Zinc Hydroxide
Properties
| State | Solid (white amorphous or crystalline) |
| Color | White |
| Solubility | Insoluble in water; soluble in acids and strong bases (amphoteric) |
| Melting Point | 125 °C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Zinc Hydroxide
Zinc hydroxide is the white gelatinous precipitate you see the second you add a few drops of NaOH to any zinc salt solution — Zn(OH)2, molar mass 99.395 g/mol. It's the textbook example of an amphoteric hydroxide: keep adding NaOH and the precipitate redissolves as the soluble tetrahydroxozincate ion [Zn(OH)4]2-, while adding HCl instead dissolves it to ZnCl2 and water. That dual behavior is the standard demonstration of amphoterism in general chemistry, and it underlies the qualitative-analysis scheme for separating Zn2+ from Mg2+ and Mn2+, which precipitate as hydroxides but don't redissolve in excess base. Industrially, Zn(OH)2 is the intermediate that closes the Bayer-style loop for zinc oxide production: precipitate it from a zinc-bearing leach liquor, then calcine to ZnO. It also occurs naturally as the rare minerals ashoverite, sweetite, and wulfingite. In alkaline-battery chemistry, Zn(OH)2 forms as the discharge product at the zinc anode of every Zn-MnO2 alkaline cell, and the morphology of the precipitate — porous and accessible vs. dense and passivating — is one of the things battery engineers tune to extend cycle life.
Where you'll encounter it
If you've ever run the qualitative-analysis lab in undergraduate inorganic chemistry, the Group II separation step is when you learn this compound by feel: add NH3 to your unknown, see a white gel form, then add excess NaOH and watch it redissolve to a clear solution — that's the diagnostic for Zn2+ in a mixture with Mg2+ and Mn2+. In a battery R&D facility, when an alkaline AA cell is discharged, the zinc anode oxidizes through a Zn(OH)4(2-) intermediate that eventually precipitates as Zn(OH)2 and ZnO inside the porous anode structure — the way that precipitate forms determines whether the cell cycles cleanly or develops a passivation layer that kills capacity. In a wound-care setting, the white paste under a calamine lotion bottle or a zinc-oxide diaper-rash cream is partially Zn(OH)2 in equilibrium with ZnO, acting as a mild astringent on irritated skin.
Common Uses
- Intermediate in zinc oxide production via precipitation from leach liquors followed by calcination
- Discharge product at the zinc anode in alkaline Zn-MnO2 batteries — morphology determines cycle life
- Mild astringent in wound-care creams and surgical bandages, often with calamine
- Qualitative-analysis diagnostic for separating Zn2+ from Mg2+ and Mn2+ via amphoteric redissolution
- Rubber-compounding additive that releases ZnO slowly during vulcanization
Safety Information
GHS: Aquatic acute Category 1 (H400), Aquatic chronic Category 1 (H410). Acute toxicity is low — Zn(OH)2 is essentially insoluble in water at neutral pH and not significantly absorbed orally. The ecotoxicity hazard is the bigger concern: free Zn2+ released in waterways is highly toxic to fish and invertebrates, with 96-hour LC50 values for trout below 1 mg/L. There is no specific OSHA PEL for zinc hydroxide; treat as a particulate not otherwise regulated, 15 mg/m3 total / 5 mg/m3 respirable. Avoid generating airborne dust, and route any waste streams through a wastewater treatment that precipitates and removes zinc rather than discharging to surface water.
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.