Video Transcript
When sulfite ions are treated with hydrochloric acid and gently heated, a gas is
produced. What is the identity of this gas? (A) Sulfur trioxide, SO3; (B) chlorine, Cl2; (C) hydrogen sulfide, H2S; (D) sulfur
dioxide, SO2; or (E) oxygen, O2.
Hydrochloric acid is used to qualitatively test for the presence of sulfite ions in a
substance. When a sample containing sulfite anions is gently heated and dilute hydrochloric acid
is added, an observable reaction occurs. The reaction produces a smelly, colorless gas, but we cannot simply smell the gas to
identify it, as this would be a very unsafe practice.
We can use a filter paper that is soaked with an oxidizing agent, such as potassium
dichromate, that will change color when reacting. If potassium dichromate changes from yellow to green, a redox reaction has
occurred. The gas is oxidized by the oxidizing agent on the filter paper. This gas is produced by the reaction between sulfite ions and hydrogen ions from the
acid. This smelly, acidic gas is sulfur dioxide.
It is worth mentioning that just like with other qualitative tests, to definitively
determine the identities of substances, confirmatory tests are needed.
So what is the identity of gas produced when sulfite ions are treated with
hydrochloric acid and gently heated? The correct answer is answer choice (D) sulfur dioxide, SO2.
