Intermediate Topic 308 of 1102 States of Matter & Solutions
Freezing Point Depression
A dissolved solute lowers the freezing point by Kf times molality.
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In-depth explanation of Freezing Point Depression
Freezing point depression is the decrease in the freezing temperature of a solvent caused by a dissolved solute. When a solution freezes, pure solvent crystallizes out and leaves the remaining solution more concentrated, which lowers the freezing point. The change is given by ΔTf = i Kf m, where Kf is the molal freezing point depression constant, which is 1.86 °C kg/mol for water. Freezing point depression is used in antifreeze mixtures for car engines, in salting roads in winter and in cryoscopy to find molar mass. It is a colligative property that depends on the number of dissolved particles.
Going deeper: Freezing Point Depression
Key ideas: Steps: calculate molality, multiply by Kf and i, then subtract from the normal freezing point. Kf values: water 1.86, benzene 5.12, camphor 40 C kg/mol, which makes camphor valuable for small samples. Applied in antifreeze, 50 percent ethylene glycol in water freezes near -37 C.
Common mistakes and connections: Students add ΔTf rather than subtracting, or ignore i for electrolytes. Another error is expecting the freezing point of the solution to stay constant while freezing; as pure solvent crystallizes, the solution concentrates and the point keeps dropping. Salt-ice mixtures reach about -21 C for NaCl. Links to colligative properties, molar mass determination and eutectic mixtures.
Visualization of Freezing Point Depression
Freezing point depression = i Kf m; camphor's huge Kf of 40 makes it useful for molar mass determination, and road salt exploits water's 1.86. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A 0.5 molal sodium chloride solution, with i = 2, freezes at about -1.86°C because the dissolved ions double the effect.
Frequently asked questions about Freezing Point Depression
What is Freezing Point Depression?
A dissolved solute lowers the freezing point by Kf times molality.
Is Freezing Point Depression a beginner, intermediate or advanced topic?
Freezing Point Depression is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
What should I learn before Freezing Point Depression?
Study these earlier topics in States of Matter & Solutions first: Azeotropes, Colligative Properties, Boiling Point Elevation.
What comes after Freezing Point Depression?
The next topics in States of Matter & Solutions are Osmosis and Osmotic Pressure.
Can I practise Freezing Point Depression with quizzes?
Yes. The interactive lesson includes a self-check question on Freezing Point Depression, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Freezing Point Depression fits in States of Matter & Solutions
Freezing Point Depression is topic 89 of 109 in States of Matter & Solutions.