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 of the following concentration term is temperature dependent?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Molarity
The passage lists the usual concentration terms: molality, molarity, mole fraction, ppm and mass %. They fall into two groups, according to what they are built from:
Built from volume: molarity, the moles of solute in one litre of solution. A liquid expands on heating and contracts on cooling, so the same solution has a different volume, and so a different molarity, at a different temperature.
Built from mass or moles only: molality, mole fraction, ppm and mass %. Mass and the number of moles do not change with temperature, so these terms are temperature independent.
\( \displaystyle \text{Molarity}=\frac{\text{moles of solute}}{\text{volume of solution in L}} \)
\( \displaystyle \text{Molality}=\frac{\text{moles of solute}}{\text{mass of solvent in kg}} \)
So the temperature dependent term is molarity, because its denominator is a volume.
Why the other options are incorrect
Option (1) Molality — Incorrect. Molality uses the mass of the solvent in kg, not a volume. Heating does not change the mass, so molality stays the same.
Option (2) Mole fraction — Incorrect. Mole fraction is a ratio of moles. The number of moles of each component does not change with temperature.
Option (4) ppm — Incorrect. ppm (by mass) is the mass of solute per 106 parts by mass of the solution. Both masses stay fixed when the temperature changes.
NCERT (2026-27 edition):
Expressing Concentration of Solutions – “Each method of expressing concentration of the solutions has its own merits and demerits. Mass %, ppm, mole fraction and molality are independent of temperature, whereas molarity is a function of temperature. This is because volume depends on temperature and the mass does not.”