Abstrakt
The topic Chemical Reactions belongs to both the key and critical ones within the learning content at the lower secondary school level. As a solution, we proposed to include chemical experimental cycles into the instruction reflecting valid legislative documents and real teacher's conditions. The experimental cycles are created as sets of simple, easily conducted experiments based on mutual change of chemical matters. Initial matter is a final product of the experimental cycle which; the cycle includes various types of chemical reactions (acid-base reactions, precipitating reactions etc.). The cycles are simple, or consisting from two or more separated ones. Various factors should be considered within the process of designing the cycles, colourful changes of the matters, which is the reason why cycles exploiting copper are so favourite. Other popular one are as follows:
1. Reaction of copper and nitric acid resulting in copper nitrate,
2. Reaction of cooper nitrate and sodium hydroxide resulting in cupric hydroxide,
3. Thermal dissociation of cupric hydroxide into cupric oxide,
4. Reaction of cupric oxide and sulphuric acid into copper sulphate,
5. Reaction of copper sulphate and zinc resulting in copper and zinc sulphate. However, some of these experiments do not meet safety requirements under the school conditions, which must be taken into consideration before conducting the experiments. Reflecting these pre-conditions, we proposed the calcium cycle, which is not accompanied by clear colourful changes but supported by single tests documenting the process of reaction. The calcium cycle consists of following steps:
1. Thermal decomposition of calcium carbonate resulting in calcium oxide,
2. Reaction of calcium oxide and water resulting in calcium hydroxide,
3. Reaction of calcium hydroxide and hydrochloric acid resulting in calcium chloride and
4. Reaction of calcium chloride and sodium carbonate resulting in calcium carbonate as the final product. This experimental cycle works as a contribution to learning chemical transformations of matters reflecting lower safety risks. In conclusion, we would emphasize that except of the cycles representing chemical changes of inorganic matters, there exist cycles focusing on organic ones, or recycling wastes etc. The proposed calcium cycle will be evaluated from the view of its exploitation in teaching practice reflecting the main intention - researching critical and key topics of the learning content in lower secondary chemistry curriculum.