LIME & THE FORMATION OF SALTS
The uses of lime in everyday life, and its technical uses, are explained in another lesson. After it has been quarried from the earth, chalk is taken to a limekiln. If possible, the children should be shown over a lime works, where they might make a drawing of the kiln, whose design is explained to them. In this kiln the chalk is heated, when the carbon dioxide escapes and burnt or quicklime remains. This is filled into sacks and despatched to masons for building. At this stage, the commercial working of a lime works and its relationship to other businesses can be explained, since it is a part of the curriculum for children of this age. The hygienic aspect may also be touched upon in explaining the dangers of the slaking process due to the corrosive action of the lime alkali. The violence of the action when large quantities of lime are being slaked will bring this home to the children. Again one asks: whence all this heat ? It is due to the great heat that was applied to the chalk in the kiln. This heat dwells in the quicklime and also in the carbonic acid gas. It is this slumbering fire which gives the quicklime its great thirst for water.
The masons mix the slaked lime with sand. The result is mortar. Here again we have an important antithesis; namely that between silica and lime. Sand is silica earth (an acid), lime, on the other hand, is a base. It is unnecessary to go into the process of glass making which brings out the acid nature of sand so strongly: it will suffice to point out the contrast between the hard brittle, shapely and immobile sand, and the greedy quicklime, which becomes so slimy and smooth when slaked.
Then there might be a demonstration of how two bricks may be united with mortar and how this quickly hardens. Something new has resulted from the interaction of the two opposites. In newly built houses this process is hastened by lighting fires in them. These not only drive out the water from the slaked lime, but also give out carbonic acid gas which unites with the lime to form calcium carbonate (chalk).
The drying out of newly built houses is also hastened when people live in them, for the air they exhale contains carbonic acid gas. Those who do this are called ‘ drying-out’ tenants, and it may be remarked here that it does no good to their health. Here we have a practical instance of the similarity of our breathing process to burning in nature; the practical use has been shown of the things we recently demonstrated in the laboratory.
On the following day we return to our starting point. Our investigation has taken us back to calcium carbonate, to the chalk from which we started. Originally it lies in the chalk pit, now, in union with the sand it holds our houses together. Why have we gone to all the bother of splitting up the chalk, if in the end we are going to arrive back at the same chalk we set out from ? Because the lime and the carbonic acid gas in the process of reuniting have bound together the stones of our houses. Man has split chalk into its hidden and opposite factors and then utilised the force of their reunion to cement his houses. It is as though the stones of a quarry, which had been scattered all over the world were to draw together again uniting to form the houses of mankind. It is an impressive moment for the student when he first realises how in his technical processes, man forces apart natural powers which are striving to be united and then uses the energy of their rapid effort at reunion to carry out his various tasks. The dammed-up lake for instance gives water-power when its pent waters are released and so, too, here in the field of chemistry.
Our investigations into chalk have also included the vital processes. The pupils have learnt that chalk is the source of the bony framework of their bodies, and how it is associated with animal processes and has its
home in the animal kingdom. Throughout all nature and bound up with man, chalk formation proceeds.
The concept of salt formation may now be extended to include the salt of the sea. Salt is an inseparable part of the sea. The technique of salt manufacture from sea water, rock-salt mines and brine springs is explained. The slowness of the evaporating and crystallisation of salt are demonstrated. It is contrasted with chalk. This salt—our ordinary cooking salt—has much less to do with the life processes than has chalk. It is essentially mineral, indeed the only mineral nutriment man requires. Other needful minerals are to be found in the normal diet, only salt must be provided independently. If he could get no salt at all, man would die, for salt preserves; it retards the processes of decay. The salting and pickling of foodstuffs is based on this knowledge. Sea air also has this effect; it makes man more awake. On the other hand an excess of salt in the sea will kill every living thing, as happens in the Dead Sea.
What happens if we treat the salt as we treated the chalk ? Heated in the furnace it melts and even passes off in steam, but very great heat is required to drive out from it any gas. Instead of using high temperatures it is more simple to pour sulphuric acid over the rock salt. A white vapour with a strong pungent smell is driven off. When led off into a vessel of water, this vapour becomes Hydrochloric acid, or spirits of salt. But this process will not serve for obtaining the alkaline base. To obtain this by heating as with chalk, requires tremendous heat, and so an alternative method is used. The children will hear more about this electrical method later on.