Most metals, with the exception of gold, silver and in some cases, copper, do not occur in a pure state in nature. Instead, they are usually combined with other elements such as sulphur and oxygen to form naturally occurring compounds known as ores. Examples of ores include cassiterite (tin oxide), haematite (iron oxide), galena (lead sulphide) and malachite (copper carbonate hydroxide)
The Magic of Metal Production
So how do we produce a solid, shiny metal with a high melting point from what appears, in most cases, to be a brownish sample of dirt? The answer lies in a chemical process termed "reduction." Reduction involves the addition of negative electrons to a positively charged atom (or "ion") to form a neutral atom of a particular element.
When metals occur as ores, they are in the form of positive ions combined with negative non-metal ions such as oxides and sulphides. When electrons are supplied to these metal ions, they form metal atoms and acquire all the properties typical of metals. Before this process can occur, however, a source of electrons is required.
Oxidation and Reduction Reactions
Carbon can act as such a source because it is a more reactive element than many metals and consequently has a greater tendency to lose electrons. When this happens, we say that carbon has been "oxidised" and the metal ions which accept these electrons have been "reduced."
Less reactive metals are relatively easy to extract from their ores in this manner – in the case of copper and iron, for instance, the pure metal can be produced by simply heating the ore in the presence of carbon. The equation for the reduction of iron is:
Iron(III) oxide + carbon → iron + carbon dioxide
2Fe2O3(s) + 3C(s) → 4Fe(s) + 3CO2(g)
Iron Extraction on a Match Head – Materials and Teaching Method
The charcoal that remains after burning a match provides a source of carbon in this experiment, while the sodium carbonate powder provides an interface between the iron oxide and the carbon when it melts.
The following materials and equipment are required per group of around four students:
• watch glass
• iron (III) oxide powder
• sodium carbonate
Students should be instructed to copy down the following directions, which could be followed by a teacher-led explanation.
• Immerse the head of a match in water. Roll the match first in sodium carbonate and then in iron oxide powder.
• Using metal tongs, hold the match in a Bunsen flame until it catches fire. Allow to burn until about half way down the length of the match.
• Place the match on a watch glass and crush the blackened head with a spatula. Spread the resulting particles evenly over the surface of the watch glass.
• To test whether any metallic iron has been produced, move a magnet around under the watch glass. Any iron particles should move with the magnet.
The following questions could be written on the board after students write up the experiment and their observations.
1. How do you know whether or not iron has been produced?
2. What did the charcoal from the burnt match help to do?
3. In this oxidation-reduction reaction, what was oxidised and what was reduced?
4. Name one other metal that can be extracted by heating in the presence of carbon.
5. How are more active metals such as aluminium extracted from their ores?
Iron Extraction on a Match Head – Follow Up Activities
Copper can be extracted from black copper oxide powder in a similar manner. For best results, one spatula of copper oxide powder should be placed in a test tube above one spatula of charcoal powder. After heating for several minutes in a Bunsen flame a copper coloured ring should appear at the junction between the two powders.
Dunne, Kevin, 2003, 'Metal Ores', caveman chemistry.com
Iron Extraction on a Match Head, youtube.com
Royal Society of Chemistry, 2010, ‘The extraction of Iron’ rsg.org