Relative humidity and vapor-pressure in equilibrium

Question

I have quite an unclear understanding of relative humidity. I am writing my understanding on the topic and please correct me wherever I'm wrong:

What I understood:

Let's say $\ce{H2O}$ has a (saturated) vapour pressure of $x~\pu{atm}$ at a certain temperature. Now, relative humidity (RH) is the ratio of the partial pressure of water vapour to the equilibrium vapour pressure of water at a given temperature.

So what this means is that if RH = 100% then the maximum amount of water vapours is present in the air. Now if RH = 50% means that the amount of water vapours present in the air is only half of the maximum possible.

• So, does this mean the water has not been fully evaporated up to the limit it can?
• Does this mean that the equilibrium has not yet been established, and the reaction tends to shift in the forward direction? If yes, then what is stopping the reaction to move forward?
• Does 100% RH mean the water and it's vapour are in equilibrium?

Can relative humidity be compared with an example of a sponge? There's a limit of the amount of water a sponge can absorb water. If it has only absorbed 50% means there is still room for more absorption until 100% capacity is reached.

Answer 1

What I understood:

Let's say $\ce{H2O}$ has a (saturated) vapour pressure of $x$ atm at a certain temperature. Now Relative humidity (RH) is the ratio of the partial pressure of water vapour to the equilibrium vapour pressure of water at a given temperature.

So what this means is that if RH = 100% then the maximum amount of water vapours is present in the air. Now if RH = 50% means that the amount of water vapours present in the air is only half of the maximum possible.

• So, does this mean the water has not been fully evaporated up to the limit it can?

Less than 100% humidity would mean exactly that. Assuming that there is still liquid water left, then an equilibrium has not been reached.

• Does this mean that the equilibrium has not yet been established, and the reaction tends to shift in the forward direction? If yes, then what is stopping the reaction to move forward?

Exactly. Assuming an isothermal system with more liquid water available, then yes, more water would evaporate. Remember when water evaporates it cools the remaining water.

• Does 100% RH mean the water and it's vapour are in equilibrium?

Yes, assuming both phases exists, and that the gas and liquid phase are at the same temperature.

Some extra points:

• The saturated vapor pressure of water is a complex function of temperature. So it is $x_1$ at $T_1$ but $x_2$ at $T_2$, and so on.

• Ice sublimes and thus will also have an equilibrium water vapor pressure. The equilibrium water vapor pressure again depends on the temperature of the ice.