# Why water vapors exerts pressure rather than diffusing in air before reaching the boiling point of water?

I am discussing only two types of vaporization in my question that are, evaporation and boiling. We know that evaporation is the phenomena which describes vaporization of liquid under a certain temperature. And boiling point is the temperature at which vapors of liquid exceeds their pressure by $760\ \mathrm{Torr}$ and starts escaping. The liquid doesn’t absorb more heat and utilize all of the heat to change its state and is said to boil.

There are more details about these two kinds but there is no need to discuss them right now so we should go back to the question that asks that why vapors exerts pressure rather than diffusing in atmosphere.

• What do you mean by "diffusing in air". If you consider evaporation diffusing, it does occur. However, boiling only occurs when the vapor pressure equals that of the atmosphere. However, water molecules are constantly lost even before the boiling point. I'm sorry if I misunderstood you but it would be nice if you could clarify. – Andy Apr 12 '15 at 20:53
• NO you get me exactly right! you have made my thought bit stronger. I must be doing a mistake! – Sufyan Naeem Apr 12 '15 at 22:03

## 3 Answers

I be mistaken, but are you referring to vapor pressure?

Vapor pressure or equilibrium vapour 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. (http://en.wikipedia.org/wiki/Vapor_pressure)

What is a closed system?

A system in which no mass is transferred.

Thus, vapor pressure would not be exerted if the container is open and the liquid is constantly diffusing. It would only occur in a closed system.

Therefore, liquids DO diffuse (evaporate) in air, rather than exert vapor pressure.

You can leave a bottle of water, one with a lid and one without. The one without the lid will soon be without water, but the one with a lid would remain with water.

*Vapor pressure does relate to evaporation rate, which occurs in the open systems we normally observe in life

• Vapors must exert a pressure on the blanket of atmosphere to get escaped. However during this rising-temperature process, some of the vapors get diffused in air. When there is a lid, vapors will exert pressure on the container and when there is no lid, vapors must exert pressure against the heavy blanket of atmosphere. When these pressures are equaled, all the vapors are free to escape now and liquid is said to boil. Irrelevant to the diffusion, some vapors must get collided with atmosphere particles and stay over the liquid and below the atmosphere and they are responsible for vapor pressure. – Sufyan Naeem Dec 13 '15 at 8:17
• Your comment above is actually addressing my question. And, I got my answer from it. If I am right at my explanation above then I am right to accept your answer. – Sufyan Naeem Dec 13 '15 at 8:21

Pictures are incomplete: they don't show molecules rebounding from others and pressing against all surfaces, both the container and the liquid.

The ideal gas laws can be derived from thermodynamics; see Kinetic theory. There's a nice animated GIF there that could help you to understand this.

• Well... My pictures are not expressing my question. – Sufyan Naeem Mar 13 '15 at 19:05

As I understand it, the distinction between evaporation and boiling is that evaporation only happens at the surface of the liquid whereas boiling happens in the bulk of the liquid (or at least where it contacts the heat source). This is what forces the temperature to remain constant until the liquid is completely gone - each and every point within the bulk of the liquid where the temperature is high enough to produce a vapor pressure equal to or greater than the hydrostatic pressure in the liquid, will immediately form a bubble, rise to the top, and leave. Any time a bubble is formed in a region where the temperature is below the boiling point, the vapor inside the bubble has insufficient pressure to oppose the hydrostatic pressure (which for a small container is equal to the atmospheric pressure) and the bubble collapses.