Why does salt lower the freezing point more than it raises the boiling point?
"Why does salt lower the freezing point more than it raises the boiling point?" is a free, interactive Class 12 Chemistry simulation for NEET and JEE, built on the NCERT section "Depression of freezing point" in Solutions. Vapour-pressure curves of pure solvent, solution and solid solvent against temperature: adding solute drops the liquid curve, so it meets 1 atm later (ΔTb) and meets the solid curve earlier (ΔTf), with ΔTf > ΔTb for water. NEET has asked about this idea 3 times in 10 years of papers. JEE Main asked 53 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Depression of freezing point in Solutions. Change the values and watch what happens.
What it shows
NCERT §1.6.2; §1.6.3
Vapour-pressure curves of pure solvent, solution and solid solvent against temperature: adding solute drops the liquid curve, so it meets 1 atm later (ΔTb) and meets the solid curve earlier (ΔTf), with ΔTf > ΔTb for water.
Controls
4 to try
- Slidermolality 0–2 mol kg⁻¹
- Togglesolvent water (Kb 0.52, Kf 1.86) / benzene (Kb 2.53, Kf 5.12)
- Togglesolute glucose (i=1) / NaCl (ideal i=2) / CaCl₂ (ideal i=3)
- ReadoutsΔTb, ΔTf, new boiling and freezing points
Try this
Using NaCl in water, make the solution freeze at −3.72 °C (1 m NaCl at ideal i = 2), then note how little the boiling point moved.
Common mistake
The solute raises the freezing point / Kb and Kf are equal / forgetting i for electrolytes.
In NEET
10 years of papers
Asked 3 times in 10 years of NEET.
See the past questions: Solutions and Colligative Properties, Solutions (chapter)
In JEE Main
42 shifts analysed
53 questions from Solutions across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT §1.6.3
- At the freezing point, solid and liquid are in dynamic equilibrium, which means their vapour pressures are equal. A solution freezes when its vapour pressure equals that of the pure solid solvent.
- A non-volatile solute lowers the liquid's vapour pressure, so it meets the solid's vapour-pressure curve at a lower temperature: the freezing point drops.
- For dilute solutions the depression ΔTf = Tf° − Tf is directly proportional to molality: ΔTf = Kf m.
- Kf, the molal depression (cryoscopic) constant, has units K kg mol⁻¹ and depends only on the solvent: 1.86 for water, 5.12 for benzene, 20.00 for cyclohexane and 31.8 for carbon tetrachloride.
- Molar mass from freezing point: M₂ = (Kf × w₂ × 1000)/(ΔTf × w₁).
- Kf = R × M₁ × Tf² / (1000 × ΔfusH), with Tf the solvent's freezing point in kelvin and ΔfusH its enthalpy of fusion.
- For water Kf (1.86) is much larger than Kb (0.52), so the same solution lowers the freezing point more than it raises the boiling point. Ethylene glycol in car radiators uses this as antifreeze.
Questions
5 answers
- What does the "Why does salt lower the freezing point more than it raises the boiling point?" simulation show?
- Vapour-pressure curves of pure solvent, solution and solid solvent against temperature: adding solute drops the liquid curve, so it meets 1 atm later (ΔTb) and meets the solid curve earlier (ΔTf), with ΔTf > ΔTb for water.
- What mistake does the "Why does salt lower the freezing point more than it raises the boiling point?" simulation fix?
- A common mistake it clears up: the solute raises the freezing point / Kb and Kf are equal / forgetting i for electrolytes.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 3 times in 10 years of papers.
- Does JEE Main ask questions from Solutions?
- Yes. JEE Main asked 53 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Depression of freezing point" in Solutions (Class 12 Chemistry).
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