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 occupational exposure limit specific to zinc hydroxide; treat it as a particulate not otherwise regulated, under the general total and respirable dust limits. 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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.