What is Vapor pressure: Definition and 113 Discussions

Vapor pressure (or vapour pressure in British English; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid). A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The pressure exhibited by vapor present above a liquid surface is known as vapor pressure. As the temperature of a liquid increases, the kinetic energy of its molecules also increases. As the kinetic energy of the molecules increases, the number of molecules transitioning into a vapor also increases, thereby increasing the vapor pressure.
The vapor pressure of any substance increases non-linearly with temperature according to the Clausius–Clapeyron relation. The atmospheric pressure boiling point of a liquid (also known as the normal boiling point) is the temperature at which the vapor pressure equals the ambient atmospheric pressure. With any incremental increase in that temperature, the vapor pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form vapor bubbles inside the bulk of the substance. Bubble formation deeper in the liquid requires a higher temperature due to the higher fluid pressure, because fluid pressure increases above the atmospheric pressure as the depth increases. More important at shallow depths is the higher temperature required to start bubble formation. The surface tension of the bubble wall leads to an overpressure in the very small, initial bubbles.
The vapor pressure that a single component in a mixture contributes to the total pressure in the system is called partial pressure. For example, air at sea level, and saturated with water vapor at 20 °C, has partial pressures of about 2.3 kPa of water, 78 kPa of nitrogen, 21 kPa of oxygen and 0.9 kPa of argon, totaling 102.2 kPa, making the basis for standard atmospheric pressure.

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  1. 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...
  2. 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...
  3. 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...
  4. 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)...
  5. 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...
  6. okandrea

    Engineering How do I calculate the overall vapour flow?

    Step 1: Values are from textbook 'Building for a Cold Climate' Mbrick = 46 ng/s*Pa*m^2 Mgypsum = 2870 ng/s*Pa*m^2 (for 9.5mm) took the above value and used ratios to determine permeance for 10mm Gypsum board (2870/9.5 = X/10) Mgypsum (new value) = 3021.05 ng/s*Pa*m^2 Step 2...
  7. 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...
  8. M

    How does butane gas flow from a canister to refill a lighter?

    when butane lighters are refilled by external canister, HOW and WHY butane gas flows from canister to lighter Is it due to GRAVITY or VAPOR PRESSURE? If GRAVITY is the reason I have posted a link where torch is held above the canister? If it is vapor pressure, it is going to be same is both...
  9. 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...
  10. 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} =...
  11. 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?
  12. 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...
  13. D

    Exploring the Effects of Vacuum Pressure on Boiling Oil | Dilip

    Hi, We have seen water boiling at low pressures (vacuum jars), Will Oil also boil the same way in vacuum jar? Dilip
  14. 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...
  15. 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...
  16. 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...
  17. 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...
  18. Umar Awan

    Fractional distillation calculations

    Say I have got a super cooled cylinder of half hydrogen and half helium. This cylinder has a pressure of 100,000 Pascal's. At this pressure, hydrogen boils at 20 Kelvin and helium at 4.21 Kelvin. I hope to separate helium by cooling gas down to 4.21 Kelvin but I know that even at the...
  19. 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...
  20. 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...
  21. Ravi Singh choudhary

    What pressure not increases if I seal the container?

    CASE1: A container with some water is open to atmosphere, pressure is definitely atmospheric, no somehow I put cover over it and sealed it also. Pressure would increase because new equilibrium will establish between water and its vapour so pressure will increase in the closed container. CASE2...
  22. Ravi Singh choudhary

    Velocity of liquid from bottom pinhole of closed container

    What I know: Below link is about Torricelli's law. Velocity of liquid coming out of bottom of the tank i.e comes after using Bernoulli's equation square root of (2*g*h*) where "h" is height of fluid in the container and "g" is acceleration due to gravity...
  23. 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)
  24. 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...
  25. 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...
  26. 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...
  27. 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...
  28. 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...
  29. 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...
  30. 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...
  31. B

    How Do Mole Fractions Determine Vapor Pressure in a Binary Solution?

    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%...
  32. 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...
  33. 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...
  34. 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 ?
  35. 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)...
  36. 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...
  37. 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...
  38. 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...
  39. 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...
  40. J

    Vapor Pressure and non ideal gases

    Hi, I understand that vapor pressure is independent of initial pressure, and depends only on temperature. However, is this true of a non ideal gas at high pressures? (I am specifically interested in the vapor pressure of a meg/water mixture at approx. 100 bar), Thanks
  41. B

    Vapor Pressure, Open Container, and Boiling

    I've been confused about this for a while now and haven't found anything that directly addresses this problem. Please correct any faults you see in my reasoning. My understanding of vapor pressure is that it is the pressure exerted by the vapor phase of a substance above the liquid phase...
  42. aleksbooker

    How do you calculate density of a vapor only vapor pressure and temp.

    Hello all, The question given is: Mercury and many of its compounds are dangerous poisons if breathed, swallowed or even absorbed through the skin. The liquid metal has a vapor pressure of 0.00169mmHg at 24 degrees Celsius. If the air in a small room is saturated with mercury vapor, how many...
  43. Q

    Solving a Vapor Pressure Problem: Find Hydrogen Volume at 23°C

    Homework Statement A 425-mL sample of hydrogen is collected above water at 35°C and 763 torr. Find the volume of the hydrogen sample when the temperature falls to 23°C, assuming the barometric pressure does not change. (vapor pressures of water : at 35°C, 42.2 torr ; at 23°C, 21.1 torr)...
  44. Q

    How Much CCl4 Remains Liquid at Equilibrium in a Sealed Flask?

    Homework Statement If 5.00 mL of liquid carbon tetrachloride (CCl4, density = 1.587 g/mL) was injected into a sealed 5.00 L flask at 30.0°C, what volume (if any) of the CCl4 would remain as liquid after equilibrium is reached? (the vapor pressure of CCl4 at 30.0°C is 143.0 mmHg) Homework...
  45. Q

    Vapor Pressure and Raoult's Law

    Homework Statement The vapor pressure of pure water at 45 degrees C is 71.9 mm Hg. What is the vapor pressure when 18g of sucrose (MM = 342.3g/mol) is added to 82g of water? Homework Equations VP of solution = mole fraction of solvent * VP of pure solvent The Attempt at a Solution...
  46. Q

    How Does Raoult's Law Apply to Calculating Vapor Pressure in Solutions?

    Homework Statement http://i.minus.com/juL6gvGzvWdm.png Homework Equations Raoult's law: Pressure of solution = Mole fraction of solvent multiplied by vapor pressure of pure solvent. We're assuming non-volatile solutes. The Attempt at a Solution My solution...
  47. LunaFly

    Vapor Pressure Curve on the PVT Diagram

    Hi all, I am wondering about where the vapor pressure curve would be located on the PVT surface of a substance. I know that as the temperature of a liquid increases, its vapor pressure increases. This continues until the vapor pressure is equal to the atmospheric pressure, at which point...
  48. M

    How Does Pressure Remain Constant During a Phase Transition?

    In reference to the graph at the bottom, my textbook has a statement '''..A process of vaporization or condensation at a constant temperature is illustrated by the lines G-H-I or I-H-G, respectively, in figure 16.3. Water would vaporize or condense at a constant temperature as the...
  49. J

    Vapor pressure + air pressure?

    Hi, could someone please tell me if the vapor pressure should be combined with the air pressure in a sealed vessel to give the total pressure? For example, if I put room temperature water in a test tube (at sea level) with some head space for air and then seal it, the pressure inside would be 1...
  50. J

    Predict which of the solutions below will exert the highest Vapor pressure

    Homework Statement Hi I have a question. Predict which of the solutions below will exert the highest Vapor pressure O2 in HOH NH3 in HOH and CH3OH in HOH Homework Equations I drew them out. So I know that CH3OH HOH exhibits h bonding. So does NH3 HOH I was wondering about O2...