Vaporization vs Evaporation vs Boiling

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While working at my previous organization where I was a development editor for school books, I had quite a lot of interaction with school teachers. The topics of these interactions vary from new teaching methods to the content which goes into the text books to various other subject related matter. Mostly, the discussions were on their views regarding the explanations of certain “confusing” topics.

Teachers can be intransigent sometimes. Teachers, especially the experienced ones, usually follow only those books they have started their teaching career with. So, they get upset if the new content in our books deviate slightly from the wordings they are familiar with, or the artwork or diagrams they are used to. There were certain topics where the rewording confused the teachers and they ended up confusing us with their corrections. A set of topic I came across while working on the Physics books was vaporization/evaporation/boiling. These terms drove my series editor to a confused state. She sent different sets of corrections. I managed to “negotiate” with this middle school teacher and a general explanation was given for the book. But I realized, looking up in internet, many people are confused as well: the difference between vaporization, evaporation and boiling. So, in this post, I briefly discuss what these are and how they differ from each other.

Vaporization: It is nothing but the change of state of matter from a liquid state to a gaseous state. For example, change of water to steam. This change of state from liquid to gas can happen by two methods: evaporation and boiling.

Evaporation: It is the change of state of liquid to gaseous at a temperature below the boiling point of the liquid. There is no boiling of liquid involved in evaporation. Evaporation takes place at the surface of liquid which is exposed to the surroundings.

Boiling: When the liquid is heated and attains the boiling point, the vaporization process is called boiling. Boiling happens throughout the liquid. This is because, the heating process (convection) heats up the entire liquid.

Let us understand these better:

What causes a liquid to change its state to gas?
We know that the various states of matter are characterized by the arrangement of molecules in them and also the intermolecular force of attraction.

In solids, the intermolecular force of attraction is strong. That is why the molecules are held together tightly and the matter is rigid.
In liquids, the intermolecular force of attraction is less, which gives the matter the ‘fluid’ characteristics.
In gases, the intermolecular force of attraction is least. That is the reason, they don’t usually hold together as a substance and easily spread everywhere.

For a solid molecule to change its state to liquid, it should have enough kinetic energy to overcome the intermolecular force of attraction which binds it to other molecules. You know that heating can provide that kinetic energy. Similarly, heating provides enough kinetic energy to a liquid molecule to overcome the intermolecular force of attraction and change to gas.

During evaporation, the liquid absorbs heat from the surroundings. The molecules at the surface of the liquid acquire enough kinetic energy and get converted to vapor and escape. The more heat the liquid can absorb, the more will be the rate of evaporation: On hot summer days, we see the water in the pond getting evaporated faster than other seasons.

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Evaporation of lake water

When we apply heat externally, for example, heating up a pan of water, the kinetic energy of the molecules increases rapidly and they start to vaporize. There will come a point at which all the molecules in the liquid have enough kinetic energy to change their state to gas. This is called the boiling point and the process is called boiling. Vaporization during boiling will be quite rapid.

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Boiling of water

Vapor Pressure
Another factor which differentiate these processes is vapor pressure. It is the pressure exerted by the liquid on the walls of a container. Liquids which can vaporize easily will have a higher vapor pressure. During evaporation, the vaporization is a slow process and the vapor pressure will be less as compared to the atmospheric pressure.

During boiling, at the boiling point, the vaporization is fast and the vapor pressure is equal to the atmospheric pressure. That is the reason, the bubbles formed (with vapor inside) during boiling rise up and burst at the surface of the liquid.

Summing up, both evaporation and boiling are same kind of phenomenon where the liquid change its state to gas. The difference depends on the supply of heat: Evaporation can take place at any temperature while boiling happens only at the boiling point. Another difference is, evaporation takes place only at the surface of the liquid while the boiling is a bulk process where vaporization takes place throughout the liquid. Also, during evaporation no bubbles are formed, but, bubbles are formed during boiling.

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Evaporation and boiling

So, the concluding question: What causes the formation of bubbles during boiling? That calls for another blog post.

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