Evaporation: Definition, Meaning, Water Cycle, Endo or Exothermic? Physical or Chemical Change?


In this article, we will learn what is evaporation, its definition, meaning, water cycle, is evaporation endothermic or exothermic or is evaporation a physical or chemical change, application, advantages, disadvantages, etc.

Let’s explore!

What is Evaporation? Definition, Meaning

What is Evaporation? Definition

Let’s try to understand the definition of evaporation. The transformation of an element or compound from a liquid state into a gaseous state is called evaporation.

  • It is a surface phenomenon, and the conversion of liquid into gas takes place below the boiling temperature of the liquid.
  • When the molecules gain enough energy, they will escape as vapors. 

Meaning of Evaporation

When liquid or water changes into gas or vapors, the process is termed evaporation. It’s a natural process that takes place both in nature and in artificial systems.

  • It is an essential phenomenon playing an essential role in exchanging energy between Earth and the atmosphere.
  • Several factors govern the evaporation process and result in an increase or decrease in the evaporation rate. 

What is the Process of Evaporation? 

The phenomenon of evaporation is an endothermic process. It’s an endothermic process because the system needs the energy to break bonds, and this way, the liquid changes into a gaseous state. “Endo” meaning within, and “therm” meaning heat.

  • The boiling point is the temperature at which a liquid converts into a gas.
  • When a liquid reaches its boiling point, the molecules gain more energy from heat and vibrate faster.
  • At the boiling point or the melting point, the temperature doesn’t change; instead, the heat provided is used in breaking the bonds to change the state.
  • When the molecules gain sufficient energy, they break the attractive forces that hold them together and become vapor.
  • The molecules now move faster than gas because the forces of attraction between gas molecules are weaker than in liquids.

Hence, in this way, the molecules gaining sufficient energy will escape and change into vapors. The escape of these molecules results in “evaporative cooling”. Evaporation can take place at room temperature also without providing any heat. 

Factors Affecting Rate of Evaporation

From the evaporation definition, we can say that the evaporation process is a surface phenomenon dependent on several factors. When the liquid particles gain enough kinetic energy, they break the liquid bonds converting them into gas.

The factors affecting the rate of evaporation are:

  • Temperature
  • Surface area 
  • Humidity/dryness in the surrounding  
  • Wind 
  • Saturated air 
  • Vapor pressure 

Temperature 

The rate of evaporation is directly proportional to temperature. When the temperature is hot, it results in the high kinetic energy of molecules of liquids.

  • Due to higher kinetic energy, the molecules can overcome the intermolecular forces of attraction, which hold them together.
  • In higher surrounding temperatures, more heat is transferred to molecules, resulting in bond breakage and converting them into vapors.
  • So Hot environments increase the rate of evaporation.

Surface area 

As mentioned above, evaporation is a surface phenomenon. The rate of evaporation is directly proportional to the surface area of the container holding the liquid.

  • Liquids don’t have a definite shape, and they attain the shape of the container in which they are stored.
  • If the container has a larger surface area, more liquid molecules are exposed and are more likely to escape. 

During evaporation, the surface molecules may gain enough energy and escape as vapors.

More surface area means more molecules are present at the surface of the container, ultimately resulting in more evaporation. Hence, in this way, greater surface area results in more evaporation. 

Humidity and dryness in the surrounding 

The rate of evaporation is inversely related to humidity in the surrounding environment. The lesser the humidity, the greater the evaporation rate.

If the surrounding is humid, it means more water vapors are present in the air which will cause a decrease in the rate of evaporation. 

  • There is a limit to the number of water vapors present in the air.
  • If the surrounding air is dry, the rate of evaporation will be greater.
  • Dry air means fewer vapors and also less humidity.
  • Therefore, the phenomenon of evaporation takes place at a high rate.
  • So, evaporation is inversely related to the dryness in the surroundings. 

Wind 

Water vapor present in the air results in humidity. The moisture remains in the air if the air is still.

  • When the wind is blowing, the molecules will disperse, resulting in less humidity, which means there is room for vapors in the surrounding air.
  • Therefore, evaporation from the surface of the liquid can take place efficiently. 
  • Hence, the rate of evaporation is directly proportional to the speed of wind/ blowing wind.

Saturated air  

When the air is saturated, the rate of evaporation slows down, or it may be zero.

  • When the air is clogged with other molecules, there will be no more room for water molecules to evaporate and form vapors.
  • For example, when the air is entirely clogged with water molecules, it means humidity is 100%; hence, evaporation doesn’t occur.

Vapor pressure 

Vapor pressure is the measure of pressure exerted by a gas in a closed container on the liquid.

  • If the intermolecular forces are strong, the vapor pressure will be less, and hence the rate of evaporation will also be below.
  • Therefore, weak intermolecular forces result in high vapor pressure and evaporation.
  • Apart from this, vapor pressure is also affected by temperature. Increasing the temperature will cause an increase in vapor pressure. 

Rate of Evaporation Calculation

From the definition of evaporation, we understood that the rate of evaporation is influenced by certain factors, as discussed above. For example, if ethanol and water are taken in the exact quantities in different containers, they will evaporate faster.

On the contrary, water and liquids evaporate at different rates. The rate can be calculated by simple means. 

  • Firstly note whether you are inside or outside. 
  • Secondly, note all conditions like temperature, time, relative humidity, barometric pressure, average wind speed, and weather (sunny or cloudy). 
  • It is better to perform indoors because the conditions are controllable. 
  • Thirdly, fill a graduated cylinder up to 500 ml with the liquid for which you want to calculate the evaporation rate and start your stopwatch.
  • Fourthly, note the amount of liquid left in the cylinder. For example, after one hour, it’s 490 ml. 
  • Lastly, Subtract the initial reading from the final reading and divide the result by time taken. In this case, it’s 1 hour. So the final result will be 10ml/hour. 

It is a simple calculation for checking out the rate of evaporation. However, on a large scale, different scientific formulas are used.

For example, the Penman formula is used to calculate the rate of evaporation from lakes.

Evaporation Water Cycle 

Water is an essential component that sustains life on Earth. Evaporation plays a vital role in the water or hydrologic cycle, water circulation between the earth-atmosphere system.

Precipitation, transpiration, freezing, melting, condensation, and runoff are also a part of this cycle.

  • The water evaporates from sea and ocean bodies.
  • It occurs when some molecules gain enough kinetic energy to escape into a gaseous state.
  • The process is affected by factors like humidity, temperature, wind, and solar radiation.
  • Transpiration from plants occurs in the process.
  • The evaporation and transpiration from water, soil, ice, vegetation, etc., are collectively known as evapotranspiration.
  • Further on, precipitation, condensation, and other processes accompany it, which are involved in the hydrologic cycle.
  • In this way, evaporation is a significant process that initiates the water cycle. 

Is Evaporation Endothermic or Exothermic?

What do you think “is evaporation endothermic or exothermic“? Let’s try to understand the basics with proper explanation.

We will know that, the meaning of the endothermic and exothermic processes.

  • Endothermic Process: The process where energy or heat you can say here, is absorbed from the atmosphere or surrounding.
  • Exothermic Process: The process where energy or heat you can say here, is released to the atmosphere or surrounding.

Now, as per the definition of evaporation, water changed into its phase from liquid water to water vapor.

The evaporation equation becomes,

H2O (liquid) → H2O (vapor)

We know from the molecular structure of water, hydrogen bonds are kept each water molecule together and it exists as a liquid.

Now, when liquid water is changed into vapor, definitely its hydrogen bond needs to be broken so that each water molecule will be freed as can exist as vapor or gaseous phase.

To break this bond, energy is required and this energy is absorbed from the surrounding. So, we can say, the evaporation process is an endothermic process.

Is Evaporation a Chemical or Physical Change?

Many times, it’s really confusing to understand that “is evaporation process is a physical change or a chemical change“. Let’s try to explain it clearly.

Liquid water, H2O (liquid), changed its phase from liquid to H2O(vapor). Here, hydrogen bonds between water molecules are broken and molecules are separated but atoms are not broken.

  • It doesn’t change its checmial properties
  • H2O (liquid) becomes H2O (vapor) means no change in H2O or any chemical change happens.
  • No break between Hydrogen atom or Oxygen atom.  
  • Only physical change that from liquid to vapor.

Hence, we can conclude that the evaporation process is a physical change instead chemical change.

Applications of Evaporation

Evaporation has widespread use in many areas. Some of them are discussed below. 

Food and Beverage Industries

Evaporation is used in the food and beverage industries. The industries employ thin-film evaporation technology for temperature-sensitive products to prevent viscosity, fouling, or foaming.

In Food industries, liquid products can be concentrated from 5% dry solids to 72% or even higher. The technology is used to achieve:

  • Dry coffee products 
  • Purified botanical extracts and oils 
  • Dry soy lecithin having less than 1% moisture 
  • The concentration of cheese products and sugar concentrates 
  • The concentration of fruit and vegetable purees
  • Pre-concentrate the food 
  • reduced water content
  • change in color of the  food
  • the increased solid content in food 

Chemical Industries 

The thin-film evaporation technology is applicable in chemical industries. It is used :

  • To Attain Better quality products by lessening the residence time
  • To Purify and separate chemical products like pharmaceuticals, food, plastics, resins, polymers, petrochemicals, bio-based chemicals, etc. 
  • To Complete reactions
  • To De-monomerize resins and plastics 
  • In deep vacuum levels to distillate high-boiling or temperature-sensitive materials
  • To increase the flowability of viscous concentrated products 
  • To attain highly purified products by removing impurities to very low residual levels
  • To process foaming or foul-prone products

Plastic and Polymer Industry

The thin-film evaporation technique is used in the plastic and polymer industry also to:

  • Improve heat transfer and mass transfer 
  • Enhance flowability of viscous concentrated products 
  • Concentrate and devolitize Polyethylene, polypropylene, phenolics, polystyrene, polycarbonate, polyesters, PVA, polydextrose, epoxies, silicones, acrylics, and various engineered thermoplastics
  • Recover solvents or monomers and polymers from waste streams 

Purification of Silicone 

The thin-film evaporation technology is used in silicone purification also. It is used to purify and devolitize silicone to safe PPM levels.

The technology helps remove silicone cyclic from polysiloxane oils to provide silicone manufacturers with products that meet safety standards. 

Resource Recovery 

Through evaporation technology, concentrated waste streams are used to recover solvents that are reused in the process.

It is a cost-effective technique often used commercially. The method can is to:

  • Recover Catalysts from reactions 
  • Regain Polymer used in plastic coating paper recycling 
  • Regain Acids from process streams 
  • Recover Organic products from tars and residues 
  • Recover Glycol so that it is reused
  • Remove solvents from bitumen or other petroleum products 
  • Reduce volatiles to less residual levels hence maximizing the recovery
  • Refine oil/ perform vacuum distillation 
  • Minimize waste streams and recover solvents from foul-prone waste products like resins, oils, inks, paints, etc

Agrochemicals

The thin-film evaporation technology is applied in agrochemicals also. The engineered systems and the technology are used to attain:

  • Concentrated AI (active ingredients) 
  • Distilled temperature-sensitive products 
  • Solvents that are efficiently recovered
  • Clean products like proteins or other fouling prone products 
  • Concentrated botanical extracts and fermentation broths

Water Desalination 

With the increasing population, there is an immense need for freshwater. Some areas don’t have freshwater sources like lakes or rivers.

They rely on limited underground water reservoirs and are rapidly becoming brackish as water extraction from aquifers continues.

  • Therefore, solar desalination evaporation is employed, which produces rain.
  • This water is then used as a primary artificial freshwater source on Planet earth. 
  • This process is pretty old.
  • The past civilizations used it as a technique to convert seawater to drinkable water while on ships.
  • However, modern times use desalination plants to carry out the process on a large scale. 

Evaporative Cooling 

Evaporative cooling or adiabatic cooling is a highly energy-efficient cooling technique. It ventilates 100% fresh, filtered, and cooled air in a room or building.

The working principle of Evaporative cooling is water evaporation which is used to cool down the air to a comfortable temperature.

In this way, it provides a comfortable indoor air quality in office buildings.

  • It is an old technique that was used by early civilizations.
  • Back in 2500 BC, Egyptian murals had porous clay vessels placed beneath air inlets so that when the air passed through them, they would produce a cooling effect.
  • At that time, the technique of evaporative cooling was employed in housing and architecture.
  • Similarly, evaporative cooling uses wet rags with wind towers to provide cooling in buildings. It’s a traditional Iranian architectural design. 

Evaporators 

Evaporation is used in many industries, especially in food industries. Evaporators are devices used for dewatering, concentration, and crystallization processes in chemical, food, petroleum, and many other industries.

They are available in different types, including: 

  • Falling Film Evaporators 
  • Rising Film Evaporators 
  • Forced Circulation Evaporators 
  • Plate Evaporators 
  • Thermal and Mechanical Vapor Recompression (TVR & MVR)

The typical applications of these evaporators are Product concentration, Dryer feed pre-concentration, Volume reduction, Water/solvent recovery, Crystallization.

Advantages of Evaporation

Evaporation is a naturally occurring phenomenon. It has many advantages; therefore, it is widely used commercially. The advantages include:

  • It is used to remove liquid from many products.
  • Concentrates such as juice concentrates are created in beverage industries. 
  • The process is used to concentrate whole milk, skim milk, whey, or lactose. It is either sold as concentrated liquids or added to products like protein shakes and infant formulas. 
  • There are several advantages to non-food processing facilities.
  • Industries use large amounts of water, and it also results in large amounts of wastewater. Draining the wastewater is harmful to the environment. So manufacturers use evaporation to produce condensed steam from wastewater on site. It is very beneficial because the water is reused in processing plants. It’s very cost-effective and helps save the environment from harmful wastes. 
  • The process is advantageous for the ethanol industry. In ethanol industries, a syrupy liquid is left after the extraction of biofuels from food by-products. So, the manufacturers remove water from the syrup; hence the concentrated syrup can be utilized for animal feed. 
  • Evaporation plays a vital role in the water cycle. 
  • Seawater can be converted to freshwater using water desalination plants based on the principle of evaporation. 
  • It’s a widely used process for steam generation in power plants, water desalination, thermal management, and membrane distillation.
  • Evaporation is commercially used in pharmaceutical, food, beverage, agrochemical, plastic and polymer, and chemical industries.
  • It is a significant phenomenon utilized by homeotherms to lose heat in warm conditions by physiological processes such as sweating and panting. 
  • Similarly, in human beings, the average body temperature is 36.5 or 37 degrees celsius. When outside temperatures are higher, sweating starts, which helps to remove the excess heat from the body and produces a cooling effect.
  • The phenomenon results in heat loss and hence produces a cooling effect. 
  • It is used for the treatment of liquid radioactive waste along with non-volatile radionuclides and salt. 

Disadvantages of Evaporation

  • Evaporation leads to water loss from open storage.
  • A great deal of water evaporates from irrigational channels.
  • About 40 % of water is evaporated from the reservoirs yearly. 
  • Some technologies like windbreaks, floating covers, etc., may be used to overcome evaporation losses, but these techniques are costly, difficult to install, and threaten the environment. It results in high decontamination factors. Moreover, it also provides a good concentration of salts and radionuclides in the bottom concentrate. 
  • Excessive sweating from the body can disturb the water/solute balance and cause dehydration in humans sometimes. 

Conclusion 

Evaporation is a naturally occurring phenomenon. It is pretty beneficial; therefore, several artificial techniques also use evaporation. It’s a widespread commercial technique used in several industries. Evaporation is an essential phenomenon in the water cycle as well. We have learned the definition, meaning, and detailed process.

The phenomenon is an endothermic process. Energy is required to overcome the intermolecular forces so that water droplets can escape and convert to gas. As the molecules gain sufficient kinetic energy, they will leave the liquid as vapors and produce a cooling effect in the system. 

Evaporation has been utilized since ancient times by people of older civilizations. In older times, porous vessels filled with water were placed in inlets of buildings to keep the buildings cool. The process has been used for years to achieve evaporative cooling. It is also known as adiabatic cooling, a modern technique to produce remarkable and fresh indoor air in buildings and offices.

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