Question

Explain the terms : 

(1) Isotonic solutions 

(2) Hypotonic solutions 

(3) Hypertonic solutions.

Answer 1

(1) Isotonic solutions : The solutions having the same osmotic pressure at a given temperature are called isotonic solutions.

Explanation : If two solutions of substances A and B contain n_A and n_B moles dissolved in volume V (in dm^-3) of the solutions, then their concentrations are, 

C_A = \(\frac{n_A}{V}\)(in mol dm^-3) and 

C_B =  \(\frac{n_B}{V}\) (in mol dm^-3)

If the absolute temperature of both the solutions is T, then by the van't Hoff equation, π_A = C_ART and π_B = C_BRT where π_A and π_B are their osmotic pressures.

For the isotonic solutions,

π_A = π_B

∴ C_A = C_B

∴ \(\frac{n_A}{V}\) = \(\frac{n_B}{V}\)

∴ n_A = n_B

Hence, equal volumes of the isotonic solutions at the same temperature will contain equal number of moles (hence, equal number of molecules) of the substances.

(2) Hypotonic solutions : When two solutions have different osmotic pressures, then the solution having lower osmotic pressure is said to be a hypotonic solution with respect to the other solution.

Explanation : Consider two solutions of the substances A and B having osmotic pressures π_A and π_B. If π_B is less than π_A, then the solution B is a hypotonic solution with respect to the solution A.

Hence, if C_A and C_B are their concentrations, then, C_B < C_A . 

Hence, for equal volumes of the solutions, n_A < n_A.

(3) Hypertonic solutions : When two solutions have different osmotic pressures, then the solution having higher osmotic pressure is said to be a hypertonic solution with respect to the other solution.

Explanation : Consider two solutions of substances A and B having osmotic pressures π_A and π_B . If π_B is greater than π_A , then the solution B is a hypertonic solution with respect to the solution A.

Hence, if C_A and C_B are their concentrations, then C_B > C_A. 

Hence, for equal volume of the solutions, n_B > n_A.