What it does
When electricity is passed through brine (a strong solution of common salt), three useful chemicals come out: chlorine, hydrogen and sodium hydroxide. Because it makes chlorine and an alkali, the industry is called the chlor-alkali process. This page shows it in a simple membrane cell:
- an animation of Cl⁻ ions moving to the positive electrode, Na⁺ ions crossing the membrane, OH⁻ ions forming at the negative electrode, and bubbles of chlorine and hydrogen,
- the reactions in words (Class 10) or as half equations (Class 12),
- what each product is used for, including bleaching powder,
- at the Class 12 level, a calculator: set the current, the time and the current efficiency, and get the charge, the moles of electrons, and the grams and litres of chlorine, hydrogen and sodium hydroxide made.
It follows the Class 10 chapter on acids, bases and salts (sodium hydroxide from common salt) and the Class 12 chapter on electrochemistry (electrolysis and Faraday’s laws).
The opposite of electrolysis, a cell that makes electricity, is in the galvanic cell builder tool; how alkaline the sodium hydroxide is shows on the pH scale explorer, and sodium hydroxide is what turns fats into soap in how soap cleans.
How to use it
- Watch the cell. Pause stops the animation; untick Show ion labels for a cleaner picture.
- Read the steps under the picture. Switch to Class 12 for the half equations and the calculator.
- In How much is made, set I (amperes) and t (minutes) with the sliders or boxes, the current efficiency, and whether gas volumes use 24 L/mol (room conditions) or 22.4 L/mol (0 °C).
A worked example
At Class 12 level the page starts with 10 A for 60 minutes.
- Charge: 10 A × 3 600 s = 36 000 C.
- Electrons: 36 000 ÷ 96 485 = 0.37311 mol.
- Chlorine: half that, 0.18656 mol × 71 g/mol = 13.246 g, which is 4.477 L at room temperature.
- Hydrogen: the same 0.18656 mol, only 0.373 g, but also 4.477 L.
- Sodium hydroxide: one per electron, 0.37311 mol × 40 g/mol = 14.925 g.
To make 40 g (one mole) of sodium hydroxide at 10 A takes 96 485 ÷ 10 s, about 2.68 hours.
Tips
- In electrolysis, the positive electrode is the anode and attracts negative ions (anions); the negative electrode is the cathode.
- Sodium is not formed at the cathode from brine: water is easier to reduce, so hydrogen comes off.
- Doubling the current or the time doubles every amount; that is Faraday’s first law.
Check yourself
Four fresh questions each time: one about what forms where, then moles of electrons and grams of chlorine and sodium hydroxide for a current and time. Nothing is saved.
FAQ
Why is there a membrane? It keeps the chlorine away from the sodium hydroxide. If they mix, they react (to make bleach), and the hydrogen and chlorine must also be kept apart.
Can I try this at home? No. Chlorine is a toxic gas. This is an industrial process; school demonstrations are done by a teacher with care.
Is this from the NCERT book? It covers the same topic with fibub’s own drawings and numbers. fibub is not part of NCERT.
Where the numbers come from
- The chlor-alkali reactions (2Cl⁻ → Cl₂ + 2e⁻; 2H₂O + 2e⁻ → H₂ + 2OH⁻) and the uses of the products are standard school chemistry. The Faraday constant 96 485.332 12 C/mol is from CODATA (exact since the 2019 SI). Molar masses are the rounded school values. Written for fibub.
- Checked against Python: 10 A for 1 hour moves 0.37311 mol of electrons, making 13.2455 g of chlorine, 14.9245 g of sodium hydroxide and 4.477 L of each gas at 24 L/mol.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
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Last updated 2026-11-09.