[kapital]

Why in three phase diagram of water(or other compound) it is said that in the line of solid and liquid there is equilibrium of solid and liquid, if liquid water always has some vapor pressure and is therefore always in equilibrium with vapor?

[Arkcon]

Why in three phase diagram of water(or other compound) it is said that in the line of solid and liquid there is equilibrium of solid and liquid, if liquid water always has some vapor pressure and is therefore always in equilibrium with vapor?

Is it true that there always is vapor above the liquid?  What is the phase diagram trying to tell you?

[kapital]

That the vapor pressure at some conditions is 0 therefore?

[Arkcon]

From here:

http://serc.carleton.edu/research_education/equilibria/phaserule.html

What happens at the lowest temperatures?

[kapital]

Water is solid, why is this important?

[Enthalpy]

if liquid water always has some vapor pressure

Yes.

and is therefore always in equilibrium with vapor

No.

Liquid water at +20°C and 1atm is not in equilibrium with its vapour.

[kapital]

Liquid water at +20°C and 1atm is not in equilibrium with its vapour.

Why not?

[Enthalpy]

Because water uses to evaporate, so it's not at equilibrium with the atmosphere.

The vapour pressure of water at +20°C is small, far less than 1atm, and though you can have water at +20°C and 1atm, so
- The equilibrium is not with the liquid's (water) or gas (air) pressure, but with the vapour IF any equilibrium is here
- The pressure of the vapour (here a partial pressure) is not the air's pressure but far less
- A liquid doesn't even need to be in equilibrium with its vapour. Here water evaporates because the partial pressure of its vapour is less than the equilibrium.

A common case is when evaporation is so slow that it can't provide enough vapour to reach an equilibrium. Ice should evaporate from comets to vacuum but far from the Sun, the evaporation rate from cold ice is very slow.

As well, you can have a liquid without its vapour, for instance in a hydraulic circuit, so the liquid needs not an equilibrium, and the liquid pressure can (does) exceed a lot the equilibrium vapour pressure.