Question

Which of the following associated with isotonic solutions is not correct
A. They will have the same osmotic pressure
B. They will have the same weight concentration
C. Osmosis does not take place when the two solution are separated by a semipermeable membrane
D. They will have the same vapour pressure

Answer 1

Hint: Solution are of three types on the basis of concentration of solution. If any solution has less number of solute molecules as compared to other solution (hypotonic which is highly concentrated) then the solution is said to be hypertonic solution (dilute or less concentrated). If the concentration of both the solutions is same then the solution is said to be isotonic solutions.

Complete Step by Step Answer:
Let us take two solutions in one beaker separated through a semipermeable membrane, one solution containing fewer solute particles (less concentrated or dilute), hypertonic solution and the other containing a large number of solute particles (more concentrated), hypotonic solution.

It is very interesting to know that a semi-permeable membrane only allows the passage of solvent molecules (liquid) from the solution but not solute (solid) inside or outside of the cell. Now the solvent of the solution which is less concentrated moves towards the solution which is highly concentrated through a semipermeable membrane. And this process is known as osmosis or forward osmosis and it is a spontaneous process.

As solvent molecules move toward the highly concentrated solution thus we fit the piston on the highly concentrated solution and push it until osmosis stops. Thus, the pressure we apply to stop the osmosis process will be equal to the force with which solvent molecules move towards a concentrated solution. This pressure which we applied is known as osmotic pressure.

This is the case when one solution is hypotonic and the other is a hypertonic solution. If both the solution is isotonic then concentration of both solutions will be the same (number of solutes per volume are same). If concentration is the same, then osmotic pressure will be equal for both solutions and osmosis occurs with the same speed and with same force in both solutions.
Thus, the correct option is B and C.

Note: It is important to note that when pressure is applied to the highly concentrated solution, we need to take care not to apply pressure more than required to stop osmosis otherwise reverse osmosis will take place, and solvent molecules from highly concentrated solution move towards a low concentrated solution through a semipermeable membrane. Osmosis is the reason due to which beans (rajma) swell when left in water for night.