Introduction
Some ionic compounds don't just crystallise on their own — they trap water molecules inside their crystal structure as they form. These are called hydrates, and today we'll learn how chemists name them systematically. This follows the same spirit of clear, unambiguous naming recommended by IUPAC that you'll see across all of chemistry nomenclature (Talbot, p.172).
What is a hydrate?
A hydrate is an ionic compound that has a fixed number of water molecules incorporated into its crystal lattice. The water is called water of crystallisation. When this water is removed (usually by heating), the compound becomes anhydrous (meaning "without water").
A common example you'll meet in the lab is copper(II) sulfate:
- Hydrated form: CuSO₄·5H₂O — a blue crystalline solid
- Anhydrous form: CuSO₄ — a white powder
The dot (·) in the formula is important — it separates the ionic compound from the water molecules and shows that the water is loosely associated with the lattice, not chemically bonded into the ion itself.
Naming rules for hydrates
Naming a hydrate follows a simple pattern:
name of the anhydrous compound + "-hydrate" with a prefix for the number of water molecules
The prefixes are the same Greek number prefixes used elsewhere in chemical nomenclature:
- 1 = mono
- 2 = di
- 3 = tri
- 4 = tetra
- 5 = penta
- 6 = hexa
- 7 = hepta
So CuSO₄·5H₂O is named copper(II) sulfate pentahydrate.
Worked Examples
Example 1 Formula: CuSO₄·5H₂O
- Anhydrous compound: copper(II) sulfate
- Number of water molecules: 5 → penta
- Name: copper(II) sulfate pentahydrate
Example 2 Formula: MgSO₄·7H₂O
- Anhydrous compound: magnesium sulfate
- Number of water molecules: 7 → hepta
- Name: magnesium sulfate heptahydrate (this is the compound known as Epsom salt)
Example 3 Formula: Na₂CO₃·10H₂O
- Anhydrous compound: sodium carbonate
- Number of water molecules: 10 → deca
- Name: sodium carbonate decahydrate (washing soda)
Example 4 (name → formula) Name: calcium chloride dihydrate
- Anhydrous compound: CaCl₂
- "di" = 2 water molecules
- Formula: CaCl₂·2H₂O
Why this matters
Writing correct hydrate formulas matters a lot in quantitative chemistry, especially when you calculate molar mass for mole calculations (n = m / M). If you forget the water molecules in the formula, your molar mass — and therefore every calculation based on it — will be wrong. For example, the molar mass of CuSO₄ is very different from CuSO₄·5H₂O, because each water molecule adds roughly 18 g mol⁻¹.
You'll often meet hydrates in practical work too, such as heating a hydrated salt to determine the number of water molecules experimentally by measuring mass loss.
Key takeaways
- A hydrate is an ionic compound with a fixed number of water molecules built into its crystal structure, shown using a dot in the formula (e.g., CuSO₄·5H₂O).
- Hydrate names use Greek prefixes (mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, deca-...) followed by "-hydrate" to show the number of water molecules.
- Removing the water (usually by heating) gives the anhydrous form of the compound.
- Always include water of crystallisation in molar mass calculations — leaving it out gives incorrect mole and mass results.
- This naming system reflects the same IUPAC goal of clear, consistent, and unambiguous communication used throughout chemical nomenclature.