Calcium chloride (CaCl₂) commonly exists in anhydrous (CaCl₂) and dihydrate (CaCl₂·2H₂O) forms due to its strong hygroscopic nature and hydration stability.
1. Anhydrous CaCl₂ (No Water)
- Formation: Produced by dehydrating hydrated forms at high temperatures (>200°C) or synthesizing it from dry processes.
- Stability: Highly unstable in humid air—rapidly absorbs moisture to form hydrates.
- Use Cases:
- Industrial drying (desiccant)
- Chemical synthesis where water-free conditions are needed
2. Dihydrate (CaCl₂·2H₂O)
- Formation: When anhydrous CaCl₂ absorbs 2 moles of water, it reaches a stable crystalline stateat room temperature.
- Why Not Higher Hydrates?
- Hexahydrate (CaCl₂·6H₂O) exists but is less stable at room temperature (melts at ~30°C).
- Dihydrate is more resistant to further water absorption under typical storage conditions.
- Use Cases:
- Road de-icing (less aggressive moisture absorption than anhydrous form)
- Food additive (E509)
Which Form is More Hygroscopic?
Anhydrous CaCl₂ is far more hygroscopic than the dihydrate because:
- Stronger Water Affinity:
- Anhydrous CaCl₂ has empty coordination sites for water molecules, making it aggressively absorb moisture.
- Dihydrate is already partially saturated, reducing its ability to absorb more water.
- Deliquescence:
- Anhydrous CaCl₂ can absorb so much water that it dissolves into a liquid brine.
- Dihydrate resists deliquescence unless exposed to very high humidity.
Practical Implications
| Property | Anhydrous CaCl₂ | Dihydrate (CaCl₂·2H₂O) |
|---|---|---|
| Hygroscopicity | Extremely high | Moderate |
| Stability in Air | Poor (absorbs water quickly) | Good (resists further absorption) |
| Common Uses | Desiccants, drying agents | De-icing, food industry |
Conclusion
- Anhydrous CaCl₂ is used where maximum moisture absorption is needed (e.g., drying gases).
- Dihydrate CaCl₂·2H₂O is preferred for storage stability and applications like de-icing.
- Hexahydrate (CaCl₂·6H₂O) exists but is less common due to its low melting point.
