What is the characteristic of the gas liberated when dilute hydrochloric acid is added to Iron(ii)sulphide?

Step 1: Analyze the question We need the gas produced when dilute HCl reacts with iron(II) sulphide, and the test that identifies it.

Step 2: The concept Metal sulphides react with dilute acids to give hydrogen sulphide (H₂S), a colourless gas with a rotten-egg smell.

Step 3: The reaction

FeS(s) + 2HCl(aq) → FeCl₂(aq) + H₂S(g)

Step 4: Test for H₂S H₂S reacts with lead(II) ethanoate (lead acetate) paper to form black lead(II) sulphide:

Pb(CH₃COO)₂ + H₂S → PbS (black) + 2CH₃COOH

So the paper turns black.

Step 5: Check the other options

• A. Turns lime water milky: this is CO₂, which comes from carbonates and not from sulphides.
• B. Decolorizes bromine water: this is typical of unsaturated hydrocarbons (alkenes) or SO₂, not the gas from this reaction.
• C. Gives a pop sound: this is H₂, which comes from active metals and acids, not from FeS.
• D. Turns lead(II) ethanoate paper black: this is H₂S, so it is correct.

(H₂S can also decolorize acidified KMnO₄, but that is not what option B describes.)

Answer: D

Exam tip: Remember the gas tests as pairs: carbonate → CO₂ → lime water milky; sulphide → H₂S → lead ethanoate paper black; active metal → H₂ → pop sound. The "S" in sulphide goes with the black lead sulphide.