Which of the following statements are correct?
(A) A solution which obeys Raoult's law strictly is called an ideal solution.
(B) A solution which shows deviations from Raoult's law is called the Real solution.
(C) If \( T_{b} \) is the boiling point of the solvent and T is the boiling point of the solution, the difference in the boiling points \( (\Delta T_{b}=T_{b}-T) \) is called the elevation of the boiling point.
(D) The elevation of the boiling point is related to the lowering of vapor pressure.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B) and (D) only
Raoult’s law says that the partial vapour pressure of each component of a solution is proportional to its mole fraction. A solution that obeys it over the whole range of concentration is an ideal solution; one that does not is a non-ideal (real) solution. When a non-volatile solute is added to a solvent, the vapour pressure is lowered, so the solution must be heated to a higher temperature before its vapour pressure reaches the atmospheric pressure. The boiling point of the solution is therefore higher than that of the pure solvent:
\( \displaystyle \Delta T_b = T_b - T_b^{\circ} \quad (\text{solution} - \text{solvent}) \)
(A) A solution which obeys Raoult's law strictly is called an ideal solution. — Correct. This is the definition of an ideal solution; for it \( \Delta_{\text{mix}}H = 0 \) and \( \Delta_{\text{mix}}V = 0 \).
(B) A solution which shows deviations from Raoult's law is called the Real solution. — Correct. Real (non-ideal) solutions show positive or negative deviations, because the A–B interactions differ from the A–A and B–B interactions.
(C) If \(T_{b}\) is the boiling point of the solvent and T is the boiling point of the solution, the difference in the boiling points \((\Delta T_{b}=T_{b}-T)\) is called the elevation of the boiling point. — Incorrect. The solution boils at the higher temperature, so the elevation is the boiling point of the solution minus that of the solvent, \( \Delta T_b = T - T_b \). The expression printed, \( T_b - T \), would be negative. (The book uses \( T_b^0 \) for the solvent and \( T_b \) for the solution.)
(D) The elevation of the boiling point is related to the lowering of vapor pressure. — Correct. The boiling point rises only because the vapour pressure of the solution is lower than that of the solvent at every temperature.
NCERT (2026-27 edition):
Ideal Solutions – “The solutions which obey Raoult’s law over the entire range of concentration are known as ideal solutions.”
Non-ideal Solutions – “When a solution does not obey Raoult’s law over the entire range of concentration, then it is called non-ideal solution. The vapour pressure of such a solution is either higher or lower than that predicted by Raoult’s law (equation 1.16). If it is higher, the solution exhibits positive deviation and if it is lower, it exhibits negative deviation from Raoult’s law.”
Elevation of Boiling Point – “… We have also learnt in the last section that vapour pressure of the solvent decreases in the presence of non-volatile solute. … Let Tb0 be the boiling point of pure solvent and Tb be the boiling point of solution. The increase in the boiling point ΔTb = Tb − Tb0 is known as elevation of boiling point.”