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The boiling point of a substance is defined as the temperature at which the vapor pressure in equilibrium with said substance in a liquid state becomes equal to the external pressure. This means that, unlike what we were taught as children, substances do not have a single boiling point. For example, the boiling point of water is not always 100 °C, since the boiling point depends on the pressure external to the liquid.
In fact, water only boils at 100°C when the external pressure is exactly 1 atm, and contrary to what we’ve been told, atmospheric pressure is almost never exactly 1 atm, even at sea level.
What is not boiling point?
Despite the fact that we have already clarified the real definition of the boiling point of a liquid, it should be noted that there is another very widespread definition, but that, however, is completely wrong. This is the idea that the boiling point is the temperature at which a liquid changes to a gaseous state or the temperature at which the phase change from the liquid state to the gaseous state occurs.
This idea is based on everyday experience according to which, when water is heated to its boiling point, it begins to boil and continues until it completely evaporates. However, there are several reasons why these observations are leading us to the wrong conclusion.
In what follows, we will explain why this definition is wrong, and also provide two additional concepts related to boiling point.
Boiling vs. Evaporation
The main reason many people have the wrong idea about boiling point is because they confuse boiling with the process of evaporation or vaporization. Indeed, both processes involve a step from the liquid state to the gaseous state. However, the main difference is that evaporation or vaporization is a process that is constantly occurring at any temperature. Evaporation is the reason clothes dry out after washing just by airing them out. As we can easily verify, wet clothes do not reach the boiling point of water when drying in the sun, much less in the shade.
Evaporation is generally considered to be a surface phenomenon, which is why water evaporates faster when it is spread over a wide surface than when the same amount of water is contained in a glass, for example.
On the other hand, the boiling process, understood as the moment in which boiling breaks and bubbles of water vapor are formed within the liquid, is a mass phenomenon, not a surface one. During boiling, steam bubbles are generated whose internal pressure, being equal to atmospheric pressure, is capable of withstanding said pressure without collapsing. In fact, the pressure of these bubbles is actually slightly higher than atmospheric pressure, since it must also withstand the pressure of the column of liquid above it.
So, during boiling, small pockets of vaporization form throughout the liquid, greatly speeding up the vaporization process that would otherwise only occur at the surface.
normal and standard boiling points
As we have just seen, the boiling point of a liquid varies with pressure. The lower the external pressure, the lower the boiling point is usually, since less increase in temperature is required for the vapor pressure to reach the external pressure.
But then why is it always said that water boils at 100 °C? And why can we find entire tables where the boiling points of different substances are reported?
The reason is because there are two additional definitions of the boiling point that refer to values that are unique and characteristic for each substance. We are talking about the normal and standard boiling points .
Definition of normal boiling point
The normal boiling point is defined as the boiling point of a substance when the external pressure is 1 atm. Therefore, and given the concept of the boiling point already presented, we can define the normal boiling point as the temperature at which the vapor pressure of a liquid becomes equal to 1 atm.
The fact that this definition specifies the pressure at which the boiling point is determined ensures that each substance has only one normal boiling point. So when we say that the boiling point of water is 100 °C, we are making a mistake. We should say that the normal boiling point of water is 100 °C. The vast majority of boiling points that we find in tables of physical and chemical properties are actually normal boiling points.
Standard Boiling Point Definition
For many years 1 atm was considered the standard atmospheric pressure. However, the atmosphere as a pressure unit presents some drawbacks related to its conversion to different unit systems, since it does not correspond to an integral multiple or submultiple of any of the pressure units of the other unit systems. For example, 1 atm equals 101.325 or 1.01325. 10 5 Pa. On the other hand, the bar is a unit of pressure whose value is very similar to that of 1 atmosphere, but which represents exactly 100,000 or 10 5 Pa , which is the unit of pressure in the International System of units.
In view of the small difference and the benefits of using the bar instead of the atmosphere, the standard pressure is now considered to be 1 bar, and the pressure of 1 atm was renamed normal pressure. Many of the properties that were measured in the past at 1 atm pressure are now reported at 1 bar and are often referred to as standard quantities. That said, the standard boiling point is defined as the boiling point of a substance at a pressure of 1 bar or 10 5 Pa . This is the same as saying that the standard boiling point is the temperature at which the pressure of vapor of a substance reaches 1 bar.
References
specialized glossaries. (2017, June 12). Normal boiling point [Normal boiling point] (Chemistry) . https://glosarios.servidor-alicante.com/quimica/punto-de-ebullicion-normal
Ondarse Álvarez, D. (2021, July 15). Boiling Point – Concept, how it is calculated and examples . Concept. https://concepto.de/punto-de-ebullicion/
STREPHONSAYS. (nd). Difference between normal boiling point and standard boiling point . https://en.strephonsays.com/normal-boiling-point-and-standard-boiling-point-11457
