Question

What are drying agents, desiccating agents, and dehydrating agents? Explain how they work, their nature, uses, and give examples for the same.

Answer 1

A drying agent is a chemical used to remove water from an organic compound that is in solution. In making or isolating chemical compounds they often become contaminated with water. If you want to dry a compound that is in solution, a saturated (highly concentrated) solution of sodium chloride can be used to remove the extra water. Water will move from an area ofhigher concentration (the compound solution) to an area of lower concentration (the saturated salt solution). You will get 2 layers: the salt solution layer and the layer of "dried " organic compound solution. The pure compound can then be separated from the salt solution layer. Drying agents can also be solid like Calclium chloride, Potassium chloride, or Magnesium sulfate. These are all types of salts (ionic compounds where metals are bonded to nonmetals.) These drying agents are added to the solutionof a compound that needs to be dried. The agents will easily pick up any extra water from the compound solution and become hydrated. The hydrated salts form clumps which can be filtered out or left behind during decanting resultingin a "dry" compound. Desiccants are drying agents that extract water from a wide range of materials. They are either soluble or insoluble substances that adsorb water due to their chemical properties. Examples include silica gel, bauxite, calcium sulfate and montmorillonite clay. Soluble agents include calcium chloride and glycerol. Molecular sieves, alumina and activated charcoal also make exceptional desiccants. Desiccants are used to prevent humidity from damaging the contents of sealed packages. This is the reason that so many products include small packages of silica gel and similar materials. These desiccants can hold a large percentage of their weight in moisture, and will adsorb the great majority of humidity within the package. Desiccants also find use in industrial applications, such as humidity detection, waste removal and cleaning, and regenerative drying. Regenerative dryers send air to a molecular sieve (or other) desiccant bed, where the moisture is adsorbed. Later, this moisture is purged by sending heated air through the system; this lowers the moisture retention capabilities of the desiccant and forces it to release stored moisture into the heated air. Eventually, this moisture laden air is expelled and the regenerative dryer cycle is complete. Desiccants come in a wide range of shapes, from perforated canisters containing silica gel to silica gel beads encased in specialized bags. Various substances, such as paper, cotton and Tyvek, are used as desiccant packaging material. Typically commercial desiccants of this nature will change color as they become full with water, thereby alerting the user that a new desiccant is required.

A dehydrating agent, actually acts as a reactant. It helps formation of a new substance by removing some parts of the other reactant as water. It effectively decomposes the other reactant by breaking chemical bonds.For example, phosphorus pentoxide dehydrates nitric acid to dinitrogen pentoxide when it is distilled with the latter. 2HNO3 + P2O5 = 2HPO3 + N2O5 Alumina (Al2O3) and conc.H2SO4 are also widely used as dehydrating agents, especially in converting alcohols to alkenes. H2SO4 extracts water from alcohol to form ether.