Igniting a crucible containing cyclohexane, showing its incomplete combustion. The black soot around the sides of the crucible reveals the presence of carbon in the flame. The yellow colour of the flame itself is indicative of incomplete combustion, the colour being produced by incandescent particles of soot in the flame. Incomplete combustion occurs when a substance burns in a relative lack of oxygen. For a hydrocarbon like cyclohexane, complete combustion would convert all the cyclohexane to carbon dioxide and water. In a lack of oxygen, the products include carbon dioxide, carbon monoxide and carbon soot.