Heat It Up: Roast or Calcine?
Convert sulfides and carbonates into metallic oxides using heat.
What this lesson covers
Why it matters
Zinc blende gives off sulphur dioxide with a suffocating smell when roasted in air, while limestone quietly releases gas that milks limewater. Both yield oxides, but the paths differ.
The idea in plain words
Balance check: confirm the redox swap. Note the yellow-hot/white-cold ZnO and the suffocating SO₂.
Predict first
Which process uses excess air to convert a sulfide ore into an oxide?
Roasting treats sulfide ores in excess air. Calcination treats carbonates in limited air.
- Roasting — correct
- Calcination
- Smelting
What you do
Toggle between ZnS roasting and CaCO₃ calcination. Compare the gases and conditions.
Check yourself
Why is roasting done in excess air?
Excess air provides the O₂ needed for the redox reaction: 2ZnS + 3O₂ → 2ZnO + 2SO₂.
What confirms the gas from calcining limestone is CO₂?
CaCO₃ → CaO + CO₂. The CO₂ turns limewater milky.
ZnO appears yellow when hot and white when cold. What does this indicate?
ZnO exhibits thermochromism: yellow when hot, white when cold.
- To ensure complete oxidation of sulfide to oxide — correct
- To prevent the ore from melting
- To remove moisture only
- It turns limewater milky — correct
- It has a suffocating smell
- It relights a glowing splint
- It is a reversible thermal color change — correct
- It has decomposed into zinc metal
- It is contaminated with sulfur