SOME CHEMISTRY OF EVERYDAY LIFE 179 



The chemical changes which occur in the use of sodium 

 carbonate to soften waters containing in solution small 

 quantities of calcium suphate (CaSO^ are thus expressed: 



CaS0 4 (soluble) + Na 2 CO 3 (soluble) = Na 2 SO 4 (soluble) 

 + CaCOs (insoluble). The sodium sulphate which remains 

 in solution does not interfere with the action of the soap, 

 and the calcium carbonate as a finely divided white solid may 

 be ignored in the further stages of the washing. Soap can 

 then be used without waste. Magnesium sulphate (MgSO) 

 present in the water is disposed of at the same time, magne- 

 sium carbonate (MgCO 3 ) being precipitated. Any carbonates 

 of calcium and magnesium in the water at first are precipi- 

 tated as the water is heated. Hardness, due to these carbon- 

 ates is said to be temporary, while that from the presence of 

 the sulphates of calcium and magnesium is said to be perma- 

 nent. In laundries where thousands of gallons of water are 

 used daily extensive plants for softening the water are 

 installed at large outlays. 



SUMMARY 



In an emulsion of oil or fat the finely divided particles are held in 

 a suspended rather than a dissolved state. The milk-like appearance 

 of the liquid is due to the diffusion of light from the suspended particles 

 just as fog, and the steam escaping from locomotives, commonly 

 appear white by reason of diffusion of light from the minute water 

 particles composing them. 



The cleansing power of water in laundry uses necessitates the freeing 

 of all adhering dirt and greasy matter from the fabric. This is accom- 

 plished by both mechanical and chemical means. In both cases there 

 must be the least possible injury to the fabric itself. 



The carbonates and the sulphates of calcium and magnesium are 

 soluble to some small extent in water, and constitute the "lime" in 

 water. The carbonates largely separate out when the water is heated. 

 They are said to make the water temporarily hard. The sulphates 

 must be gotten rid of by chemical means. 



