Sulfur dioxide is an acidic gas. When sulfur dioxide gas dissolves in water, it forms sulfurous acid (H2SO3) which is a dibasic weak acid. Sulfur dioxide has the capability to form acid rain. In areas that have high sulfur dioxide emissions, there is a higher possibility of occurrence of acid rains.
Chemical balanced equation is given here. Sulfurous acid is soluble in water and partially dissociate to give H+ ions to water.
Sulfurous acid partially dissociates in two steps to release H3O+ ions. Due to formation of 3O+ ions, aqueous solution become acidic.
H2SO3(aq) + H2O(l) ⇌ HSO3-(aq) + H3O+(aq)
In the first step, bisulfite ion (HSO3-) and H3O+ ions are formed.
In the second step, sulfite ion (SO32-) and H3O+ ions are formed.
We know when an element forms several oxides and the oxidation number of that element is high in the oxide, it increases the acidic properties. In sulfur dioxide, sulfur exists at +4 oxidation and maximum oxidation number of sulfur is +6. Because the oxidation number of sulfur in sulfur dioxide is close to +6, sulfur dioxide should have acidic properties.
As well, Sulfur dioxide will give sulfurous acid. However, sulfurous acid can be oxidized to sulfuric acid by oxygen gas or oxidizing agent. Sulfuric acid is also a strong acid that can harm the environment as nitric acid.
According to the theories, sulfur trioxide (SO3) should be more acidic than sulfur dioxide becaause sulfur trioxide reacts violently with water and form sulfuric acid which is a stronger acid than sulfurus acid. Therefore, SO3 is more acidic than SO2.
Wet blue litmus will turn into red when SO2 gas is contacted.
In acidic conditions, colourless solution will be given by phenolphthalein. Therefore, when SO gas is sent to a aqueous solution with phenolphthalein, the colourless state is not changed.
Questions asked by Students
Because Sulfur dioxide gas give a weak acidic aqueous solution, Sulfur dioxide is considered as a weak acidic gas.
Send some amount of SO2 gas to the aqueous sodium hydroxide solution and check pH value of the solution. You may see pH value is decreased to value below 10.5
It means, OH- concentration has been reduced in the solution. To happen that, H3O+ ion should be supplied and SO2 should have the ability to do that.
Therefore, SO2 can give H3O+ ions in the water and can prove that SO2 is an acidic gas.