rail car. We are experiencing a large amount of shink and the accountant want to get to the bottom of it. With propane...
  18. J

    Adiabatic expansion of a saturated vapor mix: Δx, ΔS

    Homework Statement A saturated water vapor mix in a 0.05m3 piston-cylinder at 200C with a quality of 0.5 expands adiabatically, producing 500kJ work with a final temperature of 50C. Find: a) final quality b) change in entropy[/B] Homework Equations Q-W=ΔU x=(vavg-vf)/(vg-vf) uavg=uf+xufg...
  19. B

    How Do I Analyze a Vapor Power Cycle for My Thermodynamics Project?

    Hey guys, I've been given this mini project for my thermodynamics2 class. It's supposed to be a really good example of how the upcoming exam will be. The downside is i don't really know what values I should pick. I really need help with strictly the analysis part of this project. If anyone could...
  20. J

    Reading Steam Tables for Saturated Liquid Not Vapor

    Hello, I have an assignment that centers around the Rankine cycle and requires me to find the enthalpy of water entering the boiler. My professor has told me to take the enthalpy value at boiler conditions for a saturated liquid. I'm not really sure why I should make this assumption as I was...
  21. G

    Mercury Vapor Lamp: Why 20-30 Minute Delay?

    I've been reading up on mercury vapor lamps, which are often used as a light source in fluorescence microscopes. I still don't understand why it decreases the lamp's lifetime if you turn it on and switch it off without waiting 20 - 30 minutes. Is there a reason for this?
  22. J

    Vapor Pressure of saturated/unsaturated solution

    Homework Statement : As the picture shows, there are two NaClaq solutions. One is saturated while the other is not and both of them are at the same temperature. There are three questions asked: a) Above which solution is the vapor pressure of water, PH2O, greater? Explain. b) Above one of these...
  23. Tam Le

    Gas State of Liquid Substances: Exploring Water Vapor

    Is it simply the gaseous state of a liquid substance? For instance, water vapor, in this context, would be the gas state of water after having transitioned from a liquid state.
  24. B

    Understanding Vapor Pressure in Open Systems

    I learned that vapor pressure is defined in a closed system, but some questions ask me to consider vapor pressure in an open system. How is vapor pressure defined exactly? And where is this pressure acting against? Also why is it that at the boiling point, the vapor pressure and the atmospheric...
  25. MexChemE

    Converting humidity ratio to specific humidity

    Hello PF! I have a very simple question on psychrometry. I had an argument with one of my professors regarding these two quantities, humidity ratio and specfic humidity. Humidity ratio is defined as mass of water vapor per unit mass of dry air. It's the one you can read from a psychrometric...
  26. M

    Is there a difference between water vapor and wet steam?

    Both water vapor and wet steam contain both contain tiny droplets of water particles(correct me if I'm wrong).Does it mean both water vapor and wet steam are same and one ?
  27. P

    Is the mass flow rate in a regenerative vapor power cycle constant?

    Does the mass flow rate differ depending on whether it's after the feedwater heaters or before the turbines or is it just constant throughout the entire system?
  28. Sirluke

    Vapor pressure curve for carbon dioxide

    Hello to everyone. I would like to ask a question: I'm analyzing the properties of CO2 for a project and I have noticed that for temperatures between 220 -300 K the Clasius Clapeyron curve is linear on 1/T, that is in the Antoine equation of the form logP = A-B/(T+C), C is almost 0. In my...
  29. U

    Is an asthma inhaler (MDI) an example of vaporization?

    I am trying to give some context to medical vaporizers in a literature review I am composing and initially decided to give asthma inhalers and nasal inhalers as examples. I realized however, that some sources on-line were referring to these metered dose inhalers as vaporization devices when...
  30. J

    Unbiased Physical Vapor Deposition

    I apologize if this seems ignorant, but I am at a loss and I need some assistance. I recently built a small vacuum PVD system (Ion source, argon, target material, substrate cradle, etc.) and was showing it to a colleague when it was noticed that my system was unbiased. It actually didn't occur...
  31. Z

    Ideal gas law combined with vapor pressure

    I am working on a DIY project and want to understand the system/mathematical model of a traditional steam espresso machine. How I have started to think about the problem is a closed container with liquid and gas. At room temperature I know the vapor pressure of the water. As I increase the...
  32. B

    Solutions and vapor pressure

    Homework Statement Benzene and toluene combine to form an ideal solution. At 80 C, vapor pressure of pure benzene is 800 mmHg and the vapor pressure of pure toluene is 300 mmHg. If the vapor pressure of the solution is 400 mmHg, what are the mole fractions of benzene and toluene? (A) 60%...
  33. Elvis 123456789

    Grams of solute needed to lower vapor pressure of solvent?

    Homework Statement Some KCl is dissolved in water at 25°C, where it completely dissociates. The vapor pressure of pure water at 25°C is 28.3 mmHg. Estimate the mass in grams of KCl needed per liter of pure water to reduce the vapor pressure of water at 25°C by 5%. Homework EquationsThe Attempt...
  34. MexChemE

    Psychrometry -- Ethanol in dry air

    Homework Statement Ethanol evaporates in a 10 x 20 x 50 ft3 room. Air is initially dry at a temperature of 30 °C and 765 mmHg of pressure. a) If the partial pressure of ethanol after vaporization is 40 mmHg, how many pounds of ethanol evaporated? b) The same air is compressed until it reaches a...
  35. gracy

    Vapor pressure in open container

    Vapor pressure in closed container is well understood by me.But in closed vessel ,it is a nightmare for me.However, in an open container (e.g. pot on a stove) equilibrium cannot be reached due to the vapor being dispersed into the atmosphere .The definition of boiling is when vapor pressure...
  36. K

    Partial pressure of water vapor

    Homework Statement The partial pressure of water vapor in air at 200 is 10[mm] mercury. according to the table of partial pressures we have to cool the air to 11.40 in order to bring the air to saturation, that is100% relative humidity. this according to the book. But when we cool the pressure...
  37. gracy

    Vapor Pressure: Closed-Systems Only? Open or Closed?

    Vapor pressure possible only in closed system? b.p is when vapor pressure become equal to atmospheric pressure in which system closed or open ?
  38. M

    Thermodynamics [Constant Vapor Pressure Problem]

    Homework Statement There are expanded 0.90kg/s of steam at constant pressure from 3MPa and 70% quality to a final state. If the process is nonflow for which W = 121.3kJ/s, find (a) The final temperature, (b) Q, (c) the available part of Q for a sink temperature of to = 27 Celsius Answers : a)...
  39. P

    What is the relation between the density of vapor and the

    hi guys my question is if you have few grams of alkali metals and vapored it , what is the mathematical equation that links between these variables density vapor , the mass and the temperture ?? can you help me ?
  40. H

    Ammonia Gas: Vapor Pressure > Atmospheric Pressure

    This is for "ammonia". The problem says that ammonia is a gas at room temperature. And that this tells us: "the fact that ammonia is a gas at room temperature tell us that vapor pressure of ammonia must be greater than atmospheric pressure". I know that for water, when the boiling point is...
  41. R

    Solving Air Lock / Vapor Lock Puzzle: A Guide

    Hello- First post here, I can't seem to wrap my head around this concept and how to overcome it. I read here: https://www.physicsforums.com/threads/air-lock-concept.83124/ that air's compressibility is what stops flow, why is this? I also read that a stronger pump (in a vehicle's fuel system...
  42. M

    Which Liquid Would Form the Densest Vapor Upon Heating?

    Homework Statement Which of the following liquids would form the densest vapor upon heating? A. CH3CH2CH2OH B .CH3(CH2)3CH2Cl C. CH3(CH2)4CH2Br D. (CH3)3CCH2OH Homework Equations None really The Attempt at a Solution Now I know the answer already, its C. My question is why? I got B, my...
  43. E

    Can You Explain Vapor Pressure and Partial Pressure in Simple Terms?

    I have recently been reading about vapor pressure and my mind's already blown. I just want to listen from you guys what is vapor pressure in a simple manner. I want to know where is it's direction. Towards the liquid. Let's consider an opened bottle of water which doesn't seem to evaporate at...
  44. H

    Vapor Pressure vs Atmospheric Pressure

    Homework Statement If the boiling point is the point at which vapor pressure > atmospheric pressure, so all of the water molecules can break free and fly into the atmosphere (i.e. overcoming the atmospheric pressure), then why is it that when atmospheric pressure > vapor pressure, the...
  45. W

    Pressure, temp, volume change of water vapor, and steam table clarity

    Homework Statement Two pounds of water vapor at 30 psia fill the 4ft^3 left chamber of a partitioned system. The right chamber has twice the volume of the left and is initially evacuated. Determine the pressure of the water after the partition has been removed and enough heat has been...
  46. M

    Water vapor below 100 degrees?

    hi all My question is: In a pressurized (close to 1.1 atm; B.P. 109 degrees), closed system of water mixed (in a ratio of 1000 lts to 20lts) with Boron Nitrite (yes I am talking about cooling water system for automobiles) where the temp of the mixture does not exceed 96 degrees Celsius; will...
  47. W

    How high can you push water vapor from solarthermal desalination?

    Hi all, Yet another noob question from me... I been thinking of Solarthermal desalination plant lately but I'm surprised that no plan seems to take into account the possibility of letting the vapor rise to a much higher level, thus creating a potential energy source. Is there something...
  48. powerof

    Calorimetry: mix of ice, water vapor inside a copper container

    Homework Statement We insert into a copper container (weighing 1.5 Kg) 3 Kg of water vapour at 100 ºC. Inside the container there are 10 Kg of ice at -10ºC. Find the ΔT when the system reaches the equilibrium. Known data: the specific heats of water, copper and ice and the latent heat of...
  49. A

    How can i separate water vapor from biogas

    Hi I need to know how i can separate the water vapor from biogas i know that there is separation of H2S and Co2 but what the procedure to remove water vapor from biogas ? thank you :)
  50. U

    Variation of vapor pressure with temperature

    Homework Statement The following link shows the variation of vapor pressure with temperature. In which way the data should be plotted to obtain a straight line? http://www.chem.purdue.edu/gchelp/liquids/vpress.html The Attempt at a Solution For this, I need to know exactly the...
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