Anhydrous calcium chloride effectively absorbs water from its environment by chemically reacting with it and becoming a hydrated salt.
Anhydrous calcium chloride (CaCl₂) is well-known for its strong ability to absorb moisture, acting as a powerful desiccant. This process isn't just about trapping water physically; it involves a significant chemical transformation.
The Process of Absorption: Hydration
The absorption of water by anhydrous calcium chloride is a chemical process called hydration. When anhydrous calcium chloride comes into contact with water vapor or liquid water, it attracts the water molecules very strongly. These water molecules then become incorporated directly into the crystal structure of the calcium chloride.
As stated in the reference, anhydrous calcium chloride "effectively traps that water by changing its molecular make-up." This change is the formation of calcium chloride hydrates. Instead of remaining as simple CaCl₂ molecules, the substance transforms into compounds like calcium chloride monohydrate (CaCl₂·H₂O), dihydrate (CaCl₂·2H₂O), tetrahydrate (CaCl₂·4H₂O), or hexahydrate (CaCl₂·6H₂O).
- Initial State: Anhydrous Calcium Chloride (CaCl₂)
- Absorption: Water molecules (H₂O) are attracted.
- Chemical Change: Water molecules bond with CaCl₂.
- Final State: Hydrated Calcium Chloride (CaCl₂·nH₂O, where n is the number of water molecules).
This means the water is not just passively held, but becomes a part of the new molecular structure. This reaction is also exothermic, releasing heat as the hydration occurs.
Why is it an Effective Desiccant?
Calcium chloride's high hygroscopicity (the ability to attract and hold water molecules from the surrounding environment) and its tendency to form stable hydrates are what make it so effective.
- Strong Attraction: CaCl₂ has a very high affinity for water molecules.
- Stable Hydrates: The hydrated forms are stable compounds, effectively locking the water away.
- High Capacity: A single molecule of CaCl₂ can absorb up to six molecules of water.
As it absorbs water, anhydrous calcium chloride can eventually dissolve in the water it has absorbed, forming a brine solution. This characteristic is known as deliquescence.
Practical Applications
This water-absorbing property makes anhydrous calcium chloride useful in various applications:
- Desiccant: Used to dry air, gases, and liquids in laboratories and industrial settings.
- Dehumidifiers: Commonly found in moisture absorbers for homes, basements, and storage areas to prevent mold and mildew.
- Road De-icing: While not directly related to absorption from the environment, its ability to dissolve and lower the freezing point of water is why it's used on roads in winter. However, its hygroscopic nature does play a role as it attracts moisture to form the brine.
- Food Packaging: Small packets are sometimes included in food packaging to keep contents dry.
By changing its molecular make-up to form hydrates, anhydrous calcium chloride provides a powerful method for removing and trapping moisture.
