# Boiling Point Elevation

## The boiling point of a solvent is elevated in the presence of solutes.

#### Key Points

• One of the colligative properties of a solution is boiling point elevation. The increase is directly proportional to the amount of solute present and not the identity of the solute.

• The amount that the boiling point increases in the presence of solute can be calculated by the formula: $\Delta Tb = Kb · bB$.

• The addition of solute dilutes the solvent molecules and makes it harder for the solvent molecules to escape into the gaseous phase. Therefore, a higher temperature is required to increase the vapor pressure, correlating to an increase in boiling point.

#### Terms

• The property of solutions that is directed by the ratio of solute to solvent, regardless of the identity of the solute.

• the temperature at which a liquid boils, with the vapor pressure equal to the given external pressure

• the phenomenon that the temperature at which a substance's vapor pressure equals the external pressure increases when another compound is added

#### Figures

1. ##### Colligative Properties

The introduction of solute elevates the boiling point and lowers the freezing point of chemical solutions.

## Intensive, Extensive, and Colligative Properties

Most physical properties can be classified as intensive or extensive. Intensive properties are characteristic of the substance and do not depend on the size of the material being studied; an example of this is density. Extensive properties are properties that are directly linked to the size of the material being studied; an example of this is mass. A third category of properties can only be applied to solutions; these are known as colligative properties. Properties can be considered colligative only if their properties are dependent on the amount of solute present in the solution, disregarding the identity of the solute itself.

The boiling point of a solution in the presence of a solute increases. This is referred to as boiling point elevation. Because the elevation of the boiling point is directly dependent on the amount of solute present in the solution but not based on the identity of the solute, it is considered a colligative property. This phenomena happens because of the displacement of solvent molecules by the solute; it can occur in both electrolyte and non-electrolyte solutions.

## The Relationship Between Boiling Point Elevation and Vapor Pressure

The boiling point elevation can be explained in terms of vapor pressure. Vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature. In layman's terms it is simply a measure of the tendency of the solution molecules to escape by entering the gas phase. A liquid boils when its vapor pressure is equal to the surrounding pressure. The addition of a solute to a solvent dilutes the solution, and decreases the vapor pressure: the molecules escape into the environment more slowly. A non-volatile solute has zero vapor pressure, so the solution consisting of both the solute and solvent has lower pressure than the pure solvent. For the vapor pressure to exceed the external pressure, a higher temperature is required; thus, a higher boiling point is observed.

(Figure 1)

## Calculating Boiling Point Elevation

The extent of the boiling point elevation can be calculated. It is directly proportional to the molal concentration of the solution. The amount the boiling point is elevated determined using the equation:

$\Delta Tb = Kb · bB$, where $\Delta Tb$ is the boiling point elevation defined as Tb (solution) - TB (pure solvent), $Kb$ is the ebullioscopic constant and $bB$ is the molality of the solution. The molality portion of the equation also factors into any dissociation that the solute may undergo; since boiling point elevation is a colligative property, the number of ions present in an electrolyte would affect the elevation. Therefore, can be rewritten as , where i represents the van't Hoff factor. For any non-electrolyte substances, the van' t Hoff factor is usually 1.

#### Key Term Glossary

Boiling
Boiling is the rapid vaporization of a liquid and occurs when a liquid is heated to its boiling point. A liquid's boiling point is the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environment.
##### Appears in these related concepts:
boiling point
the temperature at which a liquid boils, with the vapor pressure equal to the given external pressure
##### Appears in these related concepts:
boiling point elevation
the phenomenon that the temperature at which a substance's vapor pressure equals the external pressure increases when another compound is added
##### Appears in these related concepts:
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:
concentration
the proportion of a substance in a mixture
##### Appears in these related concepts:
constant
Consistently recurring over time; persistent
##### Appears in these related concepts:
density
a measure of the amount of matter contained by a given volume
##### Appears in these related concepts:
dissociation
Referring to the process by which compounds split into smaller constituent molecules, usually in a reversible manner.
##### Appears in these related concepts:
electrolyte
a substance that, in solution or when molten, ionizes and conducts electricity
##### Appears in these related concepts:
equilibrium
the state of a reaction in which the rates of the forward and reverse reactions are the same
##### Appears in these related concepts:
extensive property
A property of matter that depends on the amount of matter that is being measured.
##### 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.
##### Appears in these related concepts:
intensive property
A property of matter that does not depend on the amount of matter.
##### Appears in these related concepts:
ion
An atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.
##### Appears in these related concepts:
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.
##### Appears in these related concepts:
mass
The quantity of matter that a body contains, irrespective of its bulk or volume. It is one of four fundamental properties of matter. It is measured in kilograms in the SI system of measurement.
##### Appears in these related concepts:
molality
the concentration of a substance in solution, expressed as the number of moles of solute per kilogram of solvent
##### Appears in these related concepts:
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
##### Appears in these related concepts:
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.
##### Appears in these related concepts:
physical property
A physical property is any property that is measurable whose value describes a physical system's state.
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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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solute
Any substance that is dissolved in a liquid solvent to create a solution.
##### Appears in these related concepts:
solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
##### Appears in these related concepts:
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
##### Appears in these related concepts:
substance
Physical matter; material.
##### Appears in these related concepts:
surroundings
All parts of the universe that are not within the thermodynamic system of interest.
##### Appears in these related concepts:
temperature
A measure of cold or heat, often measurable with a thermometer.
##### Appears in these related concepts:
thermodynamics
The science of the conversions between heat and other forms of energy.
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vapor pressure
The pressure that a vapor exerts, or the partial pressure if it is mixed with other gases.
##### Appears in these related concepts:
volatile
evaporating or vaporizing readily under normal conditions; having a low boiling point