Water of Crystallisation

Ever wondered why certain crystals have beautiful, striking colours while others don’t have any? The answer lies deep inside the mysteries of these crystals, which we call the water of crystallisation. However, this water is not free-flowing, but is tightly locked inside crystals, giving them their classic colour, structure, and stability. The concept plays a crucial role in various industries, from academic study in chemistry labs to its practical applications in medicine, construction, etc. Here, we will explore everything related to this water, trapped inside a crystal. 

1.0Water of Crystallisation Meaning

Water of crystallisation is the fixed quantity of water molecules chemically bound to the ions within a crystal. They are not loosely held but are an integral component of the compound's structure, aiding in preserving its solid state and definite properties.

In contrast to free moisture, water of crystallisation is trapped in the interior crystal lattice. When such a compound is subjected to heat or specific conditions, it may lose its water of crystallisation, which usually causes a change in physical appearance, such as colour or texture.

2.0How the Water of Crystallisation Forms

The formation of water of crystallisation is a natural process. It happens when either a saturated salt solution is cooled or a molten substance solidifies. In both situations, the ions or molecules arrange in a fixed position in a certain structure, also known as a crystal structure. At the same time, some water molecules from the surrounding become chemically bonded into the structure, forming a crystal. Therefore, the process is also known as crystallisation. 

As the water molecules bond directly with the ions of the crystal lattice, any change in their position or symmetry significantly affects their physical and chemical properties. Simply put, water of crystallisation forms when water molecules get chemically attached to crystals during their solidification. 

3.0Water of Crystallisation Formula:

The Water of Crystallisation Formula is basically the equational representation of the number of water molecules attached to each unit of the compound. This is written using a dot(.) after the molecular formula of the salt, followed by the molecular formula of water. Some important examples of these water of crystallisation formulas are: 

Note: The number attached to the H₂O molecule tells us exactly how many water molecules are attached to a single molecule of salt. 

4.0Loss of Water of Crystallisation

The loss of water of crystallisation occurs when a crystal is heated or, in some cases, left in the environment to dry. The heating gives off the energy required to break the bonds between water and crystal molecules. This results in the evaporation of water into air, leaving an anhydrous (dry) salt. 

One of the water of crystallisation examples of loss of water includes Blue crystals of copper (II) sulphate (CuSO₄5H₂O), which lose water on heating and convert into a white powder of anhydrous copper sulphate (CuSO₄).  

${\mathrm{CuSO}}_4\cdot 5{\mathrm{H}}_2\mathrm{O}(\text{Blue})⟶{\mathrm{CuSO}}_4(\text{white})+5{\mathrm{H}}_2\mathrm{O}$

Why Is This Important?

• It helps identify the water of crystallisation. This means during a laboratory test, if a substance changes colour, it confirms the presence of water of crystallisation. 
• The loss of water can also affect the rate of chemical reactions, as hydrated salts are more reactive than their anhydrous forms. 
• Although not a confirmatory test, it also helps identify certain salts in their hydrated form.  

5.0Importance of Water of Crystallisation

From their morphological features to their chemical properties, water of crystallisation has a huge impact on the structure of a crystal. Let’s take a look at some facts about why these are so important for a crystal: 

1. Stability and Structure: Water molecules stuck inside a crystal help maintain the stability and structure of that crystal by enhancing the strength of bonds between these molecules. 
2. Colour and Appearance: As mentioned earlier, in many crystals, water of crystallisation gives their parent crystal a vivid and distinct colour, which also helps identify that crystal. 
3. Chemical Properties: Although not always, the presence of water molecules in the structure of a crystal makes the crystal more stable or less reactive in comparison to its counterpart. 
4. Solubility: Many hydrated salts tend to dissolve more slowly in solvents than anhydrous salts. This ultimately affects how these salts are to be utilised. 
5. Test for Water: A Test of water is a practical experiment used to detect the presence of water molecules in the compound. 

6.0Uses of Water of Crystallisation

Water of crystallisation has a large number of practical applications, important in various industries and even in our daily lives. Here are some key uses of water of crystallisation among them: 

• Manufacturing Construction Materials: Plaster of Paris, another important water of crystallisation, is used in manufacturing industries in the making of statues, moulds, ceilings, and wall plasters. 
• Medical Applications: Water of crystallisation is also used in the manufacturing of various medical products, such as gypsum is used for making orthopaedic casts for supporting broken bones, or Epsom salt, which is used in therapeutic baths to relieve muscle soreness. 
• Cleaning Agents: Washing soda, which is used as a detergent for washing clothes or household cleaning, also contains water of crystallisation.  
• Agricultural Uses: Epsom salts are also used as fertilisers to fulfil the need of magnesium and sulfur for plants, helping them grow more efficiently. 
• Preservation and Storage: Some hydrated crystals are also good controllers of humidity and preservers of sensitive materials; hence are used for the preservation and storage of these materials.