Copper(II) Sulfate Pentahydrate
Properties
| State | Solid (large triclinic blue crystals) |
| Color | Bright blue (white when anhydrous) |
| Solubility | Soluble in water (316 g/L at 20°C) |
| Melting Point | 110°C (loses water); 560°C (anhydrous, decomposes) |
| Boiling Point | Decomposes before boiling |
About Copper(II) Sulfate Pentahydrate
CuSO₄·5H₂O is probably the most-photographed inorganic compound in the world, and its structure explains why the white-to-blue water test works the way it does. Of the five waters in the unit cell, four are coordinated directly to Cu²⁺ in a Jahn-Teller-distorted square plane (Cu–O ~1.96 Å equatorial), and the fifth is held in the lattice by hydrogen bonds to the sulfate oxygens — it isn't bound to copper at all. Heating the crystals takes the four coordinated waters off in two steps (around 30°C and 110°C) and the lattice water at about 200°C, giving the white anhydrous CuSO₄ that turns blue on contact with even trace water — that color recovery is exothermic, ΔH around -78 kJ/mol, and is the basis of the standard test for water in alcohols and other solvents. The triclinic blue crystals are the canonical seed-crystal demo for high-school chemistry: dissolve enough CuSO₄·5H₂O in hot water to saturate, dangle a string with a small seed, and over a week you grow centimeter-scale bright blue parallelepipeds. Industrially, the compound is the active ingredient in Bordeaux mixture (CuSO₄·5H₂O + Ca(OH)₂), the fungicide developed by Pierre-Marie Millardet in 1885 for downy mildew on Bordeaux grapevines and still sprayed on tens of millions of hectares globally for organic vine, potato, and fruit production. CuSO₄·5H₂O is also the cathode side of the Daniell cell, the copper source in electroplating baths, the Cu(II) reagent in Fehling's and Benedict's solutions for sugar tests, and the Cu(II) in the Biuret test where Cu²⁺ + alkali coordinates to peptide bonds and gives a violet color proportional to protein concentration.
Where you'll encounter it
If you've ever grown crystals as a kid or in a high-school chemistry class, the bright blue parallelepipeds were almost certainly CuSO₄·5H₂O. If you've sprayed Bordeaux mixture on tomatoes or grapes for blight, the blue powder you mixed with lime and water was this compound. In an analytical lab, the violet color in a Biuret protein assay comes from Cu²⁺ from CuSO₄·5H₂O coordinating to four peptide-bond nitrogens in the protein backbone. In an electroplating shop, the bath that drops a bright copper layer on PCB through-holes runs at roughly 200 g/L CuSO₄·5H₂O plus 60 g/L H2SO4 plus a few ppm chloride and a brightener. In a teaching lab, the heat-and-rehydrate demo — heat blue crystals in a watch glass until they go white anhydrous, then add a drop of water and watch them snap back to blue with audible heat release — is the canonical introduction to coordination color and crystal water.
Common Uses
- Bordeaux mixture fungicide with Ca(OH)₂ for downy mildew on grapes, potatoes, and tomatoes
- Algaecide in swimming pools and irrigation reservoirs at 0.5–2 ppm Cu²⁺ dosing
- Copper anode replenisher in acid copper electroplating baths for PCB and decorative plating
- Cu(II) reagent in Fehling's, Benedict's, and Biuret colorimetric tests in clinical chemistry
- Seed-crystal growing demonstration in school chemistry (saturated solution + dangling string)
- Mordant for fixing natural dyes onto wool and cotton in artisan textile work
- Cathode half-cell in Daniell cell electrochemistry demonstrations and battery teaching
- Drying-tube indicator: anhydrous form turns blue on water contact, signaling desiccant exhaustion
Safety Information
GHS: H302 (harmful if swallowed, LD50 oral rat ~300 mg/kg), H315 (skin), H319 (serious eye irritation — copper salts cause prompt corneal staining), H400/H410 (very toxic to aquatic life). OSHA PEL 1 mg/m³ as Cu. Emetic at gram doses, which is partially protective in accidental ingestion. The pool-treatment use can stain blonde hair green at chronic exposure — bind the copper with sequestrants like EDTA in pool chemistry. Aquatic toxicity is high enough that fish-pond use needs careful dosing; rainbow trout 96-h LC50 around 0.04 mg/L Cu. Don't mix with iron or aluminum vessels: galvanic displacement deposits copper.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.