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. Occupational exposure limits for the fume are set well below those for coarser ZnO dust, reflecting how much more of the sub-micron fraction reaches the deep lung. 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. 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.