Properties of salts
Certain crystalline salts and compounds exhibit different behaviors when exposed to the atmosphere depending on their interaction with water molecules. These behaviors include efflorescence, hygroscopy, and deliquescence.
Efflorescence
Efflorescence is the spontaneous loss of water of crystallization from a hydrated salt when exposed to air. The salt gradually becomes a powdery anhydrous form as it loses its water molecules to the atmosphere.
This occurs because the vapor pressure of the hydrated salt is higher than that of the surrounding air.
Examples:
- Washing soda – Na₂CO₃·10H₂O
- Sodium sulfate – Na₂SO₄·10H₂O (Glauber’s salt)
- Copper(II) sulfate – CuSO₄·5H₂O
Hygroscopy
Hygroscopy is the ability of a substance to absorb moisture from the atmosphere without dissolving in it. Such substances become damp or sticky but do not form a solution.
Hygroscopic substances are commonly used as drying agents in laboratories to keep chemicals and gases moisture-free.
Examples:
- Concentrated sulfuric acid (H₂SO₄)
- Quicklime (CaO)
- Phosphorus pentoxide (P₄O₁₀)
- Calcium chloride (CaCl₂)
Deliquescence
Deliquescence is the property of certain salts to absorb so much moisture from the air that they dissolve in the absorbed water and form a solution. This happens when the vapor pressure of the absorbed water becomes equal to or greater than the atmospheric vapor pressure.
Examples:
- Calcium chloride (CaCl₂)
- Magnesium chloride (MgCl₂)
- Sodium hydroxide (NaOH)
- Iron(III) chloride (FeCl₃)
- Potassium hydroxide (KOH)
