WATER 7 



A hard water may be described as one which contains dissolved 

 mineral matter in undesirable quantity. Hardness is of two kinds, 

 temporary hardness and so-called permanent hardness. 



Temporary hardness is due to the presence in the water of the 

 bicarbonates of calcium and magnesium, and is so called because 

 it can be removed by simply boiling the water. When rain descends 

 it absorbs from the atmosphere some of the carbon dioxide, the 

 acidity thus gained is sufficient to dissolve calcium and magnesium 

 carbonate, insoluble in non-acid waters, and to form the bicar- 

 bonates. The bicarbonates thus formed remain in solution, but 

 when the water is boiled the CO 2 is driven off as gas, the bicar- 

 bonate is reduced to the carbonate and, being thrown out of solution, 

 is precipitated. It is this precipitated calcium or magnesium that 

 constitutes the scale or fur in kettles, boilers, &c. 



The eroding action of acid rain water on calcium carbonate 

 is well seen in the pitting and crumbling away of marble tombstones 

 in cities. 



The following equations will illustrate what has been said 

 above : 



H 2 O + CO 2 = H 2 CO 3 

 Carbonic Carbonic 



anhydride acid 



CaCO 3 +H 2 CO 3 = CaH 2 (CO 3 ) 2 

 Calcium Calcium 



carbonate bicarbonate 



On boiling the water the calcium bicarbonate becomes reduced 

 as follows : 



CaH 2 (CO 3 ) 2 = CaCO 3 +H 2 O + CO 2 



The sulphates and chlorides of calcium and magnesium and any 

 nitrates that may be present are soluble in water whether CO 2 be 

 present or not; therefore they are not thrown out of solution 

 when the CO 2 is driven off on boiling. Hardness due to the 

 presence of these salts is on this account called permanent as com- 

 pared with the temporary hardness due to the presence of bicar- 

 bonates, which are readily removed by boiling. The sum of the 

 temporary and permanent hardness is called the total hardness. 



Iron in the form of ferrous carbonate such as is found in chaly- 

 beate waters adds to the permanent hardness; so also may the 

 zinc salts and silicates. 



Waters differ greatly in degree of hardness according to the 

 nature of the soil through which they have percolated and from 

 which they are collected. Those collected from the igneous or 



