The Fading Blue Cell

Why it matters

Run a current through blue copper sulphate solution between platinum rods. The cathode slowly turns reddish-brown, gas streams off the anode — and the famous blue quietly drains away.

The idea in plain words

Balance check: 2CuSO₄ + 2H₂O → 2Cu + 2H₂SO₄ + O₂. Count the coppers, the sulphate groups, and the oxygens that escape as gas.

Predict first

Which observation tells you the Cu²⁺ ions are being used up?

The blue belongs to the Cu²⁺ ions. As they are discharged onto the cathode as copper metal, the colour drains away with them — the anode bubbles are oxygen from the water, not from the copper.

• The blue colour of the solution fades — correct
• The solution warms up
• Bubbles appear at the anode

What you do

Switch on the current. Watch copper coat the cathode, oxygen bubble off the inert anode, and the blue weaken as the cell runs.

Check yourself

Which gas forms at the inert anode, and how do you confirm it?

At the inert anode, water is oxidised and O₂ is released: the classic glowing-splinter test confirms it. The sulphate ions stay in solution.

Why does the solution grow more acidic as the electrolysis proceeds?

The net equation 2CuSO₄ + 2H₂O → 2Cu + 2H₂SO₄ + O₂ shows H₂SO₄ as a product: each copper that deposits leaves an acid molecule behind.

Swap the inert anode for a copper anode and the blue colour no longer fades. Why?

With an active copper anode, the anode itself oxidises (Cu → Cu²⁺), topping up the solution exactly as fast as the cathode drains it. That balance is the principle behind electroplating and electro-refining.

• Oxygen — it relights a glowing splinter — correct
• Hydrogen — it pops with a burning splint
• Sulphur dioxide — suffocating smell
• Sulphuric acid accumulates in the solution — correct
• Carbon dioxide dissolves in from the air
• Copper metal is itself acidic
• The copper anode dissolves, replacing every Cu²⁺ the cathode removes — correct
• The current stops flowing through the cell
• The cathode stops accepting copper