CHEMISTRY OF COMMON THINGS 19 



The other kind of water contains chemical compounds 

 which will act on soap and prevent the formation of an 

 emulsion. This water is said to be "hard" water, and 

 a lather will not form in hard water or in water contain- 

 ing chemicals that act on soap. 



16. Hard Water. Water flowing over the ground 

 and through the ground comes in contact with more or 

 less salts that can be dissolved and carried along in 

 solution. If these salts held in solution happen to be 

 what are known as the calcium salts, as calcium hydrogen 

 carbonate [CaH 2 (COj)i], or calcium sulphate (CaSO 4 ), 

 which is also known as gypsum, the water will be "hard" 

 water. When soap is used in hard water a sticky, 

 gummy-like substance is formed which will not dissolve 

 in the water and which is known as calcium soap. Cal- 

 cium soap is not fit for washing as it will not form an 

 emulsion with fats. In hard water the soap is wasted 

 until the calcium compounds are all broken up by the 

 soap. This is an expensive way of 

 making the water "soft," so cheaper 

 chemicals are used to soften water. 



Water containing calcium hydro- 

 gen carbonate [CaHj(COj)sJ is said 

 to be "temporarily hard" because it 

 can be softened by adding a measured TEAKETTLE 



quantity of slaked lime (CaO 2 H 2 ), or 

 by boiling it for a time. The slaked 

 lime acting on the calcium hydrogen carbonate forms 

 a white precipitate, calcium carbonate (CaC0 3 ), which 

 will settle, and then you may draw off the clear, soft 

 water. The stone-like scale in a teakettle in which hard 

 water is continually heated is due to the settling of the 

 insoluble compound, calcium carbonate, (CaCO 3 ) formed 



