I have a little question regarding partial pressure. Partial pressure I understand fully, when we talk of an ideal gas mix. The partial pressure is product of the fractional concentration and the total barometric pressure. And in thinking in molecular terms that is like saying the amount of collisions between the gas molecules, for that single gas, and the container wall. Whereas the total barometric pressure is the sum of all collisions for all gases. So my question is about partial pressure in e.g. a closed system like a sealed beaker of water. Lets say the dissolved gases saturated the water (Henry's Law) and then we seal the system. So the partial pressure of the gases in the water are whatever the partial pressure of the gases were that were in contact with the liquid before it was sealed - lets assume full equilibration. At the molecular level however, the partial pressure of the dissovled gases, is like the collisions between that gas and the container walls. Just like before. My question is therefore, why do we treat the water molecules differently - why do we not have a partial pressure for the water molecules? At a molecular level the collision of the water molecules with the container wall is no different to the collision of the dissolved gas components and the container wall. What is it about the state transition of water from a gas to a liquid that makes us not consider the collisions of the water molecules against the container wall when we talk of partial pressure? Obviously we dont - since in a beaker of water - the vast majority of molecules are water - therefore the fractional concentration of water will be 99.9% and the partial pressure would be 99.9%. I understand that this is not the case - but why is it not? What am I fundamentally missing about liquid that we do not consider the molecular collisions?