Nitric acid will produce a black spot on steel; the darker the spot the harder the steel. Iron, on the contrary, remains bright if touched with nitric acid. Good steel in its soft state has a clean fracture and a uniform gray luster; in its hard state, a dull, silvery, uniform white luster. Cracks, threads, or sparkling particles denote bad quality.

Good steel will not bear a white heat without falling to pieces. It will crumble under the hammer at a bright red heat, while at a moderate heat it may be drawn out under the hammer to a fine point.

Iron is more active than copper. If a piece of iron wire is placed in a copper salt solution, the iron wire becomes coated with metallic copper and part of the iron is dissolved in the solution. Thus, when a piece of iron wire is placed in a solution of copper sulphate the copper immediately coats the iron and part of the iron takes the place of the copper which is deposited according to this equation

Fe + CuSO4 = Cu + FeSO4 These facts are taken advantage of in marking iron tools.