Difference Between Evaporation and Distillation

Main Difference – Evaporation vs Distillation

Matter is considered the basic structural component of the universe. It is present in three phases: solid, liquid and gas. A substance can change its physical state between these three phases. This phenomenon is called phase change and this can occur at different temperatures. A liquid substance can go into the gaseous phase either by evaporation or boiling. Evaporation occurs when there is sufficient heat energy to break the intermolecular attractions and release the liquid molecules into the gaseous phase. Boiling of a particular substance occurs at a fixed temperature at which, the vapour pressure exerted by the substance in the gaseous phase becomes equal to atmospheric pressure. Distillation is based on this phenomena. The main difference between evaporation and distillation is that evaporation occurs below boiling point whereas distillation occurs at the boiling point.

This article studies, 

1. What is Evaporation
      – Definition, Process, Chemistry of Evaporation
2. What is Distillation
      – Definition, Process, Chemistry of Distillation, Uses
3. What is the difference between Evaporation and Distillation

Difference Between Evaporation and Distillation - Comparison Summary

What is Evaporation

At a given temperature, a substance in the liquid phase has the tendency to change into the gaseous phase without reaching its boiling point through the process called evaporation. Liquid molecules have intermolecular bonds. With sufficient heat energy, these bonds tend to dissociate, releasing the molecules into the gaseous phase. This process happens at the surface of the liquid. This is because the surface is in close contact with the atmosphere and heat energy can be easily absorbed. Evaporation happens below the boiling point of the liquid. Only the liquid molecules which are at the surface, absorb energy from the atmosphere to break their intermolecular bonds and go into the gaseous phase. Molecules at the bulk of the liquid only get evaporated when they reach the surface and get exposed to the atmosphere.

The rate of evaporation is directly associated with the strength of intermolecular bonds between the liquid molecules. When the strength of intermolecular bonds is high, the liquid is less volatile. Liquids with weak intermolecular bonds are highly volatile. Water molecules are less volatile due to the strong hydrogen bonds between the molecules. Non-polar organic compounds do not have such strong intermolecular attractions. They have Van Der Waals bonds which are comparatively weak. Therefore, the liquid molecules can easily go into vapour phase. Most non-polar organic liquids are highly volatile.

Evaporation is a slow process. The evaporation rate of the same liquid depends on the surface area and air flow rate. When the surface area and air flow rate is high, rate of evaporation automatically increases.

Key Difference - Evaporation vs Distillation

Figure 1: Evaporation is a crucial step in the water cycle.

What is Distillation

Distillation is a modern separation technique adapted, based on the different boiling points of liquids. This is due to the strengths of different intermolecular forces of substances. Different liquids boil at different temperatures because the heat energy needed for bond breaking varies.

Distillation is used to separate mixtures of liquids. This involves boiling and condensing liquids.

The liquid is heated and boiled at its boiling point. The temperature remains constant until the relevant liquid vaporizes completely. The vapour is then turned into liquid phase with the aid of a condenser.

There are several distillation methods such as simple distillation, fractional distillation and steam distillation.

Simple Distillation

This is used to separate liquids with a significant boiling point gap. The components of the liquid mixture are separated when they boil at their respective boiling points and change into the vapour phase. The vapour is then condensed and collected.

Fractional Distillation

In this methods, a fractionating column is employed to separate two miscible liquids, which have close boiling points.

Read more: Difference Between Simple and Fractional Distillation

Steam Distillation

Steam is used to separate compounds which are immiscible with water. When such compounds are mixed with steam, they tend to vaporize at a lower temperature than their usual boiling point.

Difference Between Evaporation and Distillation

Figure 2: Fractional Distillation

Difference Between Evaporation and Distillation

Definition

Evaporation: Evaporation is the process of transforming liquid into a gas, under the influence of heat. 

Distillation: Distillation is a process that consists of obtaining gas or vapor from liquids by heating and condensing to liquid products  for purposes such as purification, fractionation.

Features

Evaporation: Evaporation only occurs at the surface. 

Distillation: Distillation does not only occur at the surface. 

Boiling Point

Evaporation: The liquid vapourizes below boiling point. 

Distillation: The liquid vapourizes at boiling point.

Time taken for the Process

Evaporation: This is a slow process. 

Distillation: This is a rapid process.

Separation Technique 

Evaporation: This is not a separation technique. 

Distillation: This is a separation technique. 

Summary  – Evaporation vs Distillation

Evaporation only occurs at the surface of the liquid by absorbing heat from the atmosphere. The liquid vaporizes at a temperature below its boiling point. Distillation involves vapour formation at the boiling point of the liquid and is a rapid process compared to evaporation. Evaporation occurs at the surface of the liquid whereas distillation occurs with the boiling of the bulk of the liquid. This is the difference between evaporation and distillation.

Reference:
1. Shipu, Sourav .”Distillation, Vaporization, Evaporation etc.” LinkedIn SlideShare. N.p., 30 Apr. 2014. Web. 23 Feb. 2017.
2. “Boiling.” Chemistry LibreTexts. Libretexts, 21 July 2016. Web. 23 Feb. 2017.
3. “Why do different substances have different boiling points?” Chemistry Stack Exchange. N.p., n.d. Web. 23 Feb. 2017.

Image Courtesy:
1. “Fractional distillation lab apparatus” By derivative work: John Kershaw (talk)Fractional_distillation_lab_apparatus.png: User:Theresa knott – Fractional_distillation_lab_apparatus.svg, (CC BY-SA 3.0) via Commons Wikimedia 
2. “Water Cycle” by AIRS (CC BY 2.0) via Flickr

About the Author: Pabasara

Pabasara posses a Bachelor's Degree in Chemistry and is reading for M.Phil. in Chemistry. She has working experience in both academic and industry environments.

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