Main Difference – Anhydrous vs Dihydrate
Compounds in their solid state can exist as either in the anhydrous form or hydrated form. The term anhydrous means without water while the term hydrated means with water. These terms are applied to crystalline structures. Some solid crystals do not have water molecules at all. These compounds are called anhydrous compounds. Hydrated molecules are composed of water molecules. These hydrated molecules can be further categorized according to the number of water molecules present in the compounds. Such categories include monohydrated compounds, dehydrated compounds, etc. The main difference between anhydrous and dihydrate compounds is that anhydrous compounds have no water molecules whereas dihydrate compounds are composed of two water molecules per formula unit of the compound.
Key Areas Covered
1. What is Anhydrous
– Definition, Explanation with Examples
2. What is Dihydrate
– Definition, Explanation with Examples
3. What is the Difference Between Anhydrous and Dihydrate
– Comparison of Key Differences
Key Terms: Anhydrous, Deuterium, Dihydrated, Drying Agents, Heavy Water, Hydrated, Monohydrated
What is Anhydrous
Anhydrous is a term used to describe the absence of water in a compound. Substances without water are called anhydrous compounds. We can obtain anhydrous compounds through different techniques. These techniques differ from each other depending on the type of substance. Most anhydrous compounds have different colors and chemical properties from their hydrous forms.
Figure 1: Copper(II) sulfate is white colored in its anhydrous form. It changes to blue color when water is added.
Sometimes, the term anhydrous is used to describe the gaseous phase of a compound. For example, anhydrous ammonia is gaseous ammonia. This is to distinguish it from its aqueous solution. However, the compound has no water molecules.
Sometimes, anhydrous compounds can be obtained simply by heating the solution until a constant mass is obtained. But this method does not always work because water molecules can sometimes get trapped during the formation of solid crystals. A compound can also be made anhydrous by adding a reagent. This added reagent should be able to absorb water.
A common compound for anhydrous is anhydrous calcium chloride. It is an anhydrous salt. It is very useful as a drying agent. It is also helpful in determining the humidity in the air due to its dehydrating capability. When it absorbs water, the anhydrous form is converted to the hydrous form.
Not only solids, sometimes we can find anhydrous solvents as well. These solvents do not contain water molecules. For example, organic solvents do not contain water molecules. They are called anhydrous solvents. These solvents are important in reactions where the presence of water is unfavorable. Anhydrous solvents can be prepared by removing water forcefully; sometimes water is separated from these solvents due to lack of polarity.
What is Dihydrate
Dihydrate is a term used to describe the presence of two water molecules per formula unit of the compound. A hydrate is also defined as a compound that can absorb water from the surroundings and include these water molecules in their structure. The nomenclature of these compounds also differs from their anhydrous forms due to the presence of these water molecules. For example, anhydrous copper (II) chloride is brown colored whereas copper (II) chloride dihydrate is blue-green in color. Therefore, when this dihydrate compound is heated, the color is faded and turned into brown colored crystals due to the removal of water molecules.
Figure 2: The dihydrated form of copper(II) chloride.
The water included in the crystal structures are called the water of crystallization. These water molecules are trapped in the crystalline structure during the crystallization process. Generally, these water molecules can be removed by heating the compound.
The term Dihydrate is used to indicate the presence of two water molecules. For example, CaCl2.2H2O is named as Calcium chloride dihydrate. But if those water molecules are heavy water molecules composed of Deuterium instead of Hydrogen atoms, then it is called deuterate rather than dihydrate.
Difference Between Anhydrous and Dihydrate
Definition
Anhydrous: Anhydrous is a term used to describe the absence of water in a compound.
Dihydrate: Dihydrate is a term used to describe the presence of two water molecules per formula unit of the compound.
Crystalline Structure
Anhydrous: The crystal structure of anhydrous compounds have no water molecules.
Dihydrate: The crystal structure of dehydrated compounds are composed of water molecules trapped inside the structure.
Water Absorption
Anhydrous: Anhydrous compounds are good water absorbing agents.
Dihydrate: Dehydrated compounds are not very good in absorbing water from the surrounding.
Applications
Anhydrous: Anhydrous compounds can be used as drying agents.
Dihydrate: Dehydrated compounds have different applications according to the chemical compound.
Heating
Anhydrous: Heating anhydrous compounds do not evolve water vapor.
Dihydrate: Dehydrated compounds can release water vapor upon heating.
Conclusion
The terms anhydrous and dihydrate are used to describe the presence or absence of water molecules in a compound. The main difference between anhydrous and dehydrate is that anhydrous compounds have no water molecules whereas dihydrated compounds are composed of two water molecules per formula unit of the compound.
References:
1. Helmenstine, Anne Marie. “What Anhydrous Means in Chemistry.” ThoughtCo, Available here. Accessed 18 Sept. 2017.
2. Crampton, Linda. “What Is a Hydrate (Chemistry)?” Owlcation, Owlcation, 7 Aug. 2017, Available here. Accessed 18 Sept. 2017.
3. “Water of crystallization.” Wikipedia, Wikimedia Foundation, 17 Sept. 2017, Available here. Accessed 18 Sept. 2017.
Image Courtesy:
1. “Hydrating-copper(II)-sulfate” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia
2. “Copper(II) chloride dihydrate” (Public Domain) via Commons Wikimedia
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