Difference Between Hydrate and Anhydrate

Key Difference – Hydrate vs Anhydrate
 

The two terms “hydrate” and “anhydrate” are two opposite words in their meaning, and the key difference between hydrate and anhydrate is that hydrates are ionic compounds containing free water molecules while anhydrates are compounds that do not contain any free water molecules. Hydrates are formed from ionic compounds when they exposed to the air, reacting with water molecules. Anhydrates are the opposite version of hydrates; they contain no water molecules. Anhydrates are also known as drying agents or desiccants.

What are Hydrates?

Water can be considered as the most abundant compound on the earth. When the chemical compounds are exposed to the air, water vapor in the atmosphere is adsorbed to the molecules. It can be either a surface reaction or a reformation of the entire chemical structure forming a chemical complex with water. In general, water molecules are bonded to cations in ionic substances. This phenomenon is called “hydration.”

There are many ionic compounds present in hydrated form; some examples are Gypsum (CaSO4. 2H2O), Borax (Na3B4O7.10H2O), and Epsom Salt (MgSO4.7H2O). The number of water molecules in hydrates varies from one compound to another in stoichiometric amounts.The molecular formula of a hydrate compound is a combination of molecular formula of the anhydrous compound and the number of molecules per mole in the hydrate. These two are separated using a “dot”; an example is given below.

Difference Between Hydrate and Anhydrate - molecular formula of a hydrate

General name: Epsom salt and Chemical name: Magnesium Sulphate Heptahydrate.

Difference Between Hydrate and Anhydrate_Hydrate Sample

A sample of the heptahydrate of magnesium sulfate

What are Anhydrates?

Anhydrates are also known as anhydrous materials; they do not contain any water molecule as in hydrates. In this category, the water molecules are removed by heating the compound to a high temperature or by suction. In general, anhydrates can be used as drying agents, because they can absorb water molecules from the surroundings. Silica gel is one of the most commonly used anhydrates. A packet of silica gel is kept inside many finished products to absorb water. It helps to keep the surrounding area dry, and it prevents the growth of molds.

Key Difference - Hydrate vs Anhydrate

Silica Gel beads

What is the difference between Hydrates and Anhydrates?

Definition of Hydrates and Anhydrates

Anhydrates: Anhydrates (also known as drying agents or desiccants) are compounds that do not contain any free water molecules.

Hydrates: Hydrates are ionic compounds containing free water molecules.

Method of Production of Hydrates and Anhydrates

Anhydrates: Anhydrates are produced by removing freely bound water molecules by suction or heating to a relatively higher temperature.

Hydrates: Hydrate compounds are formed naturally when they exposed to air. They are all ionic compounds which are formed by making bonds with the gaseous water molecules in the air. The bond is formed between the cation of the molecule and the water molecule.

Properties of Hydrates and Anhydrates

Anhydrates: Anhydrates are considered as drying agents since they are capable of absorbing water molecules from the surroundings. The water molecules can be easily removed by heating to a high temperature.

Hydrates: In general, the water molecules in hydrates can be removed by heating. The product obtained after heating is the anhydrous compound; it has a different structure from the hydrate.

Example:

CuSO4. 5H2O                →                 CuSO4             +          5H2O

(Blue)                                                   (White)

The number of water molecules trapped in hydrate crystals varies because it also follows the stoichiometric ratio rule. The number of molecules included in the molecular formula is as follows.

Prefix No of water molecules Molecular formula Name
Mono- 1 (NH4)C2O4. H2O Ammonium oxalate monohydrate
Di- 2 CaCl2 .2H2O Calcium chloride dihydrate
Tri- 3 NaC2H3O3.3H2O Sodium acetate trihydrate
Tetra- 4 FePO4.4H2O Iron (III) phosphate tetrahydrate
Penta 5 CuSO4.5H2O Copper(II) sulphate pentahydrate
Hexa 6 CoCl2.6H2O Cobolt(II) chloride hexahydrate
Hepta 7 MgSO4.7H2O Magnesium sulphate heptahydrate
Octa 8 BaCl2.8H2O Barium hydroxide octahydrate
Deca 10 Na2CO3.10H2O Sodium carbonate decahydrate
 
 
Image Courtesy:
“Silica gel pb092529″ by Wiebew – Own work. (CC BY-SA 3.0) via Wikimedia Commons
 “Magnesium sulfate heptahydrate”. (Public Domain) via Wikimedia Commons