ZnO occurs as a white powder. The mineral zincite usually contains manganese and other impurities that confer a yellow to red color. Crystalline zinc oxide is thermochromic, changing from white to yellow when heated and in air reverting to white on cooling. This color change is caused by a small loss of oxygen to the environment at high temperatures to form the non-stoichiometric Zn1+xO, where at 800 °C, x = 0.00007.
Zinc oxide is an amphoteric oxide. It is nearly insoluble in water, but it is soluble in (degraded by) most acids, such as hydrochloric acid:
- ZnO + 2 HCl → ZnCl2 + H2O
Bases also degrade the solid to give soluble zincates:
- ZnO + 2 NaOH + H2O → Na2
ZnO reacts slowly with fatty acids in oils to produce the corresponding carboxylates, such as oleate or stearate. ZnO forms cement-like products when mixed with a strong aqueous solution of zinc chloride and these are best described as zinc hydroxy chlorides. This cement was used in dentistry.
ZnO also forms cement-like products when treated with phosphoric acid; related materials are used in dentistry. A major component of zinc phosphate cement produced by this reaction is hopeite, Zn3(PO4)2·4H2O.
ZnO decomposes into zinc vapor and oxygen only at around 1975 °C, reflecting its considerable stability. Heating with carbon converts the oxide into the metal, which is more volatile than the oxide.
- ZnO + C → Zn + CO
Zinc oxide can react violently with aluminium and magnesium powders, with chlorinated rubber and linseed oil on heating causing fire and explosion hazard.
It reacts with hydrogen sulfide to give the sulfide. This reaction is used commercially in removing H2S using ZnO powder (e.g., as deodorant).
- ZnO + H2S → ZnS + H2O
When ointments containing ZnO and water are melted and exposed to ultraviolet light, hydrogen peroxide is produced.
Read more about this topic: Zinc Oxide
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