The Dilute Sulphuric Detective
One acid, four suspects, four tell-tale gases
What this lesson covers
Why it matters
Four unlabelled jars: silvery granules, a dark grey lump, and two near-identical white powders. One reagent — warm dilute sulphuric acid — makes every suspect confess with a different gas. Your job is to read the confessions.
The idea in plain words
Balance the carbonate's confession: Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂. Track where every atom of carbon and oxygen ends up.
Predict first
Warm dilute sulphuric acid is poured onto a dark grey solid. The gas that bubbles out smells of rotten eggs. The solid must be:
Rotten eggs means hydrogen sulphide: FeS + H₂SO₄ → FeSO₄ + H₂S. Each family — metal, carbonate, sulphite, sulphide — hands dilute acid a different gas. That is the whole trick of this practical.
- A sulphide, like FeS — correct
- A sulphite, like Na₂SO₃
- A carbonate, like Na₂CO₃
- A reactive metal, like zinc
What you do
Run warm dilute H₂SO₄ over each suspect, then interrogate every gas: burning splint (pop = H₂), limewater (milky = CO₂), acidified K₂Cr₂O₇ paper (orange to green = SO₂), lead acetate paper (black = H₂S).
Check yourself
Dilute H₂SO₄ on sodium carbonate gives brisk effervescence. Which pair of results confirms the gas is CO₂?
Both CO₂ and SO₂ turn limewater milky, so you need the second check: CO₂ leaves acidified dichromate paper orange, while SO₂ turns it green.
Sodium sulphite and sodium carbonate are look-alike white powders. With dilute H₂SO₄, the one sure way to tell them apart is:
Na₂SO₃ + H₂SO₄ → Na₂SO₄ + H₂O + SO₂, and SO₂ reduces orange K₂Cr₂O₇ to green Cr₂(SO₄)₃. CO₂ from the carbonate leaves the paper unchanged.
One suspect dissolves with effervescence and its gas burns with a sharp pop. Which suspect released which gas?
Zn + H₂SO₄ → ZnSO₄ + H₂. A reactive metal above hydrogen in the activity series displaces odourless hydrogen, confirmed by the pop with a burning splint.
- Limewater turns milky AND acidified K₂Cr₂O₇ paper stays orange — correct
- Limewater turns milky AND acidified K₂Cr₂O₇ paper turns green
- The gas burns with a pop near a burning splint
- Pass each gas through limewater and see which turns milky
- Hold acidified potassium dichromate paper in each gas — only the sulphite's gas turns it green — correct
- Bring a burning splint near each gas and listen for the pop
- Zinc — hydrogen — correct
- Iron(II) sulphide — hydrogen sulphide
- Sodium carbonate — carbon dioxide