school energy&environment

Hot waters, cold waters

The possibilty of water to corrode the rocks it comes into contact with depends on the chemical composition of both water and minerals building rocks. However other factors intervene to make reactions complex. In particular, temperature is a fundamental factor that acts in two ways that are apparently in contrast one with the other.  In fact, the quantity of CO₂ that can dissolve in water depends on temperature –  (at the same pressure)  the lower the temperature, the greater the amount of  CO₂ that can dissolve (also pressure is important, but as it can be affected by relatively small variations at normal environmental conditions, it is neglected for the sake of simplicity). How temperature influences the quantity of CO₂ dissolved in water can easily be seen when a bottle of carbonated mineral water is opened : the action may happen quite peacefully if the bottle has just been taken from the refrigerator, however it can have explosive results if the operation is carried out on the same bottle that has been left inside a car parked in the sun. In fact hot water cannot contain the same amount of  CO₂ as cold water,  and the excess amount is freed in the form of bubbles as soon as it is given the chance (i.e. when we remove the cap, decreasing pressure). This theoretically tends to favour karst processes in mountain areas at high altitudes, where water temperatures are low and therefore can contain a greater quantity of CO₂. However two other contrary factors must be considered :  first the speed of reaction is higher when temperature is higher, and second at high temperatures vegetation, and as a consequence the production of CO₂ in the soil are much greater, thus, in practice, making warm waters much more aggressive than cold ones (furthermore the content of CO₂ in the atmosphere decreases with altitude, therefore  mountain waters, although they are colder,  have a lower content of CO₂).
Once again boring domestic chores offer an occasion for an experiment : if we heat the products used for cleaning, the reaction, and therefore the action of removing the calcareous incrustations, is even more rapid (this operation is  not really advisable because detergents also contain other substances that can release toxic and irritating vapours when heated – therefore it’s best to carry out this experiment with a glass of hot vinegar, as our wise grandmothers used to do!). It is of fundamental importance, when studying  karstification processes, to know the diagram that shows the solubility of calcite at different temperatures and with different quantities of dissolved CO₂.
As a consequence of this, therefore,  areas that are most favourable for karstification are those in which, besides an abundance of carbonatic rocks there is abundance of water with a large amount of  CO₂ and at a high temperature. These conditions are present in the intertropical zone. Most cavities in Italy, in fact, are the relicts of caves that formed when climate was humid tropical even at our latitudes.
Complicated reactions
Another factor able increase the dissolution of calcite and dolomite in water solutions is the presence of particular ions, such as chlorine and magnesium, sulphurs or sulphates. For this reason, sea water and even more, sea water mixed with fresh water form a very aggressive mixture. Aggressive mixtures also originate when waters with different chemical properties mix. Corrosion due to water mixing, or Boegli’s effect, named after the speleologist who discovered this phenomenon, is responsible for the formation of underground galleries and conduits. In fact aggressiveness would otherwise tend to wear out as water corrodes rocks thus increasing its calcium carbonate content, but can be “renewed” by these waters. A proof of this effect is witnessed by those privileged cave-divers when they penetrate karst conduits that have been widened by sea water and observe the gradual widening of the conduits near the area in which sea water and fresh water mix. Or when, in the presence of fresh water, a sudden widening can be observed, forming particular forms of corrosion, such as dissolution pockets, cupolas, and bell holes, in galleries joining and anastomosing into each other, where waters with a different chemical content flow and mix. In order to activate this mechanism,  large quantities of water are not necessary. Forms of water mixing corrosion can be seen on the walls  or on the ceilings of galleries, close to very small conduits, at times simple fractures (cupolas and bell holes formed by corrosion due to mixing waters are recognizable from similar forms, created by mechanical erosion, as they  become narrower in the deeper parts and they always form near conduits or fractures, even of very small diameters).