What is Vapour pressure: Definition and 38 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. B

    Saturated vapour pressure vapour quality and T-S diagram location

    Hi all., Just hoping to get a better fundamental insight into a few things. If we start with this: http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/vappre.html so we have a closed container at a given temperature, then we can find it's saturation pressure. All good so far. In the following...
  2. D

    Chemistry What is the Correct Formula for Calculating Vapour Pressure of Solutions?

    Isn't the formula to be applied ##P= \chi_{solvent}P^o _{solvent}##? in which ##P^o _{solvent}##= 18.7mmHg and ##\chi_{solvent}= \frac{mol_{solvent}}{mol_{solvent}+mol_{solute}}=\frac{9.0 mol}{9.0 mol+0,50 mol}## but seems doesn't leads to the expected result
  3. J

    Understanding Vapour Pressure & Temperature Difference

    I have a problem at the very beginning. I don't know how to relate this vapour pressure to the temperature difference. I have read the hint: Recall the idea 7: for dynamical processes, at first, a mechanical equilibrium is reached, which means the equality of pressures; the other equilibria...
  4. A

    Vapour pressure of water vapour in air

    Vapour pressure in a closed system. Why does the partial pressure of water vapour in air equals to the equilibrium vapour pressure of liquid?
  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. Rahulx084

    Various Pressures in mass transfer

    I'm confused between partial pressure, total pressure, boiling point , vapour pressure , saturation pressure and saturation temperature.These things haunt me in my subject mass transfer. I get really confused when this words come in a liquid - vapour system, I cannot identify what these pressure...
  7. W

    Partial Pressures and Vapour Pressure

    Homework Statement Two systems in diffusive equilibrium have equal chemical potentials. We can use this fact to solve the following problem. We begin with a closed system consisting of a liquid such as water in diffusive equilibrium with its vapour. At the start, only the liquid and its vapour...
  8. Ravi Singh choudhary

    Vapour pressure vs saturation pressure; too much confusion

    It was only the psychrometry came; I read about vapour pressure is equal to the saturation pressure at 100% relative humidity. While before even in the textbooks both terms are used frequently as same. I fully understood what saturation pressure is; learned during phase change phenomenon of...
  9. C

    How Does Vapour Pressure Change at Dew Point for Benzene-Toluene Mixtures?

    THERE IS NO TEMPLATE BECAUSE GHIS HOMEWORK WAS PLACED IN AN INCORRECT FORUM Equimolar benzene and and toluene form essentially ideal solutions. At 20degC the vapour of benzene and toluene are 9.866kPa and 2.933kPa respectively. The solution is boiled by reducing the external pressure below the...
  10. jmarkwalker

    Evaporating water using heat and reducing vapour pressure

    I want to reduce the amount of water present in the digestate coming out of my Anaerobic Digester. The traditional method would be to boil it until I've removed as much off as I want. However, this is expensive from an energy consumption point of view. There's a great video on Youtube showing a...
  11. A

    Estimating Vapor Pressure of Mercury at 25°C Using Appendix D Data

    Homework Statement Estimate the vapor pressure of mercury at 25 ∘C (use data from Appendix D from the textbook). Homework Equations Using the appendix I can find ΔG, ΔH, and ΔS. The Attempt at a Solution I have no idea where to start. I think that maybe vapor pressure would be found when...
  12. L

    Determining boiling point from vapour pressure

    Homework Statement When benzene freezes at 5.5 C its density changes from 0.879 g cm-3 to 0.891 g cm-3. Its enthalpy of fusion is 10.59 kJ mol -1. Estimate freezing point of benzene at 1000 atm Homework Equations Solid liquid boundary: p=p*+ΔHfus/ΔV ln(T/T*) The Attempt at a Solution From the...
  13. P

    Saturation vapour pressure equation formula

    Pws=1000⋅e[[52.58]-[6790.5/T]-[5.028lnT]] T= temprerature in Kelvin Pws= Saturation vapour pressure I plugged in my temperature to the formula in Kelvin, I couldn't arrive at the correct answer. How do you get the e value of the equation (superscript above). My problem could be the trigonometry...
  14. Eclair_de_XII

    Am I doing this vapour pressure problem correctly?

    Homework Statement 'The vapour pressure of ethanol (C2H5OH) at 19°C is 40.0 torr. A 1.00-g sample of ethanol is placed in a 2.00 L container at 19°C. If the container is closed and the ethanol is allowed to reach equilibrium with its vapour, how many grams of liquid ethanol remain?' Homework...
  15. A

    Boiling point and vapour pressure

    Hi all, I am facing difficulty in understanding the reason behind the fact that a liquid boils when the atmospheric pressure is equal to its vapour pressure. We know that at the vapour pressure, the air and the liquid remain in equilibrium, so there's no net evaporation or condensation. So...
  16. S

    Non-ideal solution and vapour pressure

    When there is a positive or negative deviation it is due to the interactions between A-A and B-B being weaker or stronger than the interactions between A-B. However, are there any scenarios where the interactions between A-A are stronger than A-B while B-B weaker than A-A or vice versa? In such...
  17. A

    Finding vapour pressure using compressibility chart

    Homework Statement I was asked to find the vapour pressure and saturated liquid molar density of propane at 263.15K using a generalised compressibility chart. (not allowed to use NIST or steam tables either, the chart i was given does not have reduced volume lines) Homework Equations Tr=T/Tc...
  18. G

    Saturated vapour pressure - why an equilibrum is reached?

    Hello, I've read about the concept of saturated vapour pressure on some sites but none of them really explain why the equilibrum happens and that's what I want to ask you. Why the number of particles leaving the surface and joining the surface can't change over time and never reach the equilibrum?
  19. M

    Relationship of vapour pressure and Dew Point

    Homework Statement Moist air at 32 degrees C has a dew point of 26 degrees C. The total or atmospheric pressure is 100.7kPa. find the: Molal Humidity Relative Humidity Homework Equations Molal Humidity = \frac{P_{i}}{P_{tot} - P_{i}} Relative Humidity = \frac{P_{i}}{P_{vap}}...
  20. M

    Vapour Pressure: Exploring How Molecules Escape Into Gas Phase

    I read in my Chemistry workbook that: "At higher temperatures, vapour pressure increases as more energy is provided to the liquid molecules, allowing them to escape into the gas phase." At the boiling point, all energy is gone into breaking bonds and to convert liquid phase molecules into...
  21. J

    Vapour Pressure: Pressure Exerted on Liquid & Container

    According to Wikipedia, vapour pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. Does that mean the pressure the vapour exerts on the container or the pressure the vapour exerts on the liquid?
  22. K

    How to Calculate Vapour Pressure Changes with Temperature?

    Homework Statement This is problem in Mandl's book on Stats Phys; The vapour pressure of water at 298.15 K is 23.75 mmHg. What is the vapour pressure of water at 273.16 K, given that the latent heat of evaporation of water at 298.15 and 273.16 K is 43,991 and 45,049 J/mol respectively?Homework...
  23. A

    Vapor Pressure: What Affects Gasoline Evaporation?

    Hi all, I have a question on vapor pressure. As far as I understood, a liquid will flashes into gas when the absolute pressure it is experiencing (in a pipe / in the atmosphere) is lower than the liquid's vapor pressure (assuming temperature is constant). Everyone knows that gasoline forms...
  24. N

    Determing vapour pressure experimentally - how?

    Homework Statement I am supposed to determine the vapour pressure experimentally of a series of prepared solutions, which are a mixture of 2 different substances in different proportions. This involved attempting to create a "vacuum", placing the liquid inside and then measuring the change in...
  25. Z

    Lowering of Vapour Pressure - a C.P ?

    Is lowering of vapour pressure a colligative property? I tried proving this and found it to be true. Pa = Poaχa, where a=solvent and b=solute Substituting χa=1-χb and some rearranging, we get Poa-Pa=Poaχb As Poa is constant for a given liquid at a given temperature, lowering of...
  26. 5

    Why is the Vapour pressure of Water Constant ?

    Now if i have the vapour pressure of water to be 200mm and the water occupies 4 L out of 20 L closed vessel.Which implies that the Vapour occupies 16 L Now if i shift the water to a 30 L closed vessel.The vapour pressure of water vapour remains the same even though it occupies a greater volume...
  27. R

    Vapour pressure and volatility.

    Homework Statement (I)Explain how each of the following factors affect the vapour pressure of a liquid: (a)surface area, (b)volume, (c)attractive intermolecular forces, (d)temperature, (e)density of liquid. (II)Order the following substances in terms of volatility: CH4, CBr4, CH2Cl2, CH3Cl...
  28. T

    Osmosis, osmotic pressure, vapour pressure

    Why does osmosis happens? Does osmosis happens when we have osmotic pressure difference between the two solutions (that are connected by a membrane)? Does osmosis happens when we have two solutions of different concentrations? My professor told that osmosis happens because of the vapour...
  29. N

    Vapour Pressure & Liquid Phase: Explained

    If the vapour pressure of a component is bigger than the system pressure, does it means it will fully in vapour phase and no liquid phase are formed??
  30. R

    CO2 vapour pressure passed critical temperature

    I've been looking at phase diagrams and vapour pressure tables for CO2 and i notice that the data stops at the critical point. what would happen to the pressure of CO2 if I heated it to above the critical temperature and kept the volume constant? Would the pressure increase following the...
  31. G

    Vapour pressure problem (simple)

    The vapour pressure of pure chloroform at 70.0oC is 135.7 kPa. What mass of iodine (I2) should be dissolved in 1 L of chloroform, CHCl3 (density = 1.49 g/cm3) to lower the vapour pressure by 13.3 kPa? hey guys i really can't do this question, i think in involves raults law, P=XPA anyhelp...
  32. E

    Calculations using Saturated Vapour Pressure

    Hi all, new here, so don't know if this is the right place. All the other places in the forum seemed like advanced stuff! Anyway, i don't understand why when calculating the partial pressure of a gas in SVP conditions, you subtract 6.28kPa from the normal atomospheric pressure of 101.1kPa...
  33. T

    How Does Introducing Gas into a Torus Affect Vapour Pressure?

    Hi... I found this site a lot time ago and sometimes I feel like viewing your problems, maybe in a future I'll be active... I want to say that this IS NOT an homework, but it's an exercise I was never able to solve and so I try to put it here, even if I don't know whether it is the right section...
  34. E

    Vapour Pressure| Boiling| Atmospheric P

    Ok I do not get WHY an object boils when atmospheric pressure is equal to Vapour pressure. Can anyone explain this? In my textbook it doesent explain it at all.
  35. K

    HELP A question on vapour pressure

    HELP!A question on vapour pressure Homework Statement The vapor pressure of dichloromethane, CH2Cl2, at 0 degree Celsius is 134 mmHg. The normal boiling point of dichloromethane is 40.0 degree Celsius. Calculate its molar heat of vaporization. Homework Equations The Attempt at a...
  36. R

    Plexiglas (PMMA) Vapour Pressure: Seeking Info

    Hi everybody. I would like to know the value of vapour pressure of plexiglas (PMMA) at ambiant temperature. For the time being I've not found that characteristic on the net.
  37. H

    Vapour Pressure and External Pressure

    When the vapour pressure of a liquid equals to the external pressure, the liquid will boil. So, what if a liquid is placed in a vacum? Will the liquid boil instantly when heated?
  38. E

    How Does Surface Curvature Affect Saturation Vapour Pressure?

    The pressure exerted by a saturated vapour depends on the temperature and the curvature of the liquids surface. Why does it depend on the curvature of the liquids surface?