Zinc Oxide
Properties
| State | Solid at room temperature |
| Color | White (yellow when heated) |
| Solubility | Insoluble in water; soluble in acids and strong bases (amphoteric) |
| Melting Point | 1975 °C |
| Boiling Point | 2360 °C |
About Zinc Oxide
Zinc oxide is the white powder behind half the chemistry you can name without thinking — ZnO, molar mass 81.379 g/mol — and one of the few inorganic solids you'll meet on the same day in a paint factory, a sunscreen tube, a tire plant, and an LED foundry. It's amphoteric (dissolves in both HCl and concentrated NaOH), it's thermochromic (turns yellow when heated to a few hundred degrees due to oxygen-vacancy defects, then returns to white on cooling), and the early chemists who collected the fluffy white sublimate from burning zinc metal called it "philosopher's wool" — a name worth knowing because it still appears in old patents. ZnO is the largest single industrial use of zinc by mass: about half of the world's production goes into rubber as a vulcanization activator, where it forms the labile zinc-thiolate intermediate that crosslinks polyisoprene. The next biggest slice is mineral sunscreens — ZnO blocks both UVA and UVB by scattering and absorption, with a 3.37 eV bandgap that puts the absorption edge at 368 nm. It's also the active in zinc-oxide diaper-rash creams, the white pigment in old oil paintings (zinc white), and a wide-bandgap semiconductor that shows up in transparent conducting films, varistors, and piezoelectric sensors.
Where you'll encounter it
If you've ever painted your nose white with a thick mineral sunscreen at the beach, that opaque coating is micron-scale ZnO suspended in oil — modern formulations use 30-200 nm particles to stay transparent on skin while keeping the same UV blocking. In a tire factory, every batch of rubber has a few percent ZnO mixed in with sulfur and accelerators before vulcanization, and without it the crosslinking would be too slow to be commercially useful. In a hospital nursery, the white cream a nurse spreads on a baby with diaper rash is 10-40% ZnO in a petrolatum base, acting as both a physical barrier and a mild astringent. And in an electronics lab, the surge protector behind your power strip contains a ZnO varistor — a sintered ceramic disk whose nonlinear current-voltage curve clamps voltage spikes from lightning strikes and motor switching.
Common Uses
- Vulcanization activator in rubber compounding — about half of world ZnO production goes here
- Active UV-blocker in mineral sunscreens, blocking both UVA and UVB via scattering and absorption
- Skin protectant and mild astringent in diaper-rash creams (10-40% ZnO in petrolatum)
- Wide-bandgap semiconductor (3.37 eV) for transparent conducting films, varistors, and piezoelectric sensors
- White pigment ("zinc white") in oil paints, ceramics, and cosmetics — non-toxic alternative to lead white
Safety Information
GHS: Aquatic acute Category 1 (H400), Aquatic chronic Category 1 (H410). ZnO is generally recognized as safe (GRAS) for topical use and is approved by FDA as a skin protectant active ingredient in OTC drug monographs. The serious occupational hazard is metal fume fever from inhaling ZnO smoke generated by welding or cutting galvanized steel — sub-micron ZnO particles cause flu-like symptoms (fever, chills, muscle aches) 4-12 hours post-exposure that resolve within 24-48 hours. OSHA PEL for ZnO fume is 5 mg/m3 (8-hour TWA), STEL 10 mg/m3; for ZnO dust, 15 mg/m3 total / 5 mg/m3 respirable. Use local exhaust or supplied-air respirators when welding galvanized steel.
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.