por He Boiling point It is the temperature at which the vapor pressure of the liquid equals the atmospheric pressure existing in the place or compartment. The liquid turns into steam. During this phase, the appearance of bubbles that rise to the surface of the liquid and escape into the air occurs.
On the other hand, the normal or standard boiling point is the temperature at which a liquid boils at sea level; that is, at one atmosphere of pressure (101.325 kPa). Meanwhile, the IUPAC (1982) defines the boiling point as the temperature at which a liquid boils at a pressure of 100,000 kPa.
The normal boiling point of water is 99.97ºC. But, at the peak of Mount Everest, at a height above sea level of 8,848 m and an atmospheric pressure of 34 kPa, it is 71 ºC. The IUPAC recommended standard boiling point is 99.61ºC at a pressure of 100.00 kPa (1 bar).
From the above it can be deduced that atmospheric pressure is a determining factor in the value of the boiling point, since it is the pressure that a liquid must reach to boil. The higher the atmospheric pressure a liquid is exposed to, the higher its boiling point. The opposite also is true.
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How to calculate the boiling point?
Taking water as an example, a simple way to calculate the boiling point value is by using one of its colligative properties; that is, the increase in its boiling point by the presence of solutes in the aqueous solution.
The boiling point of water increases when solutes are added to it, due to the interaction between the water molecules and the solute molecules.
The increase in the boiling point of water is given by the following mathematical expression:
ΔTe = Ke m
ΔTe = boiling point variation
Ke = boiling constant
m = molality of the solution
boiling point increase
The boiling point itself cannot be calculated but determined. However, the previous equation allows us to calculate the increase in this value. The following exercise helps clarify this:
– Exercise
Calculate the variation in the boiling point of water by adding 30 g of sodium chloride (NaCl) to 250 g of water, knowing that the boiling constant (Ke) has a value of 0.52 ºC·Kg/mol. NaCl molecular weight = 58.5 g/mol.
If the boiling point of water is 100 ºC: What will be the boiling point value of the NaCl solution?
First step
Calculation of moles of NaCl:
moles of NaCl = 30 g / (58.5 g/mol)
= 0.513 mole
Second step
Calculation of the molality of the solution:
0.513 moles of NaCl are dissolved in 300 g of water. To obtain the molality of the solution, the moles of NaCl are brought to 1,000 g (kg).
Moles of solutes / kg of water (molality) = (0.513 moles/300 g of water) · (1000 g of water/kg of water)
= 1.71 moles/kg of water
Third step
Calculation of the boiling point increase due to the addition of NaCl:
ΔTe = m Ke
ΔTe = 1.71 (mol/kg of water) 0.52 ºC (kg of water/mol)
= 0.889ºC
Fourth step
Calculation of the boiling point of the NaCl solution:
TeNaCl = TeH2O + ΔTe
= 100ºC + 0.889ºC
= 100.889ºC
examples of boiling points
Water
The normal boiling point of water is 99.97ºC. This value is relatively high given the small size of its molecule. However, it is explained by its unusual polarity and its ability to establish hydrogen bonds with neighboring or related molecules.
The oxygen atom has a higher affinity for electrons than the hydrogen atom. Therefore, the electrons of the OH covalent bond move towards oxygen, leaving it negatively charged; while the hydrogen atom, positively charged.
As a consequence of this, water molecules are dipoles that can interact with other water molecules, constituting an intermolecular force that contributes to increasing the boiling point. Also, water uses the oxygen atom to form hydrogen bonds with other water molecules (H2O—HOH).
Alcohol
Alcohols have OH groups in their structure. These groups are polar, which generates the dipole-dipole interaction between the similar molecules. Alcohols can also form hydrogen bonds. These two interactions represent the main contributions to intermolecular forces.
These forces explain why the boiling points of alcohols are higher than the corresponding hydrocarbons. The main factors that determine the boiling points in alcohols are the molecular mass and its structure.
The boiling point increases with increasing number of carbon atoms and decreases with branching. For example: ethanol has a boiling point of 78.37ºC, but methanol of 66ºC, and isopropyl alcohol of 80.3ºC.
oils
Oils break down on heating before reaching boiling, or boiling, so estimates of their boiling points are sparse and imprecise. The estimated boiling point for soybean oil is 300ºC.
Instead of boiling points, their smoke or fire points are reported. These are achieved by heating an oil to a certain temperature, at which a bluish-colored smoke appears, indicative of the beginning of the decomposition of the oil.
The following are examples of the smoke point of some oils: almond oil 221ºC; canola oil 220ºC; coconut oil 232ºC; and olive oil (virgin) 210 ºC.
Gold
Gold is a precious metal with a density of 19.32 g/cm3. It has a high boiling point, due to the presence of the metallic bond. However, there are discrepancies between the values reported for its boiling point, which perhaps reflect differences in the degree of purity of the gold samples submitted to the study.
Milk
Milk is an aqueous solution that presents solutes of different nature and composition; salts, sugars, proteins, lipids, amino acids, etc. The boiling point of milk is slightly higher than that of water, due to the closeness of these compounds with water, making it more difficult to evaporate.
Sugar
Glucose has a melting point of 146ºC, which is the same as the breakdown point of glucose. Therefore, its boiling point cannot be obtained. The same situation occurs with sucrose, table sugar, which has a melting point of 186 ºC and a decomposition point of 186 ºC.
The melting point is the temperature at which a chemical element or compound changes from the solid state to the liquid state. Therefore, when the sugar decomposes, there is no stable liquid for the determination of its boiling point.
Iron
The boiling point of iron is 2.861 ºC. This high value is explained by the large amount of energy required to overcome the attractive force between the metal atoms. In addition, it is necessary to overcome numerous electrostatic forces due to the lattice-like structure of the metal.
References
Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
Wikipedia. (2020). Boiling point. Retrieved from: en.wikipedia.org
Helmenstine, Anne Marie, Ph.D. (February 11, 2020). Definition of Boiling Point in Chemistry. Retrieved from: thoughtco.com
Cedron J.; Landa V. & Robles J. (2011). intermolecular forces. General chemistry. Recovered from: corinto.pucp.edu.pe
Samuel Belcher. (nd). Gold. Retrieved from: chemistry.pomona.edu
Don Ulin. (December 17, 2010). What Is The Boiling Point For Candy? Retrieved from: indianapublicmedia.org
Helmenstine, Anne Marie, Ph.D. (February 11, 2020). What Is the Boiling Point of Milk? Retrieved from: thoughtco.com