How Do Hydrates Form? Process, Examples, and Importance
The crystalline chemical compound or the substances that contain a water molecule as a constituent of the compound is called hydrate. The water in these molecules combines chemically in a definite proportion. In hydrate compounds, the water molecules surround and interact with solute ions or molecules. The water molecules present in these compounds are called water of hydration. Some common example of hydrates are:
Sodium hydrate.
Copper hydrate.
Calcium hydrate.
Hydrates of carbon.
Hydration Process
The process of adding water to the compound is called hydration. This is the process in which water molecules surround and interact with solute ions or molecules.
Water of Hydration
It is the form of water that is chemically combined with a compound to form a hydrated compound is called water of hydration. The water of hydration can be expelled from the compound by simple heating. The removal of water of hydration does not alter the composition of the substance essentially.
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Such crystalline structures contain positive and negative ions. These ions are attached to the ionic bonds. The crystal lattice of these compounds contains water molecules. During the formation of the crystal lattice of these compounds, the water molecule gets traps in it. The presence of a water molecule in their crystal lattice is the main reason for the formation of such a special type of crystal structure. The mass of hydrated and dehydrated molecules will be different.
The mass of water of hydration molecule can find out by the formula:
Let the mass of the hydrated solid molecule = m1
Let the mass of the dehydrated or anhydrous solid molecule = m2
Mass of water molecule = m1 - m2
Hydrate Chemistry
When a polar compound is dissolved in water it gets split into two parts; cation and anion. The cation gets surrounded by the oxygen of the hydroxyl group (OH-) present in the water and the anion gets surrounded by the hydronium ion (H+) of the water molecule.
Example: Copper Sulphate (CuSO4 . 5 H2O)
CuSO4 .5H2O → Cu+2 + SO4-2
In this compound, the copper ion and sulphate ions are surrounded by water molecules. Copper ions are surrounded by four water molecules and sulphate ions are surrounded by one water molecule. This difference is due to the nuclear charge. The nuclear charge on a copper ion is high (high nuclear charge) due to high charge density and the nuclear charge on sulphate ion is low (low nuclear charge) due to low charge density. The ion with more nuclear charge will combine more water molecules.
Different Types of a Hydrate
Sodium hydrate.
Calcium hydrate.
Hydrates of carbon.
Sodium Hydrate-
Sodium hydrate is a hydrated form of the sodium ion. Its molecular formula is H2 NaO+. The molecular weight is 41.00 g/mol. Its hydrogen bonding donor count is one. The formal charge of the sodium hydrate molecule is one. The structure of Sodium hydrate is given below:
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Calcium Hydrate-
Calcium hydrate is generally known as hydrated lime. Calcium hydrate is a hydrated form of calcium. The molecular weight is 92.11 g/mol. Its molecular formula is CaH4O3. The chemical name of calcium hydrate is calcium dihydroxide. Its hydrogen bond donor count is three. The formal charge of calcium hydrate is zero. The structure of calcium hydrate is given below:
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Hydrates of Carbon-
The hydrates of carbon are known as carbohydrates. These are the main source of energy. It is the first respiratory substrate in the human body. In hydrates of carbon, the ratio of hydrogen and oxygen is 2: 1. The generalized formula of the hydrates of carbon is Cx (H2O)y. Simple carbohydrates that are sweet are called sugar. Carbohydrates are the main source of energy in the body. In a normal man 55-65% of energy is available to him is in the form of carbohydrates present in his diet.
Types of Carbohydrates
Monosaccharides-
These are the simplest sugar that can not be further hydrolysed. In their generalised formula x is always equal to y. It means the number of carbon and oxygen atoms is the same. All monosaccharides occur in d and l form, except the dihydroxyacetone. Examples of monosaccharides are glucose, fructose, galactose, and mannose.
Oligosaccharides-
Oligosaccharides are those carbohydrates that on hydrolysis yield 2 to 10 monosaccharide units. In this type of hydrate of carbon, monosaccharides are linked together by glycosidic linkage. Aldehyde or ketone group of one monosaccharide reacts with the alcoholic group of another monosaccharide to form a glycosidic bond. One molecule of water is eliminated during glycosidic bond formation.
Polysaccharides-
These are composed of a large number of monosaccharides units. The suffix “an” is added in their names and they are known as glycans.
Did You Know?
The human brain is 95% water.
A person can lose a pint to a gallon of urine a day.
Do you know that water regulates the internal body temperature?
The water of hydration gives colour to the compound.
Moisture is a vaporized source of water.
FAQs on Hydrate: Meaning, Types, and Key Properties
1. What is a hydrate in chemistry?
In chemistry, a hydrate is a crystalline compound that has a specific number of water molecules chemically incorporated into its crystal structure. These water molecules, known as the water of hydration or water of crystallisation, are an integral part of the hydrate's structure and are not just superficially wet. When this water is removed, usually by heating, the remaining substance is called an anhydrous compound.
2. What is meant by 'water of hydration'?
Water of hydration refers to the fixed number of water molecules that are chemically bound within the crystal lattice of a single formula unit of a salt. For example, in copper(II) sulphate pentahydrate (CuSO₄·5H₂O), there are exactly five molecules of water for every one formula unit of copper sulphate. This water is responsible for the characteristic crystalline shape and, often, the colour of the compound.
3. How is the chemical formula of a hydrate written?
The chemical formula of a hydrate is written by stating the formula of the anhydrous compound first, followed by a raised dot (·), and then the number of water molecules associated with each formula unit of the compound. For example:
Copper(II) sulphate pentahydrate is written as CuSO₄·5H₂O.
Sodium carbonate decahydrate (washing soda) is written as Na₂CO₃·10H₂O.
Magnesium sulphate heptahydrate (Epsom salt) is written as MgSO₄·7H₂O.
4. What are the main types of hydrates?
Hydrates can be broadly classified into three main categories based on their chemical nature:
Inorganic Hydrates: These are the most common types, formed when water molecules associate with an inorganic salt. The water can be directly bonded to the metal cation or held in the crystal lattice. Example: Cobalt(II) chloride hexahydrate (CoCl₂·6H₂O).
Organic Hydrates: These are formed when water molecules chemically react with an organic compound, often adding across a double bond (e.g., to an aldehyde or ketone to form a gem-diol). Example: Chloral hydrate.
Gas (or Clathrate) Hydrates: These are ice-like crystalline solids where gas molecules (like methane) are trapped within a cage-like structure formed by water molecules. They typically form under high pressure and low temperature.
5. What is the difference between a hydrate and an anhydrous compound?
The key difference lies in the presence of water molecules within the crystal structure. A hydrate contains a fixed ratio of water molecules, which gives it specific properties like a distinct crystalline shape and colour. An anhydrous compound is the same substance after these water molecules have been removed, typically by heating. This removal often results in a change of properties, such as turning from a crystalline solid to a powder and changing colour.
6. Why do some hydrated salts change colour upon heating?
The colour of many hydrated transition metal salts is due to the water of hydration acting as ligands. These water molecules interact with the d-orbitals of the central metal ion, causing them to split in energy. This allows the ion to absorb certain wavelengths of visible light, and we perceive the complementary colour. For instance, hydrated copper(II) sulphate (CuSO₄·5H₂O) is blue because the water ligands cause it to absorb orange-red light. When heated, the water is driven off, the ligand field is lost, and the resulting anhydrous copper(II) sulphate (CuSO₄) is white.
7. How are water molecules attached within a hydrate's structure?
Water molecules can be incorporated into a hydrate's crystal structure in several ways:
Coordinated Water: The water molecules are directly bonded to the central metal cation through a coordinate bond.
Lattice Water: The water molecules are not directly bonded to the cation but occupy fixed positions within the crystal lattice, held in place by hydrogen bonds to the anions or other water molecules.
A combination of both: In many hydrates, like CuSO₄·5H₂O, some water molecules are coordinated to the metal ion while others are held by hydrogen bonds.
8. What are some real-world applications of hydrates and anhydrous compounds?
Hydrates and their anhydrous counterparts have several practical applications:
Drying Agents (Desiccants): Anhydrous salts like calcium chloride (CaCl₂) and silica gel are highly hygroscopic (they readily absorb moisture) and are used to keep products and lab environments dry.
Construction: Plaster of Paris (calcium sulphate hemihydrate, CaSO₄·½H₂O) is mixed with water to form a paste that hardens into gypsum (CaSO₄·2H₂O), used for casts and moulds.
Humidity Indicators: Anhydrous cobalt(II) chloride is blue, but it turns pink as it absorbs moisture to form the hexahydrate. This property is used in indicator strips to detect the presence of water.
9. Can a substance form more than one type of hydrate?
Yes, a single substance can often form several different hydrates containing varying amounts of water, depending on the conditions of temperature and humidity during crystallisation. For example, sodium sulphate can exist as the anhydrous salt (Na₂SO₄) as well as the decahydrate (Na₂SO₄·10H₂O), also known as Glauber's salt. Similarly, copper(II) sulphate can form a pentahydrate, a trihydrate, and a monohydrate.
