Yasmin Jayathirtha
In a previous column, I had talked about experiments that lie at the interface of chemistry and other subjects like geography and environmental science. Among the most important would be the use of earth’s resources and the effect this will have on the environment. Some of the major resources are the metals, and both the occurrence of the ores and the extraction of the metals are a large part of the study of the three subjects. If one studied the topic from the point of view of the chemical principles, the concepts would be spread over the chapters of activities, oxidation-reduction, and electrochemistry and in a very peripheral way around solubilities. All these ideas then are used to describe the extraction of the three most used metals – aluminium, iron, and copper.
If the emphasis were on physical geography and environmental science the sequencing would be different – the chemical nature of ores, the occurrence at particular places, the siting of the extraction plants and the environmental consequences of the method used for extraction and the importance and feasibility of recycling. Of course, this sequence does not touch at all on the historical and political aspects, which would include colonialism and new resources being found in technologically less advanced areas. What follows is a sequence of experiments that can be performed for helping to understand the ideas behind the production of metals.
Consider the formulae of the ores commonly used; they are either the oxides (bauxite Al2O3, haematite Fe2O3, cassiterite SnO) or carbonates (malachite Cu2CO3 (OH)2, limestone CaCO3) or sulfides (zinc blende ZnS, galena PbS).
Why are there no nitrates, and very few chlorides?
Experiment 1
Lay out a reaction surface as shown in Figure 1. Mix the solutions as shown and observe the formation of precipitates. Record the observations as accurately as possible, including colour changes, gas formation (bubbles), colour and texture of precipitates, etc. The entire reaction surface can then be used to generate solubility rules.
Use the chemistry lab bench reagents for the mixing. A good set is given in the table:
| Name | Chemical Formula | Concentration (mol/dm3 |
| Sulfuric acid | H2SO4 | 1 |
| Nitric acid | HNO3 | 1 |
| Hydrochloric acid | HCl | 1 |
| Sodium hydroxide | NaOH | 1 |
| Ammonia | NH3 | 1 |
| Na2CO3 | 0.2 | |
| Na3PO4 | 0.2 | |
| AgNO3 | 0.1 | |
| PbNO3 | 0.2 | |
| CaCl2 | 0.2 | |
| MgCl2 | 0.2 | |
| BaCl2 | 0.1 | |
| CuSO4 | 0.2 | |
| FeCl3 | 0.1 | |
| FeCl2 | 0.1 | |
| AlCl3 | 0.3 | |
| Zn (NO3)2 | 0.2 |
Consider the solubilities of various salts and it becomes apparent that all nitrates and most chlorides are soluble and so get washed down to the sea and it is only in very dry areas or dried up seas that these minerals will be deposited.
Bring out samples of all the common metals. The average lab should have sodium, magnesium, zinc, iron, tin, lead, and copper. Chromium, silver, and gold are also fairly common in the surroundings as are the alloys brass, bronze, and stainless steel. It will be very obvious that many of the metals have already reacted with air; they all have a coating that makes them dull.
Figure 1
| H2SO4 + Na2CO3 | H2SO4 + Na2PO4 | H2SO4 + Ag2NO3 | H2SO4 + Pb(NO3)2 | H2SO4 + MgCl2 |
| HNO3 + Na2CO3 | HNO3 + Na2PO4 | HNO3 + Ag2NO3 | HNO3 + Pb(NO3)2 | HNO3 + MgCl2 |
| Teachers disseminate information to students; students are recipients of knowledge. | Teachers have a dialogue with students helping them construct their own knowledge. | |||
| Teacher’s role is directive, rooted in authority. | Teacher’s role is interactive, rooted in negotiation. | |||
| Students primarily work alone. | Students work collaboratively. | |||
| Knowledge is seen as inert. | Knowledge is seen as dynamic, ever-changing. |
The author works with Centre for Learning, Bengaluru. She can be reached at yasmin.cfl@gmail.com.