Non-Volatile Acid Swap
Prepare volatile acids from salts
What this lesson covers
Why it matters
Concentrated sulphuric acid is non-volatile with a very high boiling point, so on heating it displaces more volatile acids from their salts.
The idea in plain words
Map the atoms: Na and SO₄ stay put while Cl swaps with H. Verify the equation matches the observation of white fumes.
Predict first
Why does concentrated H₂SO₄ displace HCl from NaCl when heated?
It is a physical property, not just chemical strength: the non-volatile acid (high BP) displaces the volatile acid (low BP) which escapes as gas.
- H₂SO₄ is a stronger acid than HCl
- H₂SO₄ has a much higher boiling point than HCl — correct
- HCl is insoluble in sulphuric acid
- Heat breaks the H-Cl bond directly
What you do
Perform the swap with NaCl first. Observe the gas evolution. Then try with NaNO₃. Note the difference in visible gas tests.
Check yourself
In the reaction NaCl + H₂SO₄ → NaHSO₄ + HCl, what confirms the presence of HCl gas?
HCl gas reacts with ammonia vapor to form solid ammonium chloride, visible as dense white fumes.
Why is concentrated H₂SO₄ used to prepare HNO₃ from NaNO₃?
The high boiling point of H₂SO₄ allows it to remain in the flask while the lower boiling HNO₃ vaporizes and distills off.
Under moderate heating, what is the salt formed in both reactions (NaCl and NaNO₃ with H₂SO₄)?
At moderate temperatures, the double decomposition yields the acid salt sodium hydrogen sulphate (NaHSO₄).
- It turns litmus blue
- It produces white fumes with ammonia — correct
- It burns with a pop sound
- It turns starch iodide paper blue
- Because H₂SO₄ is a dehydrating agent only
- Because H₂SO₄ is non-volatile and HNO₃ is volatile — correct
- Because HNO₃ is a stronger acid than H₂SO₄
- Because the reaction requires no heat
- Na₂SO₄
- NaHSO₄ — correct
- NaOH
- H₂SO₃