Zinc Chloride
Properties
| State | Solid at room temperature (extremely hygroscopic) |
| Color | White crystalline solid |
| Solubility | Very soluble in water (432 g/100 mL at 25 °C) |
| Melting Point | 293 °C |
| Boiling Point | 732 °C |
About Zinc Chloride
Zinc chloride is one of the most aggressively hygroscopic salts you'll handle in a chemistry lab — ZnCl2, molar mass 136.28 g/mol, and the original "zinc butter" of the alchemists, named for the way the deliquesced solid pulls so much water out of humid air that it turns into a sticky paste on the bottle threads. It's also one of the strongest mid-row Lewis acids that's still soluble in water: the small, highly charged Zn2+ ion accepts electron pairs from carbonyls, alcohols, and aromatic rings without the violent hydrolysis you get from AlCl3 or TiCl4. That combination drives most of its lab chemistry. ZnCl2 catalyzes Friedel-Crafts alkylation, Fischer indole synthesis, the Lucas test for telling primary, secondary, and tertiary alcohols apart by reaction rate, and a long list of named reactions where you want a Lewis acid that survives a trace of water. On the industrial side, zinc chloride is the standard soldering flux — it strips the oxide layer off copper joints in a few seconds — it goes into galvanizing baths to prep steel for hot-dip coating, and it's the electrolyte in the cheap dry-cell batteries still sold for clocks and remotes.
Where you'll encounter it
If you've ever soldered a copper pipe joint and watched the flux paste sizzle and pull back from a clean bright copper surface, the active ingredient was almost certainly zinc chloride dissolved in ammonium chloride. In a teaching lab, ZnCl2 is the reagent behind the Lucas test — drop tertiary butanol into ZnCl2/HCl and the solution clouds within seconds, drop primary butanol and nothing visible happens for an hour. In an organic synthesis lab, ZnCl2 catalyzes the Fischer indole synthesis from phenylhydrazone precursors, one of the most-used routes to substituted indoles in heterocyclic chemistry. And on the field side, the AA-size zinc-carbon battery in a TV remote uses a ZnCl2 paste electrolyte at the zinc-anode interface — the same chemistry Leclanche described in the 1860s, still the cheapest non-rechargeable cell on the shelf at any hardware store.
Common Uses
- Soldering and brazing flux for copper, brass, and steel — strips oxide layer in seconds
- Lewis-acid catalyst for Friedel-Crafts alkylation and Fischer indole synthesis
- Lucas test reagent in undergraduate organic chemistry for classifying alcohols
- Electrolyte paste in zinc-carbon (Leclanche) dry-cell batteries
- Galvanizing bath flux for hot-dip steel coating, also used in dyeing as a mordant
Safety Information
GHS: Skin corrosion/irritation Category 1B (H314), Serious eye damage Category 1 (H318), Acute toxicity oral Category 4 (H302), Aquatic acute and chronic Category 1 (H400/H410). OSHA PEL for zinc chloride fume is 1 mg/m3 (8-hour TWA), STEL 2 mg/m3. The corrosive hazard is real — concentrated solutions burn skin within seconds and cause severe eye damage requiring immediate flushing. Inhalation of the smoke from soldering with ZnCl2 flux causes metal fume fever, with flu-like symptoms 4-12 hours after exposure. Use a fume hood or local exhaust when soldering, wear nitrile gloves and splash goggles when handling solutions, and never mix with cyanide-containing waste.
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.