In industrial water treatment terminology, water quality with high RSC index is synonymous with the soft water but is chemically very different from naturally soft water which has a very low ionic concentration. When calcium and magnesium salts are present in dissolved form in water, these salts precipitate on the heat transfer surfaces forming insulating hard scaling /  coating which reduces the heat transfer efficiency of the heat exchangers. To avoid scaling in water cooled heat exchangers, water is treated by lime and or soda ash  to remove the water hardness.
The following chemical reactions take place in lime soda softening process which precipitates the calcium and magnesium salts as calcium carbonate and magnesium hydroxide which have very low solubility in water.
* CaSO + NaCO   ---> CaCO↓     + NaSO
* CaCl + NaCO ---> CaCO↓  + 2NaCl
* MgSO  +  Ca(OH)   +  NaCO  --->  Mg(OH)↓    +  CaCO↓   +  NaSO
* MgCl  +  Ca(OH)  +  NaCO ---> Mg(OH)↓      +  CaCO↓    +  2NaCl
* 2NaHCO  +  Ca(OH) ---> CaCO↓   +  NaCO   +  2HO
* NaCO  +  Ca(OH) ---> CaCO↓   +  2NaOH
* Ca(HCO)  +  Ca(OH) ---> 2CaCO↓  +  2HO
* Mg(HCO) + 2Ca(OH) ---> Mg(OH)↓  + 2CaCO↓   + 2HO
* MgCO + Ca(OH) ---> Mg(OH)↓  + CaCO↓
The excess soda ash after precipitating the calcium and magnesium salts is in carbonates & bicarbonates of sodium which imparts high pH or alkalinity to soil water.