Drying and Dehydrating Agent: “Drying” and “dehydrating” are the two words that seem to be quite similar to us because both are accompanied by loss of water. But there are a lot of differences between these two terms! In chemistry, we use a lot of drying and dehydrating agents for maintaining optimum conditions for chemical reactions and for preserving the chemicals. These drying and dehydrating agents are also used in our daily lives, for example, silica gel used in air conditioners and many more.

Let’s have an overview of drying and dehydrating agents. A chemical used to remove water present in the solution of an organic compound is known as a drying agent. In contrast, a substance that dries or completely removes water from a material is a dehydrating agent. Some commonly used chemicals as drying agents are calcium chloride \(\left({{\text{CaC}}{{\text{l}}_2}} \right),\) calcium sulphate \(\left({{\text{CaS}}{{\text{O}}_4}}\right),\) sodium sulphate \(\left({{\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4}} \right),\) magnesium sulphate \(\left({{\text{MgS}}{{\text{O}}_4}} \right)\) in their anhydrous form. Some dehydrating agents are concentrated sulphuric acid, concentrated phosphoric acid, hot ceramics, hot aluminium oxide, etc. Let us understand the meaning of “drying” and “dehydration” before learning about “drying and dehydrating agents.

What is Drying in Chemistry?

Drying is the process of removal of water from a solution, suspension, or other solid-liquid mixture by evaporation. Apart from solids, water can also be removed from liquids or gases by drying. For example, we dry wet clothes in sunlight.

What is Dehydration in Chemistry?

As signified by the name itself, the word “dehydration” deals with the removal of water molecules from a substance. In chemistry, dehydration reaction is a chemical reaction between two compounds that gives one of the products as water. In this reaction, two monomers react where a hydrogen atom \(\left({\text{H}} \right)\) from one monomer binds to a hydroxyl group \(\left({{\text{OH}}} \right)\) of the other monomer to form a dimer and a water molecule \(\left({{{\text{H}}_2}{\text{O}}} \right).\)

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Drying and Dehydrating Agents Definition

Drying Agents – The substance that readily absorbs moisture from other substances without reacting with them chemically are known as drying agents. In short, we can say that all hygroscopic substances are drying agents. For example, Anhydrous calcium chloride, phosphorus pentoxide, anhydrous zinc chloride, dry sodium sulphate, magnesium sulphate, etc., are used to absorb moisture from the air.

Dehydrating Agents – The substances that even absorb the chemically bound or combined water molecules from a compound are known as dehydrating agents. Some of the well-known dehydrating agents are concentrated sulphuric acid, concentrated phosphoric acid, hot ceramics, hot aluminium oxide, etc.

Drying and Dehydrating Agent Acid

Concentrated sulphuric acid can act as a drying and dehydrating agent in various conditions. Let’s discuss this property:

As a Drying agent: Concentrated sulphuric acid can act as a drying agent when it is used to remove water from other substances by evaporating the water molecules, such as drying of gases like hydrogen chloride gas.

As a Dehydrating agent: Concentrated sulphuric acid can act as a dehydrating agent when it is used to remove chemically combined water molecules from a compound due to its strong affinity towards water molecules.

We can take the example of the following chemical reactions:

1. Use of concentrated sulphuric acid as a dehydrating agent for removing water molecules from hydrated copper sulphate crystals.

\(\underset{{\left({{\text{Blue}}\,{\text{crystals}}} \right)}}{\mathop {{\text{CuS}}{{\text{O}}_4}.5{{\text{H}}_2}{\text{O}}}} \xrightarrow{{{\text{Conc}}.{{\text{H}}_2}{\text{S}}{{\text{O}}_4}}}\underset{{\left({{\text{White}}} \right)}}{\mathop {{\text{CuS}}{{\text{O}}_4}}} + 5{{\text{H}}_2}{\text{O}}\)

2. Charring of sugar is another example of dehydration reaction by concentrated sulphuric acid as a dehydrating agent.

\(\underset{{{\text{Sugar}}}}{\mathop {{{\text{C}}_{12}}{{\text{H}}_{22}}{{\text{O}}_{11}}}} \xrightarrow{{{\text{Conc}}.{{\text{H}}_2}{\text{S}}{{\text{O}}_4}}}12{\text{C}} + 11{{\text{H}}_2}{\text{O}}\)

Dehydrating Agents in Organic Chemistry

In organic chemistry, we study dehydration reactions where a dehydrating agent is used to remove water molecules from the organic compound and form a new unsaturated product. Some of the famous dehydrating agents in organic chemistry are \({\text{A}}{{\text{l}}_2}{{\text{O}}_3},\) anhydrous \({\text{ZnC}}{{\text{l}}_2},{\text{conc}}.{{\text{H}}_2}{\text{S}}{{\text{O}}_4},{{\text{H}}_3}{\text{P}}{{\text{O}}_4},{\text{NaB}}{{\text{H}}_4},\) etc.

Dehydrating Agents for Alcohol

Alcohols undergo a dehydration reaction to form alkenes. Different dehydrating agents are used for different types of alcohol. Let’s understand this with the help of the following given examples:

Dehydration of Secondary and Tertiary alcohols: Secondary and tertiary alcohols are efficiently dehydrated by heating the alcohol with concentrated sulphuric acid as it helps to remove a water molecule due to the presence of this protic acid. The dehydration of secondary and tertiary needs a strong dehydrating agent, and it is known as an \({\text{E1}}\)-elimination reaction. It is a two-step mechanism.

\({\text{C}}{{\text{H}}_3}{\text{CH}}\left({{\text{OH}}} \right){\text{C}}{{\text{H}}_3}\xrightarrow[{{\text{heat}}}]{{{\text{conc}}.{{\text{H}}_2}{\text{S}}{{\text{O}}_4}}}{\text{C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2} = {\text{C}}{{\text{H}}_2} + {{\text{H}}_2}{\text{O}}\)
Dehydration of Primary alcohols: Dehydration of primary alcohol is known as \({{\text{E}}_2}\)-elimination reaction (it’s a one-step reaction) due to the difficulty to form primary carbocations.
\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}}\xrightarrow{{{{\text{H}}^ + },{\text{heat}}}}{\text{C}}{{\text{H}}_2} ={\text{C}}{{\text{H}}_2} + {{\text{H}}_2}{\text{O}}\)

Distinguish between Drying and Dehydrating Agent

Drying Agent	Dehydrating Agent
The drying agent readily absorbs moisture (water) from other substances without reacting with them.	The dehydrating agent even absorbs the chemically combined water molecules from a compound.
Drying agents do not react chemically. Drying is a physical change.	Dehydrating agent undergoes chemical change as it combines chemically.
For example, anhydrous calcium chloride, phosphorus pentoxide, anhydrous zinc chloride, dry sodium sulphate, magnesium sulphate, etc.	For example, concentrated sulphuric acid, concentrated phosphoric acid, hot ceramics, hot aluminium oxide, etc.

Desiccating Agents

Drying agents are also known as desiccants or desiccating agents like silica gel, quicklime. Solid substances are dried by spreading and keeping them on a watch glass inside a desiccator. A desiccator is a glass chamber containing anhydrous calcium chloride that absorbs all the moisture and makes the solid compound dry.

Summary

In a nutshell, “drying and dehydrating agents” are two different things that seem to be similar due to their action of removing water molecules from a substance, but there are many differences between these two terms due to the way they react. A chemical substance used to remove water or moisture present in the solution of an organic compound is known as a drying agent, while a substance that dries or completely removes water molecules from material by chemically combining with them is known as a dehydrating agent. Apart from chemical laboratories drying agents or dehydrating agents are also used for biological functions as; in biological laboratories dehydrating agents such as alcohol, isopropanol, and butanol are used to clean and mount tissues on slides for examining them.

In air conditioners, silica gel is essentially used as a drying agent to absorb moisture in the atmosphere. Some other commonly used chemicals as drying agents in chemistry are calcium chloride \(\left({{\text{CaC}}{{\text{l}}_2}} \right),\) sodium sulphate \(\left({{\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4}} \right),\) magnesium sulphate \(\left({{\text{MgS}}{{\text{O}}_4}} \right),\) calcium sulphate \(\left({{\text{CaS}}{{\text{O}}_4}} \right)\) etc., in their anhydrous form. While concentrated sulphuric acid, concentrated phosphoric acid, hot ceramics, hot aluminium oxide, etc., are some dehydrating agents.

FAQs on Drying and Dehydrating Agents

Q.1. What is drying and dehydrating agents? Give examples.
Ans: The substance that readily absorbs moisture from other substances without reacting with them chemically is known as a drying agent. We can also say that all hygroscopic substances are drying agents. For example, anhydrous calcium chloride, phosphorus pentoxide, anhydrous zinc chloride, dry sodium sulphate, magnesium sulphate, etc., are used to absorb moisture from the air. On the other hand, the substances that remove water by absorbing the chemically bound or combined water molecules from a compound are known as dehydrating agents. Some of the well-known examples of dehydrating agents are concentrated sulphuric acid, concentrated phosphoric acid, hot ceramics, hot aluminium oxide, etc.

Q.2. What are dehydrating agents?
Ans: Dehydrating agents are substances that even absorb the chemically combined water molecules from a compound. They undergo chemical change. For example, by heating alcohol in the presence of concentrated sulphuric acid, it turns to an alkene with the removal of a water molecule (dehydration).

Q.3. What are drying agents? Give examples.
Ans: All hygroscopic substances are drying agents. This means that any substance that readily absorbs moisture or water from other substances without reacting with them chemically is known as a drying agent. For example, anhydrous calcium chloride, phosphorus pentoxide, anhydrous zinc chloride, dry sodium sulphate, magnesium sulphate, etc., are used to absorb moisture from the air. Concentrated sulphuric acid can act as a drying agent when used to remove water from other substances by evaporating the water molecules, such as drying gases like hydrogen chloride gas.

Q.4. What is a desiccating agent?
Ans: Drying agents are also known as desiccants or desiccating agents. A desiccator is a glass chamber containing anhydrous calcium chloride that absorbs all the moisture inside it and makes the solid compound dry. Some examples of desiccating agents are silica gel, quicklime. Solid substances are dried by spreading and keeping them on a watch glass inside a desiccator.

Q.5. Give an example of a substance used both as a drying agent and dehydrating agent.
Ans: One of the best examples of the substance or chemical used both as a drying agent and a dehydrating agent is concentrated sulphuric acid. This acid can act as a drying agent when it is used to remove water from other substances by simply evaporating the water molecules, such as drying gases like hydrogen chloride gas. In contrast, concentrated sulphuric acid can act as a dehydrating agent at the same time when it is used to remove chemically combined water molecules from a compound due to its strong affinity towards water molecules. For example, concentrated sulphuric acid is used as a dehydrating agent for removing water molecules from hydrated copper sulphate crystals. Charring of sugar is another example of dehydration reaction by concentrated sulphuric acid as a dehydrating agent.

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