The key difference between boiling point and evaporation is that evaporation takes place on the surface of the liquid whereas boiling point is the temperature at which vaporization takes place from the liquid mass.
Vaporization from liquids to produce vapor can happen in two ways. One way is to produce vapor at the boiling point. In the other method, the vaporization takes place below the boiling point; we name it as evaporation. Although both processes produce molecules in the vapor state, the way of producing them is different.
What is Boiling Point?
Simply, the boiling point means the temperature at which a liquid or a solvent will start to boil. We can define it for a fixed pressure; normally the atmospheric pressure. In other words, it is the temperature at which a liquid starts to vaporize. Therefore, at this temperature, the vapor pressure equals to the atmospheric pressure.
In the first place, boiling points of substances are affected by many factors. As external factors, the atmospheric temperature affects it. For example, a liquid in a vacuum has a lower boiling point than it is in the normal atmospheric pressure. Similarly, a liquid in a high pressure will have a relatively higher boiling point.
Moreover, the chemical and physical properties of the liquid itself also affect the boiling point. For example, if the molecular weight of the molecules in the liquid is higher, it will have a higher boiling point compared to a liquid having lower molecular weighted compounds. The chemical bonds also affect the boiling point. Alcohol will have a higher boiling point compared to the corresponding alkane. Here, the reason for this is the presence of hydrogen bonds between alcohol molecules. Alkanes don’t have strong hydrogen bonds; rather, they will have weak Van der Waals interactions. Therefore, the energy required to break the strong bonds is larger in alcohols, which increases the boiling point of it.
Apart from that, the boiling points are useful for separation of each substance from a mixture. The technique that we use for this purpose is distillation. It is the fundamental behind the petroleum distillation as well. There, petroleum contains a large number of hydrocarbons with differing number of carbons. Some are straight chains, some branched, and some are aromatic. Therefore, the boiling points of these differ from each other. However, it is hard to isolate every molecule separately since their boiling points are varying with small amounts. However, it is possible to purify them to some degree. Hence, in petroleum distillation, we can separate molecules with closer molecular weights in a temperature range.
What is Evaporation?
Evaporation is the process of changing a liquid into its vapor stage. We use the word “evaporation” specifically when the vaporization happens from the surface of the liquid. Liquid vaporization can also happen at the boiling point where evaporation happens from the whole liquid mass. But then, we do not call it evaporation.
Furthermore, evaporation can be influenced by various factors such as the concentration of other substances in the air, surface area, pressure, the temperature of the substance, density, flow rate of air, etc.
What is the Difference Between Boiling Point and Evaporation?
The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into vapor. Whereas, evaporation is the process of changing a liquid into its vapor stage. Hence, the key difference between boiling point and evaporation is that evaporation takes place on the surface of the liquid whereas, at boiling point, vaporization takes place from the whole liquid mass. Here, evaporation of a particular liquid takes place below the boiling point.
Moreover, at boiling point, the liquid forms bubbles and there is no bubble formation in evaporation. Hence, this is an observable difference between boiling point and evaporation. Further, at boiling point, the heat is supplied for the molecules, and that energy is used to form vapors. But in evaporation an external heat is not supplied. Rather, molecules get energy when they collide with each other, and that energy is used to escape to the vapor state. Therefore, this is a significant difference between boiling point and evaporation.
Below is an infographic on the difference between boiling point and evaporation that tabulates all these differences.
Summary – Boiling Point vs Evaporation
Boiling point is the temperature at which vaporization occurs when we provide the liquid with external heat energy. However, evaporation is a spontaneous process in which we do not provide any external energy. In summary, the key difference between boiling point and evaporation is that evaporation takes place on the surface of the liquid whereas boiling point is the temperature at which vaporization takes place from the whole liquid mass.
1. Helmenstine, Anne Marie, Ph.D. “Definition of Boiling Point in Chemistry.” ThoughtCo, Dec. 24, 2018. Available here
2. Libretexts. “12.4: Evaporation and Condensation.” Chemistry LibreTexts, National Science Foundation, 26 Nov. 2018. Available here