# Vapor pressure of boiling water

• silversurf
In summary: But the main dependence of vapour pressure is on temperature and (weakly) on intermolecular forces.In summary, the vapor pressure of a substance only depends on its temperature and intermolecular forces, not on external pressure. Therefore, the vapor pressure of water in a half-filled balloon at 25°C and sea level will remain at 24 mm Hg, even if it is taken to a depth where the external pressure is 2 atm. The decrease in boiling point at higher elevations is due to the external pressure being lower, not a change in the vapor pressure.
silversurf
Why does the answer say that vapor pressure only depends on temperature and intermolecular forces substance experiences? I thought external pressure did affect vapor pressure because when you heat a pot of boiling water at higher elevation the boiling point decreases, and vapor pressure increases, due to less atmospheric (external pressure). What am I missing?

Question
Inside a half-filled water balloon at 25°C and sea level, the vapor pressure of water is 24 mm Hg. What will the vapor pressure of water in the balloon be if a diver takes it to a depth where temperature is 25°C and pressure is 2 atm?

B.

24 mm Hg

Explanation:
B. The vapor pressure of a substance depends only on the temperature and the intermolecular forces that substance experiences. In particular, it does not depend on external pressure. Therefore, the vapor pressure of water will not change and 24 mm Hg is the correct answer.

Last edited:
silversurf said:
I thought external pressure did affect vapor pressure because when you heat a pot of boiling water at higher elevation the boiling point decreases, and vapor pressure increases, due to less atmospheric (external pressure). What am I missing?

You are mixing something up here. The boiling point decreases, that´s true. At the boiling point the vapour pressure is equal to the external pressure. At higher elevation, the external pressure is lower and hence boiling occurs at a lower pressure. This is not due to the boiling pressure changing with external pressure.

However you are right in that the vapour pressure also changes with in principle external pressure. This effect is quite small and you need high pressures to observe it.

## 1. What is the definition of vapor pressure of boiling water?

The vapor pressure of boiling water is the pressure exerted by the gaseous form of water molecules at the point of equilibrium between liquid water and water vapor at a given temperature.

## 2. How does the vapor pressure of boiling water change with temperature?

The vapor pressure of boiling water increases as the temperature increases. This is because higher temperatures provide more energy to water molecules, allowing them to break free from the liquid phase and enter the gaseous phase.

## 3. Is there a limit to the vapor pressure of boiling water?

Yes, there is a limit to the vapor pressure of boiling water. This limit is known as the critical temperature, and it is where the distinction between liquid and gas phases of water becomes indistinguishable. At this point, the vapor pressure is equal to the surrounding atmospheric pressure.

## 4. How does altitude affect the vapor pressure of boiling water?

Altitude has a significant impact on the vapor pressure of boiling water. As altitude increases, the atmospheric pressure decreases, which means that the water molecules require less energy to reach their boiling point. Therefore, the vapor pressure of boiling water decreases with altitude.

## 5. What other factors can affect the vapor pressure of boiling water?

Aside from temperature and altitude, other factors that can affect the vapor pressure of boiling water include the presence of other substances in the water (such as salt or sugar), the surface area of the water, and external pressure changes.

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