BOILING POINT: CONCEPT, CALCULATION AND EXAMPLES - CHEMISTRY - 2025

The boiling point is the temperature at which the vapor pressure of the liquid equals the atmospheric pressure in the room or compartment. The liquid turns into vapor. During this phase, bubbles appear that rise towards the surface of the liquid and escape into the air.

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.

All liquids will begin to boil as their vapor pressure equals the external pressure. Source: Ervins Strauhmanis via Flickr (https://www.flickr.com/photos/ervins_strauhmanis/18775075796)

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 follows 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.

How to calculate the boiling point?

Taking water as an example, a simple way to calculate the value of the boiling point is by using one of its colligative properties; that is, the increase in its boiling point due to the presence of solutes in the aqueous solution.

The boiling point of water increases with the addition of solutes, 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:

ΔT _e = K _e m

ΔT _e = boiling point variation

K _e = boiling constant

First step

Calculation of the moles of NaCl:

moles of NaCl = 30 g / (58.5 g / mol)

= 0.513 moles

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:

ΔT _e = mK _e

ΔT _e = 1.71 (mol / kg of water) · 0.52 ºC · (kg of water / mol)

= 0.889 ºC

Fourth step

Calculation of the boiling point of the NaCl solution:

T _e NaCl = T _e H ₂ O + ΔTe

= 100 ºC + 0.889 ºC

= 100.889 ºC

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 the increase in the boiling point. In addition, water uses the oxygen atom to form hydrogen bonds with other water molecules (H ₂ O-HOH).

Alcohol

Alcohols have OH groups in their structure. These groups are polar, which generates the dipole-dipole interaction between 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 has a boiling point of 66ºC, and isopropyl alcohol has a boiling point of 80.3ºC.

Oils

Oils decompose by heating before reaching boiling or boiling point, so estimates of their boiling points are scarce and imprecise. The estimated boiling point for soybean oil is 300ºC.

Instead of boiling points, its smoke or burn points are reported. These are achieved by heating an oil to a certain temperature, at which a bluish smoke appears, indicative of the beginning of the oil's decomposition.

The following are examples of the smoke point of some oils: almond oil 221ºC; canola oil 220 ° C; coconut oil 232 ° C; and olive oil (virgin) 210 ºC.

Gold

Gold is a precious metal with a density of 19.32 g / cm ³. 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 subjected 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 affinity these compounds are with water, making it more difficult to evaporate it.

Sugar

Glucose has a melting point of 146ºC, which coincides with the decomposition point of glucose. Therefore, you cannot get its boiling point. The same situation occurs with sucrose, table sugar, which has a melting point of 186ºC and a decomposition point of 186ºC.

The melting point is the temperature at which an element or chemical compound goes from the solid state to the liquid state. Therefore, when the sugar decomposes, there is no stable liquid for determining 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. Furthermore, numerous electrostatic forces need to be overcome due to the metal's web-like structure.

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Wikipedia. (2020). Boiling point. Recovered from: en.wikipedia.org
3. Helmenstine, Anne Marie, Ph.D. (February 11, 2020). Definition of Boiling Point in Chemistry. Recovered from: thoughtco.com
4. Cedrón J.; Landa V. & Robles J. (2011). Intermolecular forces. General chemistry. Recovered from: corinto.pucp.edu.pe
5. Samuel Belcher. (sf). Gold. Recovered from: chemistry.pomona.edu
6. Don Ulin. (December 17, 2010). What Is The Boiling Point For Candy? Recovered from: indianapublicmedia.org
7. Helmenstine, Anne Marie, Ph.D. (February 11, 2020). What Is the Boiling Point of Milk? Recovered from: thoughtco.com