Evaporation and boiling are two different processes. Evaporation is the change of a liquid into the gaseous phase by natural method. Boiling is a process in which liquid is converted into a vapor phase by heating up. For example, water is converted into vapors by evaporation and boiling.
Difference between evaporation and boiling
| Evaporation | Boiling |
| A process in which the liquid phase changes into the gases phase. | A process in which liquid is heated up and vaporized. |
| Bubbles do not formed in evaporation | Bubbles formed in boiling. |
| Occurs on the surface of liquid. | Entire liquid is involved in this process. |
| It is a slower process | It is a fast process |
| It is a natural process | It is an unnatural process. |
| This is spontaneous | This is non-spontaneous |
| No specific temperature is required | Specific temperature is required |
| No specific external energy source is required | An external energy source is required |
| E.g. Conversion of liquid into vapor phase by the temperature of the Sun |
E.g. water turns into vapors at 100 oC |
Evaporation
- Evaporation is the changing of liquid into vapor (gaseous state) from the surface of a liquid without heating it.
- The transition of the liquid phase in the gas phase below the temperature of boiling.
- The spontaneous change of a liquid into its vapor is called evaporation.
- Conversion of liquid to gas or vapor at all temperatures.
- It is also defined in terms of the kinetic energy of molecules that can escape from the surface.
Explanation of Evaporation
- Evaporation is the process in which liquid molecules escape from the surface and enter the gas phase. It can be explained in terms of energy possessed by molecules on the surface of the liquid. Surface molecules whose kinetic energy is higher than average kinetic energy, overcome the intermolecular forces that bind them to the liquid and enter the gas phase. After their escape, the average kinetic energy of the remaining molecule decreases, therefore, the temperature of the liquid decreases, thus evaporation is a cooling process.
- Evaporation is not a heating process, it is a cooling process. when molecules of higher energy escape. The result of this higher temperature fall and the residue molecules have lower energy and lower temperature.
- Molecules of the liquid are in random motion. The energy of molecules is not equally distributed. The molecules which have low kinetic energy move slowly. Those with high kinetic energy move faster. If one high-speed molecule reaches the surface, it may escape the attractions of its neighboring molecule. In this way, it leaves the bulk liquid.
Cooling effect by evaporation
- During hot summers the water is usually kept in the earthen pot to keep it cool because it contains large pores. Water seeps via these pores to the outside of the pot. This water evaporates and takes heat from the surroundings. The result is water inside the pot becomes cool.
- Floors are often sprinkled with water drops in hot summers to keep cool.
- Evaporation also takes place on the leaves of plants which help to keep the surroundings cool and fresh.
- Cotton dresses help to keep our bodies cool by the evaporation method.
Factors Affecting Evaporation
Evaporation occurs at all temperatures from the surface of the liquid. The rate of evaporation depends upon various factors:
Temperature
Evaporation has a direct relationship to temperature. Higher the temperature, the higher the rate of evaporation.
Surface Area
The larger is the surface area of a liquid. A greater number of molecules have the chance to escape from their surface.
Wind
Wind blowing over the surface of liquid sweeps away the liquid molecules that have just escaped out. This increases the chance for more liquid molecules to escape out.
Nature of the Liquid
Liquids differ in the rates at which they evaporate.
Use of evaporation process infiltration
Evaporation to dryness
Seawater contains dissolved salts. To harvest salt from seawater, pans of seawater are evaporated to dryness using heat from the sun.
In other words, evaporation to dryness is used to obtain soluble solid from the solution by heating the solution until all the water boiled off.
For example, In the formation of pickles, vegetables took under sunlight to get rid of extra moisture from them.
Boiling
It starts when the liquid achieves boiling point. When the liquid is under the influence of heat, the vapor pressure of the liquid starts increasing. When the vapor pressure of the liquid becomes equal to the atmospheric pressure. This temperature is said to be the boiling point of the liquid.
The bubbles which are formed in the interior of the liquid have greater internal pressure as compared to external atmospheric pressure on the surface of the liquid. This makes bubbles come out and burst upon the surface of the liquid. This constant stream of bubbles comes out at the boiling point.
Explanation of Boiling point
In an open container, the atmosphere exerts pressure on the liquid surface. When the liquid is heated in an open container, the average kinetic energy of molecules increases. Therefore, the temperature of the liquid also increases. Heating allows a larger number of molecules at the liquid surface to overcome the attractive forces that are responsible for keeping them in the liquid.
This increases the vapor pressure of the liquid, the vapor pressure keeps going to increase with the increase in the temperature. The liquid boils when its vapor pressure becomes equal to the atmospheric pressure i.e 760 mhg or 101.325 kpa at sea level.
Effect of external pressure on boiling point
The liquid boils when its vapor pressure is equal to the pressure exerted on the liquid by its surrounding. The normal boiling point of water is 100oC. In the mountains, the atmospheric pressure is less than 1 atm so water boils below 100oC.
For example, Water boils at 70oC on the top of Mount Everest and 120oC in the pressure cooker because Mount Everest is about 8850 M above sea level, atmospheric pressure is only 34 kpa. Water boils at this height above sea level when its vapor pressure is 34 kpa at 70 oC. That’s why water boils at 70oC. On the other hand, the pressure cooker is equipped with a valve that controls the pressure inside the container, this valve generally exerts a pressure of 2 atm. Therefore, the valve does not allow water vapor to escape until the pressure inside the container reaches 2 atm. Vapor pressure of water reaches 2 atm when the temperature reaches 120oC, So water boils at 120oC in the pressure cooker.
The boiling point of some liquids
| Liquid | Boiling point |
| Water |
100 OC |
| Acetic acid |
118.5 OC |
| Acetone |
56.00 OC |
| Aniline |
184.4 OC |
| Benzene |
80.15 OC |
| Carbon disulphated |
46.30 OC |
| Carbon tetrachlorideMethane |
76.50 OC |
| Ethanol |
78.26 OC |
| Naphthalene |
218.00 OC |
| Phenol |
181.80 OC |
Key Difference
Concepts Berg
How is evaporation different from boiling?
Evaporation is a spontaneous process and can occur at any temperature while boiling is a nonspontaneous process that required a specific temperature.
What is the difference and similarity between evaporation and boiling?
Similarities
- Both are gaseous phases.
- Depends on vapor pressure.
Differences
- Evaporation is a cooling process while boiling is a heating process.
- Boiling is fast while evaporation is slow.
Example of boiling and evaporation?
Water converted into vapor phase during hot summer is an example of evaporation. Water boils at a specific temperature which is 100oC is an example of boiling.
Which is the slower process, evaporation or boiling?
Evaporation is a very slow process while boiling is a rapid process.
Why is it that water evaporates off of any warm surface despite its boiling point being 100 oC?
Because of evaporation which occurs at any temperature. On the other hand, boiling takes place at the temperature of 100oC.
How to make water evaporate fast?
Water can evaporate fast by increasing the temperature because it has a direct relation to the temperature.
What is the difference between drying and evaporation?
Drying means the complete removal of water. Evaporation is a conversion of liquid into vapor phase spontaneously.
What is the difference between evaporation and crystallization?
Crystallization is a process in which crystals are formed while evaporation is the change of a liquid into the gaseous phase.
Why does water evaporate in a vacuum?
Because there is no external vapor pressure acting on the water so it evaporates faster as compared to normal.
What is the difference between boiling and cavitation?
Cavitation is the formation of bubbles or cavities and boiling is the temperature at which liquid boils.
What is the difference between boiling and sublimation?
Sublimation is the conversion of a solid into a gaseous phase without into a liquid phase. While boiling is a temperature at which internal pressure becomes equal to external pressure.
When water evaporates, does its mass decrease?
Evaporation is a slow process, it required more time to convert water into the gaseous phase. As result, the mass of water decreases.
Does water evaporate without a heat source?
Yes, Evaporation does not need an external heat source. Evaporation takes place at all temperatures.
What is the difference between volatility and evaporation
Volatility is a property of liquid. It is the tendency of a liquid to turn into vapors while evaporation is the process of converting the liquid into the gaseous phase.
