Overview
This lecture covers the definitions, properties, and chemical behavior of hydrated and anhydrous salts, focusing on water of crystallisation and its representation in chemical formulas.
Hydrated and Anhydrous Salts
- Hydrated salts contain water molecules incorporated within their crystal structure during crystallisation.
- Anhydrous salts have no water in their chemical structure.
- Example: Hydrated copper(II) sulfate (blue) contains water, while anhydrous copper(II) sulfate (white) does not.
- Heating hydrated copper(II) sulfate removes its water, producing anhydrous copper(II) sulfate.
- The process is reversible: adding water to anhydrous copper(II) sulfate recreates the hydrated form.
Water of Crystallisation (Extended tier only)
- Water molecules included in some crystal structures are known as water of crystallisation.
- Hydrated compounds contain water of crystallisation; anhydrous compounds do not.
- The formula for hydrated salts includes a dot followed by the number of water molecules (e.g., CuSO₄·5H₂O).
- The number before H₂O shows moles of water per mole of salt (e.g., CuSO₄·5H₂O means 5 moles of water per mole).
- Conversion between hydrated and anhydrous salts is reversible, typically by heating or adding water.
- Example: CuSO₄ + 5H₂O ⇌ CuSO₄·5H₂O
Example: Copper(II) Sulfate
- Hydrated copper(II) sulfate: CuSO₄·5H₂O, blue in color.
- Anhydrous copper(II) sulfate: CuSO₄, white in color.
- Heating removes water; adding water reverses reaction.
Key Terms & Definitions
- Hydrated salt — a salt that contains water molecules within its crystal structure.
- Anhydrous salt — a salt that contains no water in its structure.
- Water of crystallisation — water molecules included in a salt’s structure during crystallisation.
- Dehydration — process of removing water from a hydrated salt to form an anhydrous salt.
- Reversible reaction — a reaction where products can be converted back into reactants.
Action Items / Next Steps
- Practice writing formulas for hydrated and anhydrous salts.
- Be able to describe and explain the reversible dehydration and rehydration of salts like copper(II) sulfate.
- Review example reactions for water of crystallisation.