Question

Isotonic solutions have:
[A] Same boiling point
[B] Same vapour pressure
[C] Same melting point
[D] Same osmotic pressure.

Answer 1

HINT: The prefix –iso is used when two solutions have something in common. The word isotonic means constant tension. It is also used to express liquids that have the same concentration.

COMPLETE STEP BY STEP SOLUTION: We know that the prefix –iso means similar. To solve this question let us discuss the terms mentioned in the options and then try to find the correct answer.
Firstly we have boiling point.
Boiling is the process of a liquid turning into vapour when subjected to heat. A substance boils when the heat reaches the boiling temperature of the substance. This is a phase change and it is a physical process, not a chemical reaction. Boiling point is the temperature at which the phase of any substance changes from liquid to vapour.
Next we have vapour pressure.
Vapour pressure is the pressure exerted by the gas on liquid (or solid) in a closed system at a particular temperature. It is the upward pressure that the molecules of the liquid face when they are trying to leave the surface of the liquid through evaporation. In simple words, we can say that it is the pressure which is exhibited by the vapour present above the liquid surface.
Then we have a melting point.
Melting is the process of a solid turning into liquid when heated. A substance melts when the heat reaches the melting temperature of the substance. Melting point is the temperature at which the phase of any substance changes from solid to liquid.
And lastly we have osmotic pressure.
Osmotic pressure is the minimum pressure that is required to apply to a solution for stopping the inward flow of the pure solvent across the semipermeable membrane. It is a colligative property. An isotonic solution is a solution that has the same osmolarity or solute concentration as another solution. In an isotonic solution, there is free movement of water across the membrane without changing the concentration of any of the solutes.
We can understand from the above solution that an isotonic solution is a solution having the same osmotic pressure.

Therefore the correct answer is option [D] osmotic pressure.


NOTE: There are four colligative properties and one of them is osmotic pressure. Osmotic pressure is given as-
\[\pi =iCRT\]
Where, $\pi $ is the osmotic pressure, ‘i’ is the Van’t Hoff factor, C is the concentration of the solution, R is the universal gas constant and T is the temperature. When the vapour pressure of a liquid reaches the atmospheric pressure, it starts to boil. As a result of vapour pressure the amount of atmospheric pressure exerted on the liquid decreases. Therefore, liquids with high vapour pressure will have a lower boiling point.