Copper(II) Sulfate
Properties
| State | Solid at room temperature |
| Color | White (anhydrous); bright blue (pentahydrate) |
| Solubility | Soluble in water (31.6 g/100 mL at 20 °C) |
| Melting Point | 110 °C (pentahydrate, decomposes) |
| Boiling Point | 650 °C (anhydrous, decomposes) |
About Copper(II) Sulfate
Anhydrous CuSO₄ is the white form — and almost no one ever sees it for long, because it's one of the most aggressively hygroscopic Cu(II) salts and pulls water out of solvents, air, and most things you pour onto it. That's actually its diagnostic value: a pinch of dry white CuSO₄ added to ethanol, ether, or any other organic solvent turns blue if there's free water in it, because reforming [Cu(H₂O)₄(SO₄)] coordination shifts the d-d absorption from invisible into the 700–800 nm range. The reaction is fast and visible at sub-percent water content, which is why anhydrous CuSO₄ has been the textbook qualitative test for water in alcohols since the 19th century. Anhydrous CuSO₄ has Cu²⁺ in an unusual square-planar environment with sulfate oxygens, no aquo ligands, and a d⁹ configuration that gives it the Jahn-Teller distortion in two of the three crystalline polymorphs. Heat it past about 560°C and it decomposes via 2 CuSO₄ → 2 CuO + 2 SO₂ + O₂ — historically that decomposition was used as a route to SO₂ for sulfuric acid manufacture before the contact process. Industrially, almost everything labeled "copper sulfate" in agricultural, plating, or analytical chemistry is the pentahydrate (CuSO₄·5H₂O); the anhydrous form is mostly sold for desiccant work or where the user will hydrate it down to the pentahydrate themselves. The compound is the active Cu(II) species in Bordeaux mixture (since 1885), in Fehling's and Benedict's reagents for sugar detection, and in the Biuret test where Cu²⁺ in alkali coordinates four peptide-bond nitrogens to give a violet color used in protein quantitation.
Where you'll encounter it
If you've worked in an organic chemistry lab and needed to dry an alcohol, you may have shaken it over anhydrous CuSO₄ — the white powder turns visibly blue if the alcohol is wet, and stays white if it's truly dry. In a Daniell cell electrochemistry demo, the blue half-cell solution is CuSO₄ at around 1 M. In agricultural supply stores you'll see "copper sulfate" sold by the bag for algae control in stock ponds and irrigation canals; that's the pentahydrate, not the anhydrous form.
Common Uses
- Qualitative test for water in organic solvents — white powder turns blue on hydration
- Bordeaux mixture fungicide precursor when slaked with Ca(OH)₂ for vine and orchard spraying
- Cu(II) source in Fehling's and Benedict's tests for aldehydes and reducing sugars
- Cu²⁺ reagent in the Biuret protein assay for quantifying peptide bonds at 540 nm
- Algaecide for irrigation reservoirs and farm ponds at 0.5–2 ppm copper
- Cu²⁺ source for acid copper electroplating baths in printed circuit and decorative plating
- Pyrotechnic color agent giving green-blue flame from CuOH* and CuO* emission
- Cathode half-cell active species in Daniell cell electrochemistry demonstrations
Safety Information
GHS: H302 (harmful if swallowed), H315 (skin irritation), H318 (serious eye damage from anhydrous form, which is more aggressive than the hydrate because of its dehydrating action on tissue), H410 (very toxic to aquatic life). OSHA PEL 1 mg/m³ as Cu. Emetic at gram-scale ingestion. The anhydrous form is more hazardous on skin contact than the pentahydrate because it draws water out of skin proteins on contact — wash immediately and copiously. Aquatic toxicity is the dominant environmental concern; rainbow trout 96-h LC50 around 0.04 mg/L Cu²⁺. EPA registration controls for agricultural and aquatic use.
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.