LIME & THE FORMATION OF SALTS
In a further lesson, the study of chalk may be extended in another direction. A piece of chalk or limestone is heated—preferably over a bellow-blown fire. After it is burnt, it is allowed to cool and then water is poured on it, when it is observed to sizzle and get hot. The experiment is repeated with a larger quantity of burnt chalk. It swallows the water thirstily; considerable quantities disappear into the chalk, leaving no trace. Instead, steam rises and heat is produced. We conclude that burnt chalk contains no water; the fire has separated it completely from the water in which it had its origin. That is why it is so thirsty. Long after it was connected with animal life, chalk still retains its animal nature.
If now we add more water, a milky liquid results (milk of Lime). If left to stand for some while, the slaked lime settles to the bottom, but a part still stays in solution as a clear liquid (Lime Water). We now dip a piece of litmus paper into the lime water. It is turned blue. A liquid which turns litmus blue is called an alkali. Like acids, the alkalis have a characteristic taste. Through burning chalk and slaking it with water an alkali is formed. Fire and water have changed the chalk into something else. Perhaps something has gone from the chalk while it was being heated ? This is evident when chalk is burned in a lime kiln. Carbonic Acid gas is driven off. A method of catching this gas is explained to the children, and some prepared gas is shown to them. This is the same gas that rises from mineral springs and bubbles up in mineral water. The chalk has evidently breathed out something while it was burning. The quicklime which remains is more solid. This more solid part of lime is also sometimes called a base because it is, so to say, the solid foundation of the lime salt; and as we saw, when it is dissolved in water it becomes alkaline. The carbonic acid gas which was driven off may also be dissolved in water. It gives the water a sour taste, and in it blue litmus turns red. The chalk thus gives rise to the alkaline quicklime, and the acid, carbonic acid gas. In this way the ideas of acid and alkali reach the child. Chalk had its origin in a salt forming process. It is itself a salt, because fire can change it, carbonic acid gas passing off and quicklime remaining. Thus:
Carbonic Acid Gas—Air < Chalk > Quicklime —Solid, Chalk-earth Fire has produced this division. The addition of water to each of these parts brings out their antithetical nature as acid and alkali.
These distinctions can be even more tellingly demonstrated. Two bottles are shewn, the one containing carbon dioxide in solution (ordinary mineral water), the other containing lime-water. From the first bubbles arise; they are the gas that has gone out from the chalk. The liquid tastes sour, prickly, and sharp, and it turns blue litmus paper red. The lime-water in the other bottle tastes flat and insipid and turns the litmus paper blue. Experiencing these things for themselves is a joy to the children; it seems only natural to them that the sharp and sour turns red, while the dull and insipid turns blue. What they have learnt in their painting lessons as well as their own experience of colour make this entirely comprehensible to them. Carbonic acid gas bubbles up from the carbonated water; the stopper flies out if it is not firmly fastened down, but at the bottom of the lime- water bottle a white deposit has settled. The stopper stays firm and may even’ freeze in ‘ because solids are deposited around it. In the carbonic acid bottle, gas strives to escape upwards, in the lime-water bottle the solids tend to settle down to the bottom. These two contradictory factors were originally contained in the chalk. Fire was able to drive them apart, and water could bring out the characteristics of each. But now see what happens when some of the mineral water is poured into the lime-water! It becomes cloudy, and presently a white salt settles on the bottom. The opposites are once again united, for this salt is nothing else than chalk.
Now let the children blow through a pipe into a glass of lime-water. Again the water clouds and there is a white deposit. Our breath therefore contains the same substance that chalk gives off when it is burnt—carbon dioxide gas.
Fire changed the chalk into light gaseous carbon dioxide and solid basic quicklime. The water, which was driven off, stands as a link between them. Bring each to the water independently and acid or alkali will result. But bring the two together, and chalk—or as we should more correctly say Calcium Carbonate—is formed. Fire split the chalk into two parts manifesting their different natures. Water brought these contradictions separately into evidence, but it also combined them once more. What fire has divided, the water once again unites.
It is important to notice that we did not start with acid and alkali and obtained the salt from them. Instead we took the opposite and natural way, for chalk is a natural substance which is in process of formation around us at all times. Acid and Alkali can only be produced by artificial means. They are opposites which can only be differentiated by splitting the salt. In teaching a young child to add, we do not start with the numbers to be added and deduce therefrom the total, but we make the child understand that an original whole has been divided up and that the sum of these portions must bring us back to unity— our original whole. We start with the integer and not with the integrant. So here we start with the whole and show forth the parts. Teachers should appreciate how, even at this early stage in the teaching of chemistry, the concept of a ‘ chemical combination ‘ may be regarded not merely as the sum of its parts, the chemical sum of its elements. It is something new, often indeed something primary. In this case we start with chalk as our natural substance which is taken up by the organic processes to build shells, bone, etc. Only when this has been made clear, we proceed to separate from it its different factors. Chalk itself is the resultant of organic processes. Fire, in dividing it into acid and alkali, shows that it was the chemical compound ! calcium carbonate.’