Anhydrous calcium chloride transforms into a colorless solution when exposed to air primarily because it is a deliquescent substance. This means it has an exceptionally strong affinity for water and will readily absorb sufficient moisture from the surrounding atmosphere to dissolve itself, forming a clear, liquid solution.
Understanding Deliquescence
Deliquescence is a phenomenon where a solid substance absorbs moisture from the air until it dissolves in the absorbed water, forming a solution. This process occurs when the vapor pressure of the saturated solution formed by the substance is less than the partial pressure of water vapor in the surrounding air.
- Key Characteristics of Deliquescent Substances:
- High Hygroscopy: They are highly hygroscopic, meaning they attract and hold water molecules.
- High Solubility: They are very soluble in water.
- Low Vapor Pressure: The resulting solution has a very low vapor pressure, which allows it to continue absorbing moisture.
- Appearance Change: They visibly transform from a solid to a liquid state over time.
For more information, you can explore the concept of deliquescence on Wikipedia.
How Anhydrous Calcium Chloride Forms a Solution
The transformation of anhydrous calcium chloride (CaCl₂) from a white solid lump to a colorless liquid solution is a straightforward chemical process driven by its deliquescent nature:
- Absorption of Atmospheric Moisture: When anhydrous calcium chloride is exposed to air, its surface begins to attract and absorb water vapor molecules from the humidity in the atmosphere.
- Dissolution Process: As it absorbs sufficient water from the air, this moisture acts as a solvent. The calcium chloride then begins to dissolve in this accumulated water.
- Formation of a Clear Liquid: After a relatively short while, the initial white solid lump gradually disappears. Instead of a solid, you will observe a pool of clear, colorless liquid forming, which is an aqueous solution of calcium chloride. This solution is colorless because calcium chloride ions (Ca²⁺) and chloride ions (Cl⁻) do not absorb visible light, and the solution contains no other colored impurities.
Key Properties Driving This Transformation
Calcium chloride's remarkable ability to deliquesce is due to a combination of its chemical properties:
| Property | Description |
|---|---|
| High Affinity for Water | Calcium chloride is an ionic compound with strong electrostatic forces that readily interact with polar water molecules. |
| Exothermic Hydration | The process of dissolving calcium chloride in water (hydration) releases a significant amount of heat, indicating a favorable interaction. |
| High Solubility | It has one of the highest solubilities among common salts, allowing it to dissolve extensively in even small amounts of water. |
You can learn more about calcium chloride on Wikipedia.
Practical Applications and Considerations
The deliquescent property of anhydrous calcium chloride is leveraged in various applications:
- Drying Agent (Desiccant): It is widely used in laboratories and industries to remove moisture from gases and liquids, as well as to keep humidity-sensitive items dry.
- Dust Control: On unpaved roads, calcium chloride absorbs atmospheric moisture, creating a damp surface that helps to bind dust particles, preventing them from becoming airborne.
- Road De-icer: Its ability to lower the freezing point of water and its deliquescent nature make it effective for melting ice and snow on roads and sidewalks.
Due to its strong deliquescent properties, anhydrous calcium chloride must always be stored in tightly sealed containers to prevent it from absorbing moisture from the air and turning into a solution.
