# Freezing Point Depression

## Freezing point depression is a colligative property observed in solutions, brought on by the introduction of solute molecules to a solvent.

#### Key Points

• The freezing point depression can be calculated using the formula: .

• At the freezing point, the vapor pressure of both the solid and liquid form of a compound must be equal.

• The freezing point of a substance is the temperature at which the solid and liquid forms are in equilibrium. Introduction of a solute disrupts this equilibria.

• To reattain equilibria, the freezing point of the solute and solvent mixture are lowered relative to the original pure solvent.

#### Terms

• The temperature at which a liquid freezes, and the solid and liquid phases are in equilibrium; normally the same as the melting point.

• The pressure that a vapor exerts, or the partial pressure if it is mixed with other gases.

• A measure of the effect of a solute upon colligative properties.

• the phenomenon that adding a solute to a solvent decreases the temperature at twhich the liquid solvent becomes a solid.

#### Figures

1. ##### Effect of Solutes on Physical Properties

Introduction of solutes to a solvent effectively lowers its freezing point and lowers its melting point.

Colligative properties are a collective group of properties unique to solutions. They are solely dependent on the number of solute molecules present in the solvent; effectively the solute:solvent ratio. Introduction of solute to a pure solvent will effectively lower its freezing point.

Freezing point depression describes the phenomena in which adding solute to a solvent or the mixing of two solids results in the lowering of the freezing point of the solvent or original solids. At the freezing point, molecules pass between the two phases at equal rates. When a solute is introduced to a liquid solvent and its solid in equilibrium, the solute effectively reduces the mole fraction of the liquid solvent. This change decreases the tendency of the molecules to escape from the solvent phase, not only into the air as gas, but into the solid phase. The introduction of a solute does not affect the escape of molecules from the solid into liquid or gas. Therefore, the system is no longer in equilibrium and the solid will begin to melt. To prevent the solid from melting, the temperature should be lowered. This not only prevents the solid from melting, but also lowers the escaping tendency of the molecules from the solvent. Therefore, at a new lower temperature, there exists a temperature where the two materials can coexist in both phases in equilibrium.

The freezing point depression can also be explained in terms of vapor pressure. Addition of solute to a solvent will essentially dilute the solvent molecules, and according to Raoult's law this leads to a decrease in vapor pressure. Considering the fact that the vapor pressure of the solid and liquid forms must be the same at freezing point (or else the system would not be at equilibrium), the lowering of the vapor pressure leads to the lowering of the temperature at which the vapor pressures of the liquid and frozen forms of the solution will be equal.

Figure 1

The freezing point depression can be calculated by the formula:

$\Delta TF = KF · b · i$, where $\Delta TF$ is the freezing point depression, KF is the cryoscopic constant, b is the molality and i is the van't Hoff factor.

#### Key Term Glossary

colligative properties
those that depend upon the ratio of the number of solute particles to the number of solvent molecules in a solution
##### Appears in these related concepts:
colligative property
The property of solutions that is directed by the ratio of solute to solvent, regardless of the identity of the solute.
##### Appears in these related concepts:
constant
Consistently recurring over time; persistent
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equilibrium
the state of a reaction in which the rates of the forward and reverse reactions are the same
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fraction
a part of a whole, especially a comparatively small part
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Freezing
Freezing or solidification is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point.
##### Appears in these related concepts:
freezing point
The temperature at which a liquid freezes, and the solid and liquid phases are in equilibrium; normally the same as the melting point.
##### Appears in these related concepts:
freezing point depression
the phenomenon that adding a solute to a solvent decreases the temperature at twhich the liquid solvent becomes a solid.
##### Appears in these related concepts:
gas
Matter in a state intermediate between liquid and plasma that can be contained only if it is fully surrounded by a solid (or held together by gravitational pull); it can condense into a liquid, or can (rarely) become a solid directly.
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group
A vertical column in the periodic table, which signifies the number of valence shell electrons in an element's atom.
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liquid
A substance that flows and keeps no definite shape, such as water. A substance whose molecules, while not tending to separate from one another like those of a gas, readily change their relative position, and which therefore retains no definite shape, except that determined by the containing receptacle; an inelastic fluid.
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molality
the concentration of a substance in solution, expressed as the number of moles of solute per kilogram of solvent
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mole
In the International System of Units, the base unit of the amount of substance; the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. Symbol: mol. The number of atoms in a mole is known as Avogadro’s number.
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molecule
the smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds
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mole fraction
The ratio of the number of moles of one component of a mixture to the total number of moles.
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phase
Any one point or portion in a recurring series of changes, as in the changes of motion of one of the particles constituting a wave or vibration; one portion of a series of such changes, in distinction from a contrasted portion, as the portion on one side of a position of equilibrium, in contrast with that on the opposite side.
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Pressure
the amount of force that is applied over a given area divided by the size of this area
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ratio
The relative magnitudes of two quantities (usually expressed as a quotient).
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solid
A substance in the fundamental state of matter that retains its size and shape without need of a container (as opposed to a liquid or gas).
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solute
Any substance that is dissolved in a liquid solvent to create a solution.
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solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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Solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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solvent
a substance that dissolves a solute, resulting in a solution
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system
the part of the universe being studied, arbitrarily defined to any size desired
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temperature
A measure of cold or heat, often measurable with a thermometer.
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vapor pressure
The pressure that a vapor exerts, or the partial pressure if it is mixed with other gases.