The Four Faces of Sulphuric Acid

Why it matters

Most acids have just one trick. Sulphuric acid has four. The very same liquid can behave as an ordinary acid, a fierce oxidiser, a water-stealing dehydrator, and a non-volatile acid that drives other acids out of their salts. That range is why it is called the king of chemicals. Let us meet all four faces.

The idea in plain words

Balance each face, then look closely at the oxidiser: Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O. Why two acid molecules? One gives up its sulphate to build the salt; the other is reduced to SO₂. That second job — being reduced — is exactly what acting as an oxidising agent means.

Predict first

Zinc reacts with dilute H₂SO₄ to give hydrogen gas. What happens when you warm copper with concentrated H₂SO₄?

Two different faces. Dilute H₂SO₄ behaves as an ordinary acid, so reactive zinc pushes out H₂. But hot concentrated H₂SO₄ behaves as an oxidiser: copper sits below hydrogen and cannot displace it, yet the acid oxidises the copper and is itself reduced to SO₂. Same acid, opposite behaviour.

• The same — hydrogen bubbles off
• No hydrogen — it gives sulphur dioxide instead — correct
• Nothing — copper never reacts with sulphuric acid
• It gives oxygen gas

What you do

Sulphuric acid plays four very different roles. Run each tab and watch what it does: as an ordinary acid it frees hydrogen from zinc; as an oxidiser, hot concentrated acid turns copper into a blue solution plus SO₂; as a dehydrator it chars white sugar to black carbon; and as a non-volatile acid it drives volatile HCl out of common salt.

Check yourself

Concentrated H₂SO₄ turns white sugar into a black spongy column of carbon. Here it is acting as a…

It strips hydrogen and oxygen out of the sugar as water (they are present in the 2:1 ratio of H₂O), leaving carbon behind. Removing water = dehydrating agent.

Zinc + dilute H₂SO₄ gives H₂, but copper + hot concentrated H₂SO₄ gives SO₂, not H₂. Why the difference?

Metals above hydrogen (zinc) displace H₂ from dilute acid. Copper is below hydrogen and cannot, but hot concentrated H₂SO₄ oxidises it and is itself reduced to SO₂.

Heating common salt (NaCl) with concentrated H₂SO₄ releases white fumes of HCl. Which property of H₂SO₄ makes this work?

A non-volatile acid (high boiling point) pushes a volatile acid (low boiling point) out of its salt — the HCl simply boils off as fumes. It is a physical property, not acid strength.

Which statement best captures what makes sulphuric acid special?

One reagent, four distinct behaviours — ordinary acid, oxidiser, dehydrator, and non-volatile acid-maker. That versatility is why it is called the king of chemicals.

• dehydrating agent — correct
• oxidising agent
• reducing agent
• an ordinary acid
• Dilute acid acts as an ordinary acid; hot concentrated acid acts as an oxidiser — correct
• Copper is heavier than zinc
• Concentrated acid is a weaker acid
• Zinc is a non-metal
• It is non-volatile (high boiling point), so it displaces the volatile HCl — correct
• It is a stronger acid than HCl
• HCl is insoluble in sulphuric acid
• Heat breaks the H–Cl bond directly
• Depending on conditions it can act as an acid, an oxidiser, a dehydrator and a non-volatile acid — correct
• It is the single strongest acid that exists
• It only reacts when it is dilute
• It contains no hydrogen