Boiling-point elevation

	Email		Print
	Bookmark		Link

Boiling-point elevation

Boiling-point elevation describes the phenomenon that the boiling point

Boiling point

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
of a liquid

Liquid

Liquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material....
(a solvent

Solvent

A solvent is a liquid or gas that dissolves a solid, liquid, or gaseous solute, resulting in a solution.The most common solvent in everyday life is water....
) will be higher when another compound is added, meaning that a solution

Solution

In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent....
has a higher boiling point than a pure solvent. This happens whenever a non-volatile solute, such as a salt, is added to a pure solvent, such as water. The boiling point can be measured accurately using an ebullioscope

Ebullioscope

An ebullioscope is an instrument for measuring the boiling point of a liquid. This can be used for determining the alcoholic strength of a mixture, or for determining the molecular weight of a non-volatile solute based on the boiling-point elevation....
.

Explanation
The boiling point elevation is a colligative property, which means that it is dependent on the presence of dissolved particles and their number, but not their identity.

Discussion

Ask a question about 'Boiling-point elevation'

Start a new discussion about 'Boiling-point elevation'

Answer questions from other users

Full Discussion Forum

Encyclopedia

Boiling-point elevation describes the phenomenon that the boiling point

Boiling point

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
of a liquid

Liquid

Liquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material....
(a solvent

Solvent

Solution

Ebullioscope

Explanation

The boiling point elevation is a colligative property, which means that it is dependent on the presence of dissolved particles and their number, but not their identity. It is an effect of the dilution of the solvent in the presence of a solute. It is a phenomenon that happens for all solutes in all solutions, even in ideal solutions, and does not depend on any specific solute-solvent interactions. The boiling point elevation happens both when the solute is an electrolyte

Electrolyte

An electrolyte is any substance containing free ions that behaves as an electrical conductor medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions, but molten electrolytes and solid electrolytes are also possible....
, such as various salts, and a nonelectrolyte. In thermodynamic

Thermodynamics

In physics, thermodynamics is the study of the conversion of heat energy into different forms of energy ; different energy conversions into heat energy; and its relation to macroscopic variables such as temperature, pressure, and volume....
terms, the origin of the boiling point elevation is entropic

Entropy

In many branches of science, entropy is a measure of the disorder of a system. The concept of entropy is particularly notable as it is applied across physics, information theory and mathematics....
and can be explained in terms of the vapor pressure

Vapor pressure

Chemical potential

In thermodynamics, physics and chemistry, chemical potential, symbolized by ?, is a term introduced by the American engineer, chemist and mathematical physicist Willard Gibbs, which he defined as follows:...
of the solvent. In both cases, the explanation depends on the fact that many solutes are only present in the liquid phase and do not enter into the gas phase (except at extremely high temperatures).

Put in vapor pressure terms, a liquid boils at the temperature when its vapor pressure equals the surrounding pressure. For the solvent, the presence of the solute decreases its vapor pressure by dilution. A non-volatile solute has a vapor pressure of zero, so the vapor pressure of the solution is the same as the vapor pressure of the solvent. Thus, a higher temperature is needed for the vapor pressure to reach the surrounding pressure, and the boiling point is elevated.

Put in chemical potential terms, at the boiling point, the liquid phase and the gas (or vapor) phase have the same chemical potential (or vapor pressure

Vapor pressure

Vapor pressure , is the pressure of a vapor in Thermodynamic equilibrium with its non-vapor Phase s. All liquids and solids have a tendency to evaporate to a gaseous form, and all gases have a tendency to Condensation back into their original form ....
) meaning that they are energetically equivalent. The chemical potential is dependent on the temperature, and at other temperatures either the liquid or the gas phase has a lower chemical potential and is more energetically favourable than the other phase. This means that when a non-volatile solute is added, the chemical potential of the solvent in the liquid phase is decreased by dilution, but the chemical potential of the solvent in the gas phase is not affected. This means in turn that the equilibrium between the liquid and gas phase is established at another temperature for a solution than a pure liquid, i.e., the boiling point is elevated.

The phenomenon of freezing-point depression

Freezing-point depression

Freezing-point depression describes the phenomenon that the Melting point of a liquid is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent....
is analogous to boiling point elevation. However, the magnitude of the freezing point depression is larger than the boiling point elevation for the same solvent and the same concentration of a solute. Because of these two phenomena, the liquid range of a solvent is increased in the presence of a solute.

Calculations

The extent of boiling-point elevation can be calculated by applying Clausius-Clapeyron relation

Clausius-Clapeyron relation

The Clausius-Clapeyron relation, named after Rudolf Clausius and ?mile Clapeyron, who defined it sometime after 1834, is a way of characterizing the phase transition between two phases of matter, such as solid and liquid....
and Raoult's law

Raoult's law

Established by Fran?ois-Marie Raoult, Raoult's law states:Once the components in the solution have reached chemical equilibrium, the total vapor pressure p of the solution is:...
together with the assumption of the non-volatility of the solute. The result is that in dilute ideal solutions, the extent of boiling-point elevation is directly proportional to the molal

Concentration

In chemistry, concentration is the measure of how much of a given chemical substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent....
concentration of the solution according to the equation:

?T_b = K_b · m_B

where

?T_b, the boiling point elevation, is defined as T_{b (solution)} - T_{b (pure solvent)}.
K_b, the ebullioscopic constant
Ebullioscopic constant

In thermodynamics, the ebullioscopic constant, Kb, allows one to relate molality to boiling point elevation. It is the ratio of the latter to the former:...
, which is dependent on the properties of the solvent. It can be calculated as K_b = RT_b²M/?H_v, where R is the gas constant
Gas constant

The gas constant is a physical constant which is featured in a large number of fundamental equations in the physical sciences, such as the ideal gas law and the Nernst equation....
, and T_b is the boiling temperature of the pure solvent, M is the molar mass of the solvent, and ?H_v is the heat of vaporization per mole of the solvent.
m_B is the molality of the solution, calculated by taking dissociation
Dissociation

Dissociation is an unexpected partial or complete disruption of the normal integration of a person?s conscious or psychological functioning that cannot be easily explained by the person....
into account since the boiling point elevation is a colligative property, dependent on the number of particles in solution. This is most easily done by using the van 't Hoff factor
Van 't Hoff factor

The van 't Hoff factor is a measure of the effect of a solute upon colligative properties, such as vapor pressure, osmotic pressure and freezing point depression....
i as m_B = m_solute · i. The factor i accounts for the number of individual particles (typically ions) formed by a compound in solution. Examples:

i = 1 for sugar
Sucrose

Sucrose is a disaccharide of glucose and fructose, with the molecular formula C12H22O11. Its systematic name is a-D-glucopyranosyl- -?-D-fructofuranoside ....
in water
i = 2 for sodium chloride
Sodium chloride

Sodium chloride, also known as common salt, table salt, or halite, is a chemical compound with the chemical formula SodiumChlorine....
in water, due to the full dissociation of NaCl into Na⁺ and Cl^-
i = 3 for calcium chloride
Calcium chloride

Calcium chloride, CaCl2, is a common Salt . It behaves as a typical ionic halide, and is solid at room temperature. It has several common applications such as brine for refrigeration plants, ice and dust control on roads, and in concrete....
in water, due to dissociation of CaCl₂ into Ca²⁺ and 2Cl^-

At high concentrations, the above formula is less precise due to nonideality

Ideal solution

In chemistry, an ideal solution or ideal mixture is a solution in which the enthalpy of solution is zero; the closer to zero the enthalpy of solution is, the more "ideal" the behavior of the solution becomes....
of the solution. If the solute is also volatile, one of the key the assumptions used in deriving the formula is not true, since it derived for solutions of non-volatile solutes in a volatile solvent. In the case of volatile solutes it is more relevant to talk of a mixture of volatile compounds and the effect of the solute on the boiling point must be determined from the phase diagram

Phase diagram

A phase diagram in physical chemistry, mineralogy, and materials science is a type of Graph of a function used to show conditions at which thermodynamically-distinct phase can occur at thermodynamic equilibrium....
of the mixture. In such cases, the mixture can sometimes have a boiling point that is lower than either of the pure components; a mixture with a minimum boiling point is a type of azeotrope

Azeotrope

An azeotrope is a mixture of two or more liquids in such a ratio that its composition cannot be changed by simple distillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture....
.

Ebullioscopic constants

Values of the ebullioscopic constants K_B for selected solvents:

Compound	Boiling point in °C	Ebullioscopic constant K_B in units of °(C Celsius Celsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death.... · kg) / mol or C Celsius Celsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death.... /molal
Acetic acid Acetic acid Acetic acid, CH3COOH, also known as ethanoic acid, is an organic acid which gives vinegar its sour taste and pungent smell. Pure, water-free acetic acid is a colourless liquid that absorbs water from the environment , and freezes at 16.7 Celsius to a colourless crystalline solid....	118.1	3.07
Benzene Benzene Benzene, or benzol, is an organic compound chemical compound and a known carcinogen with the molecular formula Carbon6Hydrogen6....	80.1	2.53
Carbon disulfide Carbon disulfide Carbon disulfide is a colorless, volatile liquid with the chemical formula CS2. The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical non-polar solvent....	46.2	2.37
Carbon tetrachloride Carbon tetrachloride Carbon tetrachloride, also known by many other names is the organic compound with the chemical formula CCl4. It is a reagent in organic synthesis chemistry and was formerly widely used in fire extinguishers, as a precursor to refrigerations, and a cleaning agent....	76.8	4.95
Naphthalene Naphthalene Naphthalene, also known as naphthalin, naphthaline, tar camphor, white tar, albocarbon, or antimite and not to be confused with naphtha, is a crystalline, Aromaticity, white, solid hydrocarbon with formula Carbon10hydrogen8 and the structure of two fused benzene rings....	217.9	5.8
Phenol Phenol Phenol, also known as carbolic acid, is a toxic, white crystalline solid with a sweet tarry odor, commonly referred to as a "hospital smell"....	181.75	3.04
Water Water Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....	100	0.512

Uses

Together with the formula above, the boiling-point elevation can in principle be used to measure the degree of dissociation

Dissociation (chemistry)

Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller molecules, ions, or Radical , usually in a reversible manner....
or the molar mass

Molar mass

Molar mass, symbol M, is the mass of one mole of a substance . It is a physical property which is characteristic of each pure substance. The base SI unit for mass is the kilogram but, for both practical and historical reasons, molar masses are almost always quoted in grams per mole , especially in chemistry....
of the solute. This kind of measurement is called ebullioscopy (Greek

Ancient Greek

Ancient Greek is the historical stage in the development of the Greek language spanning across the Archaic Greece , Classical Greece , and Hellenistic civilization periods of ancient Greece and the classical antiquity....
"boiling-viewing"). However, since superheating

Superheating

In physics, superheating is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling. Superheating is achieved by heating a wiktionary:Homogeneous substance in a clean container, free of nucleation sites, while taking care not to disturb the liquid....
is difficult to avoid, precise ?T_b measurements are difficult to carry out, which was partly overcome by the invention of the Beckmann thermometer

Beckmann thermometer

A Beckmann thermometer is a device used to measure small differences of temperature, but not absolute temperature values. It was invented by Ernst Otto Beckmann , a German chemist, for his measurements of colligative properties....
. Furthermore, the cryoscopic constant that determine freezing-point depression is larger than the ebullioscopic constant, and since the freezing point is often easier to measure with precision, it is more common to use cryoscopy.

A common mis-attribution of the use of boiling-point elevation is adding salt

Salt

A salt, in chemistry, is defined as the product formed from the neutralisation reaction of acids and base . Salts are ionic compounds composed of cations and anions so that the product is electrically electric charge ....
when cooking foods to elevate the temperature of the water before it boils. However, the temperature increase caused by the amounts of salt added when cooking is generally not enough to raise the temperature by a single degree, as a comparison, seawater has a boiling point of 100.6°C. The salt is added simply to season the food.

Boiling-point elevation

Explanation

Calculations

Ebullioscopic constants

Uses

See also

External links