Boiling Point vs Evaporation
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, which is known as evaporation, the vaporization takes place below the boiling point. Although both processes produce molecules in the vapor state, the way of producing them is different.
In simple words, boiling point means the temperature at which a liquid or a solvent will start to boil. This is defined at a fixed pressure, normally the atmospheric pressure. In other words, it is the temperature at which a liquid starts to vaporize. So at this temperature the vapor pressure will be equal to the atmospheric pressure. Boiling points of substances are affected by many factors. As external factors, the atmospheric temperature affects it. A liquid in a vacuum has a lower boiling point than it is in the normal atmospheric pressure. And a liquid in a high pressure will have a relatively higher boiling point. 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. An alcohol will have a higher boiling point compared to the corresponding alkane. 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.
Boiling points are useful for separation of each substance from a mixture. The technique used for this is known as distillation. This is the fundamental basis behind the petroleum distillation also. 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 each and every molecule separately since their boiling points are varying with small amounts. But it is possible to purify them to some degree. Therefore, in petroleum distillation, molecules with closer molecular weights are separated in a temperature range.
Evaporation is the process of changing a liquid into its vapor stage. The word “evaporation” is specifically used when the vaporization happens from the surface of the liquid. Liquid vaporization can also happen at the boiling point where evaporation happens from whole liquid mass. But then it is not called as evaporation. Evaporation can be influenced by various factors like concentration of other substances in the air, surface area, pressure, temperature of the substance, density, flow rate of air, etc.
What is the difference between Boiling Point and Evaporation?
• Evaporation is taking place on the surface of the liquid. At boiling point, vaporization is taking place from the whole liquid mass.
• Evaporation takes place below the boiling point.
• At boiling point, the liquid forms bubbles and there is no bubble formation in evaporation.
• 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.
• At boiling point, the vaporization happens rapidly, whereas the evaporation is a slow process.