Vapor Pressure...?

Question

I know it's really simple, but I'm having some trouble understanding the concept of vapor pressure. Could I get a quick overview or definition? Thanks.

Answer 1

Definition:

Vapor pressure is defined only for the equilibrium state, where a substance is evaporating (if a liquid) or subliming (if a solid) to the gaseous or vapor state at the same rate it is condensing back to the liquid or solid state. The vapor pressure is a function of temperature, lower temperatures producing lower vapor pressures and higher temperatures producing higher vapor pressures.

Quick overview:

Consider water, which being a liquid or a solid at commonly encountered temperatures has a measurable vapor pressure in either state. Left out in the open (not in equilibrium) water will either continuously evaporate to a gaseous vapor or, if in the form of ice or snow, more slowly sublime to a gaseous vapor. And, being a gas, it will try to fill all the space available to it, namely the entire atmosphere. It is not in equilibrium and will return from the atmosphere to a liquid or solid only if it rains, sleets, or snows. Nevertheless, we can deduce the water must exist with some vapor pressure as a gas, because all gasses exert a pressure.

Now, put the same liquid water or solid ice in a closed container, like a sealed jar, and it will very quickly come into an equilibrium state where the water or ice is evaporating or subliming at the same rate it is condensing or freezing. Obviously if the water is in an environment where the temperature is greater than 0 C any ice present will eventually melt, leaving only the liquid to evaporate. Or if the temperature is less than 0 C, any liquid present will eventually freeze leaving only solid ice to sublime.

Whichever the case, there will be water molecules in vapor form above the surface of the liquid or the ice. These molecules exert a pressure called the vapor pressure of the water at that temperature.

It is fairly easy to measure vapor pressure if a vacuum can be drawn on the container. After pumping down to a few milli-Torr, the container is sealed off and the pressure allowed to rise (because of evaporation or sublimation) until equilibrium occurs. The pressure the container eventually reaches (assuming only liquid water above the melting point, or only solid ice below the freezing point) is the vapor pressure of water at that temperature, assuming there was negligible residual gas (air) present after pumping and sealing the container. Any residual air not pumped out will have a partial pressure that must be subtracted from the pressure measured in the container to obtain the true vapor pressure.

I have used water as an example only because it is a common and familiar substance. The concept of vapor pressure applies to ANYTHING, even things you don't think of turning into vapor, like solid steel or gold jewelry. It's just that the vapor pressure for most solid things is very, very, small so as to be almost immeasurable.

Answer 2

Dorian is on track (and from Canada or the UK- when is QE2 coming to Virginia?) If a liquid is placed in a closed vessal, some of the liquid will vaporize until equilibrium is reached between the liquid and the vapor (or vapour). The amount of the substance in vapor form will exert a pressure. At about room temperature, a vapor density of 1 mole/25 L of volume of an ideal gas exerts a 1 atm (760 mm Hg) or 1.013x10^5 Pa pressure. The portion of the gas that is the substance, will exert a proportionate part of this pressure.

Answer 3

Vapour pressure can be a bit confusing, so I'll try to give you a clear explanation. Basically, vapour pressure is the amount of pressure that the vapour of a chemical must have to remain in equilibrium with some liquid of that chemical. In a non-equilibrium situation, a higher vapour pressure means a more rapid evaporation. The underlying concepts follow below.

Consider a body of liquid (such as water) that is open to the atmosphere. Every second, molecules from the liquid water will escape and become water vapour. To maintain equilibrium, an equal number of water molecules in the vapour phase must strike the liquid surface and join the liquid phase.

We can calculate the number of molecules that will escape using the temperature, the latent heat of vapourization, and the Boltzmann distribution. We then calculate the number density of vapour molecules needed to balance this. Using the ideal gas law, P = nkT, we can then calculate the necessary pressure that the vapour molecules must have. This is the vapour pressure.

The system doesn't have to be in equilibrium for the vapour pressure to be meaningful, though. If there is no water vapour in the air around a glass of water, water molecules will escape at the same rate as above. With no water vapour to balance this, however, the amount of water in the liquid phase will steadily decrease. Because the equilibrium vapour pressure is calculated directly from the number of molecules that escape in a given time, it tells us how fast the liquid will evaporate in the absence of vapour. Thus, a higher vapour pressure means the liquid will evaporate more rapidly.

Answer 4

A quick overview.

Vapour pressure: All liquids have a vapour pressure.

In a closed vessel, a liquid will vaporise until the number of molecules leaving the liquid is equal to the number returning.

The liquid and vapour are said to be 'In Equilibrium'.
The vapour pressure exerted is generally measured in mmHg
and is called the 'Saturated' vapour pressure.

As the liquid temperature is increased, the vapour pressure increases until, at a specific temperature, equilibrium is reached again.

When the vapour pressure is equal to that of the atmosphere, the liquid is said to be boiling.

E.g. At 100°C (212°F), the vapour pressure of water is 760mmHg (14.7 psi) in an open container.

In a closed container, increasing temperature to a specific temperature above the boiling point will increase the pressure until, at that specific temperature equilibrium is again reached.

In this case, the liquid is Superheated. But the vapour is still saturated,

E.g. Water at 486°F will have a Saturated Vapour Pressure of around 600 psi.

Answer 5

Hello, The partial pressure of a gas is related to its pressure in comparison to other gases around it. For example, the pressure created by oxygen in the atmosphere and the pressure created by nitrogen in the atmosphere add up to nearly the total pressure. The vapor pressure of a liquid is the pressure its vapor exerts when the liquid is placed in a closed container. In this case, the gas is on its own, and it can change with temperature. For example, there is water vapor filling up the empty space in a water bottle, it constitutes for the vapor pressure inside that bottle.

Answer 6

As you know, all the liquids tend to evaporate to form the gaseous form, the vapour, and the gases have tendency to condense back. At a certain temperature, the equilibrium is reached. This means that the number of molecules of the liquid that evaporate per time unit (let's say, 1 second) is the same as the number of the molecules of the vapour that condense, in 1 second, back to the liquid state. At this temperature, the molecules of the vapour cause the pressure that is called the vapour pressure.