Concept of Solution
There are several ways by which concentration of solution is quantitatively described: molality, molarity, mole fraction, ppm, mass % etc. Liquid- liquid solutions can be classified as ideal and non-ideal solution on the basis of Raoult's law. Suppose the molecules of two components are represented by A and B respectively. In pure components, the intermolecular attractive interaction will be of types A-A and B-B, whereas in the binary solution, A-B type of interactions will also be present. If intermolecular attractive forces between the A-A and B-B are nearly equal to those between A-B, ideal solution is formed.
The properties of solution which depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution are called colligative properties.
Which one of the following is not correct for an ideal solution?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Δmix H ≠ 0, and Δmix V = 0
The passage gives the molecular condition for an ideal solution: the A–B attractions are nearly equal to the A–A and B–B attractions. When the forces do not change on mixing, no heat is taken in or given out and the molecules take up the same space as before. So an ideal solution has three properties together:
\( \displaystyle p_A=x_A\,p_A^{0},\quad p_B=x_B\,p_B^{0} \) (Raoult’s law at every composition)
\( \displaystyle \Delta_{mix}H=0,\quad \Delta_{mix}V=0 \)
A non-zero enthalpy of mixing means the A–B attractions are weaker (ΔmixH > 0) or stronger (ΔmixH < 0) than in the pure liquids. That is a non-ideal solution, so ΔmixH ≠ 0 cannot describe an ideal one, even though the second half (ΔmixV = 0) is true.
Option (1) It must obey Raoult's law. — True for an ideal solution. Obeying Raoult’s law over the entire range of concentration is the definition of an ideal solution.
Option (2) Δmix H = 0 — True for an ideal solution. The attractions are the same before and after mixing, so no heat is absorbed or evolved.
Option (3) Δmix V = 0 — True for an ideal solution. The volume of the solution equals the sum of the volumes of the two liquids.
Option (4) Δmix H ≠ 0, and Δmix V = 0 — Not true for an ideal solution. For an ideal solution ΔmixH must be zero; a non-zero value belongs to a solution that deviates from Raoult’s law.
NCERT (2026-27 edition):
Ideal Solutions – “The solutions which obey Raoult’s law over the entire range of concentration are known as ideal solutions. The ideal solutions have two other important properties. The enthalpy of mixing of the pure components to form the solution is zero and the volume of mixing is also zero, i.e.,
\( \displaystyle \Delta_{mix}H=0,\quad \Delta_{mix}V=0 \) (1.21)
It means that no heat is absorbed or evolved when the components are mixed. Also, the volume of solution would be equal to the sum of volumes of the two components. …”