Water of Crystallisation: The Hidden Structure of Crystals
What Exactly is Water of Crystallisation?
Imagine building a house with bricks. Now, imagine that you need to include some specific-sized sponges between certain bricks to keep the whole structure stable. If you remove those sponges, the wall might crumble or change shape. In chemistry, some crystals are like that wall. The "sponges" are water molecules, and they are called water of crystallisation or hydration water.
These are water molecules ($H_2O$) that are chemically bonded into the crystal structure of a salt or another compound. They are not liquid water floating around; they are precisely positioned within the 3D arrangement of ions or molecules. The compound that contains them is called a hydrate. When the water is removed, the resulting substance is called an anhydrous compound (meaning "without water").
How Water Molecules Bind Inside Crystals
The binding of water in crystals can happen in a few different ways, which are crucial for the crystal's stability.
- Coordination Bonds: Water molecules can act as ligands, directly bonding to a central metal ion. For example, in $CuSO_4 \cdot 5H_2O$, four water molecules are bonded to the copper ion ($Cu^{2+}$), and the fifth is held by hydrogen bonds[1] to the sulfate ion.
- Hydrogen Bonds: This is a weaker but very important bond. Water molecules, with their positive and negative ends, can form bridges between different ions in the lattice, acting like glue that holds the structure together.
- Occupying Spaces: Sometimes, water molecules simply fit into specific cavities or channels within the crystal framework, stabilized by surrounding ions.
The energy required to remove this water is usually moderate, meaning heating the crystal gently can often drive the water off as vapor.
Common Hydrates and Their Transformations
Many common substances we use or study in the lab are hydrates. Their properties can change dramatically when they lose or gain water.
| Common Name | Chemical Formula (Hydrate) | Color (Hydrate) | Anhydrous Form | Observable Change |
|---|---|---|---|---|
| Blue Vitriol | $CuSO_4 \cdot 5H_2O$ | Bright Blue | $CuSO_4$ | Turns white or greyish upon heating. Reverses to blue with water. |
| Epsom Salt | $MgSO_4 \cdot 7H_2O$ | Colorless/White | $MgSO_4$ | Becomes a fine, dry powder. The process is called efflorescence. |
| Gypsum | $CaSO_4 \cdot 2H_2O$ | White | $CaSO_4$ (Plaster of Paris) | Heating gives a powder that hardens when mixed with water. |
| Washing Soda | $Na_2CO_3 \cdot 10H_2O$ | Transparent Crystals | $Na_2CO_3$ (Soda Ash) | Crystals lose water and become a white powder in dry air. |
A Closer Look: The Copper Sulfate Experiment
A classic classroom experiment perfectly demonstrates the role of water of crystallisation. Start with blue crystals of copper(II) sulfate pentahydrate ($CuSO_4 \cdot 5H_2O$).
Step 1 – Dehydration: Gently heat the blue crystals in a dry test tube. You will observe water droplets forming on the cooler parts of the tube, and the bright blue crystals turn into a white or greyish powder. This is anhydrous copper(II) sulfate ($CuSO_4$). The water molecules, which were involved in the bonding that allowed the crystal to absorb specific wavelengths of light (giving it color), have been removed. The change is reversible.
Step 2 – Rehydration: Add a few drops of water to the white powder. You will see an immediate color change back to blue, and feel the test tube get warm. This is an exothermic[2] reaction, releasing heat as the water molecules re-integrate into the crystal lattice, reforming the hydrate.
This simple experiment shows that the water is an integral part of the substance's identity, affecting its color and energy state.
Efflorescence, Deliquescence, and Hygroscopy
How a hydrate behaves in air depends on the vapor pressure[3] of its water compared to the water vapor pressure in the air.
| Term | Definition | Condition | Example |
|---|---|---|---|
| Efflorescence | A hydrate loses its water of crystallisation spontaneously to the air, becoming a powder. | Vapor pressure of hydrate's water > Vapor pressure of water in air. | $Na_2CO_3 \cdot 10H_2O$ (washing soda) left in open air forms a white powder ($Na_2CO_3$). |
| Deliquescence | A substance absorbs so much water from the air that it dissolves in it, forming a solution. | Vapor pressure of substance's solution << Vapor pressure of water in air. | Calcium chloride ($CaCl_2$) becomes wet and eventually turns into a liquid puddle. |
| Hygroscopy | A substance absorbs moisture from the air but does not dissolve (does not become liquid). | Intermediate absorption, less than deliquescence. | Silica gel packets in shoe boxes. Anhydrous $CuSO_4$ (white) slowly turns blue. |
Important Questions
No. The water molecules are an integral part of the solid crystal lattice. They are fixed in position and do not flow like liquid water. A hydrated salt like $CuSO_4 \cdot 5H_2O$ is a dry, solid crystal. The water is chemically bound, not trapped as a liquid.
Yes, through a process called "Heating to Constant Mass." You weigh a sample of the hydrate, heat it strongly to remove all water, and weigh the anhydrous residue. The mass loss is the mass of water. For example, if 2.50 g of $CuSO_4 \cdot xH_2O$ gives 1.60 g of anhydrous $CuSO_4$, then the mass of water is 0.90 g. Using molar masses, you can calculate '$x$'.
It's crucial in many areas. In construction, gypsum ($CaSO_4 \cdot 2H_2O$) is heated to make Plaster of Paris, which hardens by re-forming hydrate bonds. In food, hydrates like citric acid monohydrate control texture and stability. In chemistry, hydrates ensure consistent formulas and are used as indicators (like cobalt chloride paper) for moisture. Understanding hydration is key to storing chemicals properly.
Footnote
[1] Hydrogen Bonds: A type of attractive intermolecular force that occurs between a hydrogen atom bonded to a highly electronegative atom (like oxygen or nitrogen) and another electronegative atom. It is stronger than van der Waals forces but weaker than covalent or ionic bonds.
[2] Exothermic: A process or reaction that releases heat energy to its surroundings. The opposite is endothermic, which absorbs heat.
[3] Vapor Pressure: The pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature. In this context, it refers to the tendency of water molecules to escape from a solid hydrate or a liquid solution into the air.