Lab Prep: Salt & Acid
Generate HCl gas from NaCl and conc. H2SO4
What this lesson covers
Why it matters
Mix solid salt with thick sulfuric acid and gently warm the flask. A sharp, choking gas rises, ready to turn ammonia vapour into white smoke.
The idea in plain words
Map the swap: H from acid pairs with Cl from salt. Na pairs with HSO₄.
Predict first
What gas escapes the flask when NaCl meets hot H₂SO₄?
Sulfuric acid is less volatile than HCl. Heating drives the more volatile HCl gas out of the mixture.
- Hydrogen gas (H₂)
- Hydrogen chloride (HCl) — correct
- Sulfur dioxide (SO₂)
- Oxygen gas (O₂)
What you do
Add concentrated H₂SO₄ to NaCl. Apply gentle heat. Observe the fumes and test with ammonia.
Check yourself
Which equation correctly represents this reaction below 200°C?
Below 200°C, only one hydrogen is replaced, forming sodium hydrogen sulfate (NaHSO₄).
How do you confirm the gas is HCl?
HCl reacts with NH₃ to form solid ammonium chloride (NH₄Cl), seen as white fumes.
Why is concentrated H₂SO₄ used instead of dilute?
The reaction relies on the non-volatile nature of H₂SO₄ to drive out the volatile HCl gas.
- NaCl + H₂SO₄ → Na₂SO₄ + HCl
- NaCl + H₂SO₄ → NaHSO₄ + HCl — correct
- 2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl
- It turns lime water milky
- It produces dense white fumes with ammonia vapour — correct
- It relights a glowing splint
- Dilute acid reacts too slowly
- Concentrated acid is non-volatile and displaces the volatile HCl — correct
- Dilute acid produces SO₂ gas