Question

What are hydrates?

Answer 1

Let us first look at some examples of hydrates that can help us determine the exact definition of hydrates. Some examples of hydrates are ethanol $C{{H}_{3}}C{{H}_{2}}OH$, chloral hydrate $CC{{l}_{3}}CH{{(OH)}_{2}}$, cobalt(II) chloride hexahydrate $CoC{{l}_{2}}.6{{H}_{2}}O$ , and methane hydrate $4C{{H}_{4}}.23{{H}_{2}}O$

Complete answer:
From the examples in the hint and the name "hydrates", we can confer that substances that either contain water molecules themselves or their constituent elements are known as hydrates.
There are different types of hydrates
- Inorganic hydrates
The molecules of water in a compound are known as the water of crystallization. These water molecules are present inside the crystals of the solid inorganic salts in a definite ratio and are bound either to the metal center or form a metal crystal complex.
The term deuterate is used instead of hydrate when heavy water is involved.
For example, a red-colored compound cobalt(II) chloride hexahydrate $CoC{{l}_{2}}.6{{H}_{2}}O$ is formed when blue-colored cobalt(II) chloride $CoC{{l}_{2}}$ is hydrated.
The common notation for inorganic hydrates is hydrated compounds. $n{{H}_{2}}O$ and the nomenclature is the name of the compound followed by the prefixed-hydrate according to the table below

n	prefix
$\dfrac{1}{2}$	Hemi-
1	Mono-
$1\dfrac{1}{2}$	Sesqui-
2	Di-
3	Tri-
4	Tetra-
5	Penta-
6	Hexa-
7	Hepta-
8	Octa-
9	Nona-
10	Deca-
11	Undeca-
12	Dodeca-

- Organic hydrates
The addition of water or its elements like H and OH or hydration of a substance results in the formation of hydrate.
For example, the hydration of ethylene $C{{H}_{2}}=C{{H}_{2}}$ results in the formation of ethyl alcohol $C{{H}_{3}}C{{H}_{2}}OH$. Hence ethanol is also known as the hydrate of ethene.
Similarly, the reaction between trichloroacetaldehyde or chloral $CC{{l}_{3}}CHO$ and water results in geminal diol known as chloral hydrate $CC{{l}_{3}}CH{{(OH)}_{2}}$.
For various reasons, components like methanol and glucose were also incorrectly considered to be hydrates and methane was thought of as methyl hydrate $C{{H}_{3}}O{{H}_{2}}$ instead of $C{{H}_{3}}OH$ and glucose was thought of as a carbohydrate ${{C}_{6}}{{({{H}_{2}}O)}_{6}}$ instead of ${{C}_{6}}{{H}_{12}}{{O}_{6}}$.
Like inorganic hydrates, organic molecules form water of crystallization for example trehalose sugar and protein crystals.
- Clathrate hydrates
A clathrate is formed when gas molecules are trapped within water ice. For example, methane hydrates $4C{{H}_{4}}.23{{H}_{2}}O$.

Note:
It should be noted that when hydrated compounds are dehydrated, they lose water and are known as anhydrides compounds. This can be done by the action of dehydrating agents like sulfuric acid ${{H}_{2}}S{{O}_{4}}$ or upon heating strongly.
Compounds that do not contain any water (anhydrous) but are hydrated easily are known as hygroscopic and are used as drying agents.