Difference Between Hydrous and Anhydrous

Main Difference – Hydrous vs Anhydrous

Hydrous compounds are chemical compounds that are composed of water molecules in their structure as a constituent. Anhydrous compounds are chemical compounds that have no water molecules in the chemical structure. This is the main difference between hydrous and anhydrous compounds. Hydrous compounds are known as hydrates. Anhydrous compounds are known as anhydrates.

Key Areas Covered

1. What is Hydrous
      – Definition, Structure, Examples
2. What is Anhydrous
      – Definition, Structure, Examples
3. What is the Difference Between Hydrous and Anhydrous
      – Comparison of Key Differences

Key Terms: Anhydrate, Anhydrous, Crystal Lattice, Crystallization, Grignard Reaction, Hydrate, Hydrous, Hygroscopic

What is Hydrous

Hydrous is a term used to describe a substance that contains water as a constituent. Hydrous compounds are known as hydrates. Here, water refers to the water of crystallization. This means water can get trapped inside the crystal lattice of substances while crystallization occurs since these compounds cannot crystallize in the absence of water.

Water in these compounds exists in the form of H₂O molecules. Hydrous compounds (hydrates) can be either organic compounds or inorganic compounds. In organic chemistry, hydrous compounds are formed by the addition of water into an organic molecule. In inorganic chemistry, hydrous compounds are slats containing water molecules in definite ratios in their crystal structures.

The substances that absorb water from the air to form hydrates are known as hygroscopic compounds. This water absorption is often a reversible process. Sometimes, it appears as a color change when water is absorbed, and a hydrate is formed.

Most inorganic hydrous compounds are coordination compounds having a central metal ion bonded to several water molecules via coordinate covalent bonds. There, water molecules act as ligands. The coordination number will equal to the number of water molecules in the complex.

Figure 1: Ferrous Sulfate is a Hydrous Compound

Examples of Hydrous Compounds

Ethanol (CH₃CH₂OH)
Barium Chloride (BaCl₂.2H₂O)
Copper(II) Sulfate (CuSO₄.5H₂O)
Ferrous Sulfate (FeSO₄.7H₂O)
Calcium Sulfate (CaSO₄.2H₂O)

What is Anhydrous

Anhydrous is a term used to explain a substance that does not contain water as a constituent. It describes the absence of water in a compound. These compounds are known as 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.

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.

Anhydrous compounds are used for chemical processes where chemical reactions should be done in the absence of water. For example, Grignard reaction should be done in the absence of water; otherwise, the end product will be different from the desired end product. Therefore, anhydrous reactants are used for this reaction.

Difference Between Hydrous and Anhydrous_Figure 2

Figure 2: Anhydrous Chromium(III) Chloride

Examples of Anhydrous Compounds

Copper Sulfate (CuSO₄) is an anhydrous compound. The hydrous form is CuSO₄.5H₂
Gaseous HCl.
Chromium(III) Chloride (CrCl₃)
Calcium Chloride (CaCl₂)

Definition

Hydrous: Hydrous is a term used to explain a substance that contains water as a constituent.

Anhydrous: Anhydrous is a term used to explain a substance that does not contain water as a constituent.

Water

Hydrous: Hydrous compounds are composed of water molecules.

Anhydrous: Anhydrous compounds are not composed of water molecules.

Compounds

Hydrous: Hydrous compounds are known as hydrates.

Anhydrous: Anhydrous compounds are known as anhydrates.

Water Absorption

Hydrous: Hygroscopic compounds can form hydrous compounds by absorption of water from the air.

Anhydrous: Anhydrous compounds can absorb water from the air.

Heating

Hydrous: Hydrous compounds can release water vapor upon heating.

Anhydrous: Anhydrous compounds do not release water vapor upon heating.

Conclusion

Hydrous and anhydrous compounds are chemical compounds that are categorized depending on the presence or absence of water molecules in the chemical structure. The main difference between hydrous and anhydrous compounds is that hydrous compounds are composed of water as a constituent whereas anhydrous compounds are not composed of water as a constituent.

References:

1. Kauffman, George B. “Hydrate.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 21 Mar. 2016, Available here.
2. “Examples of Hydrated Salts.” Edurite.com, Available here.
3. Helmenstine, Ph.D. Anne Marie. “What Anhydrous Means in Chemistry.” ThoughtCo, Available here.

Image Courtesy:

1. “Iron(II)-sulfate-heptahydrate-sample” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia
2. “Chromium(III)-chloride-purple-anhydrous” By Ben Mills – Own work (Public Domain) via Commons Wikimedia