The number of water molecules associated with copper sulphate molecule to form crystals is

Copper sulphate crystals, commonly known as blue vitriol or bluestone, contain water of crystallisation which gives them their characteristic blue colour [1]. The chemical formula for these crystals is CuSO4·5H2O, indicating that five water molecules are associated with each copper(II) sulphate molecule [3]. Structurally, four of these water molecules are coordinated directly to the central copper(II) ion, while the fifth water molecule is held in the crystal lattice through hydrogen bonding. When these crystals are heated, the water of crystallisation is removed, causing the salt to lose its blue colour and turn into a white anhydrous powder [1]. This process is reversible; adding water to the anhydrous salt restores the blue colour and the crystalline structure [1]. Therefore, the hydration number for copper sulphate crystals is five.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Activity 2.15 > p. 32
2. [3] https://pubchem.ncbi.nlm.nih.gov/compound/Copper-sulfate-pentahydrate