Which substance can be used to reduce the effect of acid rain on freshwater lakes and other natural bodies without leaving behind an alkaline solution? Sodium hydroxide or Calcium carbonate? Please explain their properties as well.
Both Na(OH) and CaCO3 react with acids giving a salt and water, but Na(OH) is a very strong base soluble in water whereas CaCO3 is only a carbonate that is insoluble in water and therefore it does not leave behind alkaline traces when mixed with water.
The reactions that both substances give when mixed with the sulphuric acid from the acid rain are as follows
2*Na(OH) + H2SO4 = Na2SO4 + 2*H2O
CaCO3 + H2SO4 = CaSO4 + H2CO3 = CaSO4 + H2O + CO2
From the first reaction we can see that using Na(OH) we obtain as a reaction product Na2SO4 which in aqueous solution hydrolyzes and gives back a part of the initial H2SO4.
From the second reaction we can see that using CaCO3 we obtain as a reaction product H2CO3 which is a very weak acid and decomposes further to water (H2O) and carbon dioxide (CO2) which is a gas.
Therefore using CaCO3 to neutralize the acid rain gives much better results as regarding both initial traces of alkalinity and final traces of acids in reaction products.
NaOH is industrially obtained by NaCl electrolysis. It melts down at very low temperatures. It is a very strong base that reacts with all acids, almost all oxides and metals and is widely used to manufacture soap.
CaCO3 is a substance found in nature in some minerals like chalk, marble. At relatively high temperatures it decomposes into CaO and CO2. It reacts with strong acids and its thermal decomposition product CaO is widely used in constructions.