Nickel(II) oxide is a light green solid. It does not dissolve in water. Sometimes it is gray because all of the nickel is not in the +2 oxidation state. It reacts with acids to make nickel(II) salts. For example, it reacts with hydrochloric acid to make nickel(II) chloride. It does not react with bases easily.
Nickel(II) oxide can be made by heating nickel powder in air at 400 °C (752 °F). This normally makes a black nickel oxide. It also can be made by reacting nickel with water at 1,000 °C (1,830 °F). The simplest way to make it is to heat nickel(II) hydroxide, nickel(II) carbonate, or nickel(II) nitrate.
Nickel(II) oxide can irritate skin and make problems when breathed in.