Copper sulfate [CuSO4]

Copper salt (CuSO4) sold mainly as the pentahydrate, in bright blue crystals.

Characteristics

Highly soluble in water; dissolving it releases heat.
Supplies copper and sulfur in sulfate form.
The anhydrous form is white and turns blue when hydrated.
Historic ingredient, with lime, of traditional copper mixtures such as Bordeaux mixture.

Role in the preparation: It supplies the copper ion and reacts with slaked lime when traditional copper mixtures are prepared.

Caution: Harmful if swallowed, irritating to skin, damaging to eyes and very toxic to aquatic life: wear gloves and goggles, keep away from children and animals, and check local regulations before any use.

Sources: Copper(II) sulfate - Wikipedia

Documentary information: this is neither a recommendation for use nor a plant-protection or veterinary claim. Check your country's regulations before any use.