Chemistry - Questions Regarding Osmosis

Solution 1:

Osmotic pressure

  • is consequence of non zero net water diffusion,
  • which is consequence of non equal water activities on both sides of semipermeable membrane,
  • which is consequence of the fact dissolved solutes decrease activity of water.

Osmotic pressure of a free solution is formally an external pressure needed to be acting on this solution to keep diffusion equilibrium with pure solvent on the other side of a semipermeable membrane.

Applying external pressure then leads to the well known reverse osmosing, widely used in production of deionized water ( replacing expensive distillation ) from sweet water, or in desalination of sea water.

Osmosis continues until fully counteracted by extenal, e.g. atmospheric pressure. E.g. if there is osmotic pressure $\pu{1 atm}$, osmosis leads up to $\pu{10 m}$ high solution column ( eventually corrected by solution density ). Similarly, you would need to use external pressure $\pu{p > 1 atm}$ to start reverse osmosis.

If a book says the primary reason is hydrostatic pressure difference, it is wrong. We can have 2 solutions of the same density, but different osmotic pressures. Combining them on the opposite membrane sides would lead to disbalancing of solution levels, even if they would be initially the same and even if the hydrostatic pressure difference would be zero.

Solution 2:

In addition to Poutnik's answer, I would like to add some comments on the question:

The first statement 'Osmosis of water is not diffusion of water:....' is incorrect.

The cause of osmosis is simply diffusion: the solvent is able to diffuse through the semi-permeable membrane the solute is not. It is wrong to assume that diffusion occurs only in the direction of decreasing concentration it is rather the difference in chemical potential that drives this.

Statement 2 is also incorrect. There is no special interaction with the solute and the semi-permeable membrane that causes osmosis, other than that the solute is too big to pass through it but the solvent is not.

The pressure difference is not what causes osmosis but it is the equalisation of the chemical potential as equilibrium is reached. The pressure difference is a result of this process.

A solution made up with an involatile solute (say a protein) has a lower vapour pressure than that of the pure solvent due to the fact that some of the solution volume is now taken up with the solute. In thermodynamic terms this means that the chemical potential of the solution is less than that of the pure solvent (i.e Raoult's Law). To restore chemical equilibrium the chemical potentials have to become equal and this is achieved by solvent diffusing into the solution thereby diluting it (i.e. trying to make it pure solvent and so increase its chemical potential). The consequence is that the volume of the solution increases (e.g. as seen by pushing solution up a tube) and at a certain hydrostatic pressure, equilibrium is restored.