Chlor-alkali process

The name comes from two of its products: chlor for chlorine, and alkali for sodium hydroxide. All three products are useful, so none is wasted.

• Chlorine gas: given off at the anode, the positive electrode. It is used to make bleaching powder.
• Hydrogen gas: given off at the cathode, the negative electrode. It can be used as a fuel.
• Sodium hydroxide: forms in the solution near the cathode. It is used to make soap.

A small example

The equation is 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g). Check the balance. The left side has 2 Na, 2 Cl, 4 H and 2 O. On the right, 2NaOH has 2 Na, 2 O and 2 H. Cl₂ adds 2 Cl, and H₂ adds 2 more H. That makes 4 H in all, so it balances.

Where marks go

Question 18 of CBSE's 2026-27 Science sample paper gives a table of four rows: product, where it forms, and its use. The 2026-27 Science marking scheme marks only rows I and III as right. Chlorine does form at the anode, but it is not used to make margarine. Sodium hydroxide forms near the cathode, not the anode.

So each row has two facts to check, place and use, and one wrong fact spoils the row. It is 1 mark, with no part credit. See where the marks go in CBSE Science.

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