Question

In general, rain water is more acidic than surface water due to the properties of sulfur oxides in the air which differ from those of CO₂. State ONE property of SO₂ which cause increased acidity of rain water relative to CO₂-containing surface water.  

Answer 1

Normal rain has a pH of about 5.6; it is slightly acidic because carbon dioxide (CO2) dissolves into it forming weak carbonic acid (H2CO3)

Rain water contains oxides of Sulphur (SO2) and Nitrogen (NOx) apart from CO2. These oxides come into environment by several processes like burning of fuel , volcano eruption etc.

SO2 is oxidized to form SO3 in the atmosphere, which dissolves to make sulfuric acid (H2SO4) which is very strong acid and it makes the rain water more acidic. This property of SO2 to get oxidized easily in the atmosphere and than reacting with water to form a strong acid, makes rain water more acidic. Note that SO2 also dissolves in water to form H2SO3. This acid is somewhat stronger than H2CO3 , but weaker than H2SO4. So, main contribution of sulphur oxides in increasing acidity is due to effect of H2SO4 only.

Note that Nitric acid is also present in rain water which is also a strong acid, and contributes to increased acidity of rain water.