Limestone in your everyday life: teacher’s notes

Level This activity is most appropriate for students aged 14-16 to illustrate useful materials made from rocks. Topic Limestone has a relatively simple chemical composition – it is largely calcium carbonate, CaCO 3 . However, limestone and substances made from it have a diverse range of uses. This activity illustrates some of these uses by relating them to everyday activities of the students. Description Students fill in a timetable of their activities in the morning from waking up to leaving for school. Discussion with the teacher and / or reference book- or web-based research follows, and indicates that a surprising number of activities have a link to limestone in some way. Context This activity follows on well from The chemistry of limestone which points out some of the uses of limestone and products made from it. Teaching points The format is quite flexible. One approach is to ask the pupils to log and then report their activities from getting up in the morning until leaving for school. The degree of formality is a matter for the teacher. For example responses could be written logs or reports produced either individually or in groups, or they could be brainstormed and written up on the blackboard or presentation software. Students could then be challenged to suggest links between the various activities and limestone. To do this they can use knowledge from previous lessons, reference books and internet searches if web access is available. Entering ‘Uses of limestone’ into the Google search engine, for example, produces a large selection of websites with relevant information. Other links can be made by the teacher with reference to the notes and table below. Alternatively, the teacher could discuss with the class in general terms some of the uses of limestone and its derivatives. This could be followed by students making a list of their activities between waking and leaving for school. They could then be asked to relate these general uses to specific activities in their morning routine. For example students might make the link between the use of limestone in the blast furnace to make iron and their use of an iron bath or the transport of water in iron pipes. Another alternative would be for the teacher to supply the students with a copy of the table (or part of it) with selected parts (activities and / or applications of limestone) missing and ask the students to fill in the gaps. Note that the table is largely for the teacher’s reference. Students are unlikely to spot more than a few of the links in the Table. Many links other than those in the Table are also possible. Uses of limestone and its derivatives Some important uses of limestone follow.

Production of lime by thermal decomposition limestone → lime + carbon dioxide CaCO 3 (s) → CaO(s) + CO 2 (g) This is the main reaction in commercial lime burning to make lime (calcium oxide) which is a base and has many uses, some of which are described below. Controlling pH by the use of lime In sugar refining, for example, lime is used to make the solution alkaline, causing impurities in the sugar to separate out for removal. Carbon dioxide (also from limestone) is then used to remove the lime by reforming calcium carbonate (limestone) which precipitates out. This is the same chemistry as the limewater test for carbon dioxide. Neutralisation of acids Lime is used to neutralise acidic soil and it is also used to remove sulfur dioxide from coal-fired power station gases (known as flue gas desulfurisation, FGD). The product is gypsum, used in building materials such as plasterboard. A very simplified form of the reaction can be represented by the following equation. CaO(s) + SO 2 (g) + 2H 2 O(l) + ½ O 2 (g) → CaSO 4 .2H 2 O(s) Making iron in the blast furnace The initial reaction is the thermal decomposition of calcium carbonate to form calcium oxide (lime): CaCO 3 (s) → CaO(s) + CO 2 (g) This is followed by the reaction of lime with sandy impurities (mostly silicon dioxide, silica, SiO 2 ) in the iron ore to form calcium silicate (slag): CaO(s) + SiO 2 (s) → CaSiO 3 (l) The reactions involved in the production of glass and Portland cement (the latter involving alumina and aluminium silicate) are very similar. Indeed some blast furnace slag is used to make glass and cement. Manufacture of sodium carbonate Sodium carbonate is made from salt (sodium chloride) and limestone by the Solvay (or ammonia-soda) process. The overall reaction is CaCO 3 (s) + 2NaCl(aq) → Na 2 CO 3 (s) + CaCl 2 (aq) although the details are much more complex, see T. Lister (ed), Sodium carbonate – a versatile material, London: Royal Society of Chemistry, 2000. Limestone powder as a filler Limestone dust is frequently used as a filler to give bulk to or to extend expensive resins, adhesives, polymers etc. In the trade, fillers are often referred to as ‘inert’; they are only ‘inert’ in the sense that they do not react significantly with the other materials used in that particular process. The table gives a typical list of early morning activities and their links with limestone.