Oxidizers also generate oxygen gas in contact with water. The heat and oxygen represent two sides of the fire triangle or tetrahedron. All that is needed is fuel. If combustible materials are present, combustion can occur spontaneously. When that occurs, there is also excess oxygen present that accelerates the rate of combustion. Third, the corrosive action of a strong acid, such as nitric acid, can generate heat. The heat can be sufficient to ignite combustible materials to which the corrosive is exposed.