(1) According to the law of conservation of mass, the types of elements are unchanged before and after the reaction, and because the reactants contain iron, it is possible to generate elemental iron.

(2) The dark brown substance contains elemental iron, which reacts with hydrochloric acid to generate ferrous chloride and hydrogen. The chemical equation of this reaction is: Fe+2 HCl ═ FeCl 2+H 2 ↑.

(3) Iron powder can be attracted by magnets, and magnets can attract dark brown solids. If there is residual powder on the magnet, it means that the powder contains elemental iron.

So the answer is: (1) iron; (2)fe+2hcl═fecl 2+H2↑； (3) attract with a magnet.