Sodium Hydroxide
Every bar of soap, every sheet of paper, and every disinfected glass of drinking water owes something to one industrial reaction happening inside tall electrolysis cells: the chlor-alkali process. Pass an electric current through concentrated brine — plain salt water — and it splits apart into three separately useful products at once, none of them wasted. This is why sodium hydroxide (NaOH), the "caustic soda" of your Class 10 Acids, Bases and Salts chapter, is never taught alone. It arrives bundled with chlorine gas and hydrogen gas, made from the same feedstock in the same cell in one balanced equation. This article stays inside that syllabus scope — the reaction, its products, their uses, and NaOH's basic chemistry — and shows you how to reason through it rather than memorise it.
Electrolysing concentrated brine (NaCl solution) in the chlor-alkali process yields three co-products in fixed atomic proportion — sodium hydroxide, chlorine gas at the anode, and hydrogen gas at the cathode — each with distinct industrial uses.
The core idea
What the process does. The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride (brine). An electric current is passed through the brine, and the ions present are driven apart and discharged at the two electrodes, leaving sodium hydroxide behind in solution.
where: NaCl(aq) is the dissolved brine feedstock, is the water it is dissolved in, NaOH(aq) is the sodium hydroxide (alkali) formed near the cathode, is chlorine gas evolved at the anode, and is hydrogen gas evolved at the cathode. The up-arrows mark the two gases escaping the solution.
Why the name fits. "Chlor-alkali" is a portmanteau of its two headline products: chlorine and the alkali sodium hydroxide. Because all three products — NaOH, , — form together from a single brine feed in one cell, a plant cannot dial up just one of them; the fixed atomic ratio in the balanced equation locks their proportions together, and every co-product is sold rather than discarded.
NaOH's own chemistry. As a strong alkali, sodium hydroxide is highly soluble in water, feels soapy and slippery, turns red litmus blue, and is corrosive to skin and eyes. It shows the standard reactions of a base: it neutralises acids,
and it reacts with acidic (non-metal) oxides such as carbon dioxide, absorbing from the air if left exposed — one reason NaOH pellets are stored in airtight containers.
Where each product goes. Sodium hydroxide is used in making soap and detergents, in paper manufacturing, and for degreasing metals before painting or plating; chlorine goes into bleaching powder, disinfecting drinking water, and manufacturing PVC; hydrogen is burned as a fuel, used to manufacture ammonia, and used to hydrogenate vegetable oils into solid fats (vanaspati).
Why this reaction, and not another, is the syllabus example. Among the many electrolysis reactions you could study, the chlor-alkali process is chosen because it packs three separate learning points — an electrolysis equation, a base's properties, and everyday industrial uses — into a single balanced equation you can fully write out by hand. It is also a genuinely efficient real-world process: unlike reactions where a by-product must be safely disposed of, every one of the three chlor-alkali outputs is captured, purified, and sold, which is why chemical engineers hold it up as a model of atom-efficient industrial chemistry.
The goal is to take the unbalanced skeleton equation and turn it into the correctly balanced chlor-alkali equation using nothing but atom counting and the law of conservation of mass, so that the number of each type of atom is identical on both sides.
Step 1. Count atoms in the skeleton equation as written. Left side: 1 Na, 1 Cl, 2 H (from ), 1 O. Right side: 1 Na and 1 O (in NaOH), 2 Cl (in , since it is diatomic), and 2 H (in , also diatomic). Chlorine is short on the left (1 vs 2 needed) and hydrogen is short on the left too once you account for both product gases.
Step 2. Fix chlorine and sodium together. Since NaCl supplies one Na and one Cl per unit, and the products need 2 Cl (for one molecule), double the NaCl to 2 NaCl. This automatically gives 2 Na on the left, so the product NaOH must also be doubled to 2 NaOH to keep sodium balanced.
Step 3. Fix hydrogen and oxygen using water. With 2 NaOH on the right, the right side now needs 2 O and, from the OH groups plus one full molecule, a total of 4 H. Two water molecules supply exactly 4 H and 2 O, so set 's coefficient to 2 as well.
Step 4. Verify the full balanced equation and add state symbols and the electrolysis condition. 2\text{NaCl}(aq) + 2\text{H}_2\text{O}(l) \xrightarrow{\text{electrolysis}} 2\text{NaOH}(aq) + \text{Cl}_2(g)\uparrow + \text{H}_2(g)\uparrow Recount: left has 2 Na, 2 Cl, 4 H, 2 O; right has 2 Na (in NaOH), 2 O (in NaOH), 2 H (in NaOH) + 2 Cl (in ) + 2 H (in ) = 4 H total. Both sides now carry 2 Na, 2 Cl, 4 H, 2 O — balanced, consistent with the law of conservation of mass since electrolysis only rearranges atoms into new bonds, never creates or destroys them.
Worked example
Question: A plant electrolyses saturated brine and collects the gas at the cathode separately from the gas at the anode. Identify each gas, give one confirmatory test for the cathode gas, and name the aqueous product left behind once both gases have escaped.
Step 1. Write the balanced chlor-alkali equation to see where each product forms: 2\text{NaCl}(aq) + 2\text{H}_2\text{O}(l) \xrightarrow{\text{electrolysis}} 2\text{NaOH}(aq) + \text{Cl}_2(g)\uparrow + \text{H}_2(g)\uparrow
Step 2. Assign each gas to its electrode. Chlorine gas (), a greenish-yellow gas with a pungent smell, is evolved at the anode. Hydrogen gas () is evolved at the cathode.
Step 3. State the confirmatory test for the cathode gas. Bring a burning splint near the collected gas; hydrogen burns with a characteristic "pop" sound, confirming it is .
Step 4. Name the aqueous product left behind. Once both gases have escaped, the solution remaining is sodium hydroxide solution, NaOH(aq) — the "alkali" in "chlor-alkali."
Common traps
- Forgetting to balance the chlor-alkali equation (leaving 1 NaCl instead of 2) — easy to miss with three different products in one equation.
- Treating NaOH as the only useful product — chlorine and hydrogen from the same process are equally examinable, each with its own named uses.
- Swapping the electrodes — chlorine forms at the anode and hydrogen at the cathode, with NaOH accumulating near the cathode, not the other way round.
- Describing detailed industrial cell/membrane engineering — voltages and electrode materials are beyond Class 10; only the named reaction and product uses are required.
Ask the tutor: want a quick walkthrough of balancing the chlor-alkali equation step by step, or a rapid-fire quiz on which product goes with which electrode and use?
Why does the chlor-alkali process make three products instead of just sodium hydroxide?
Because all three come from the same brine feed through one balanced electrolysis reaction, they are locked together in fixed atomic proportion: 2 NaCl and 2 water molecules always yield 2 NaOH, one , and one . You cannot switch off chlorine or hydrogen production while keeping NaOH — the stoichiometry ties them together. This is actually the process's strength: nothing goes to waste, since chlorine and hydrogen each have valuable industrial uses of their own.
Is sodium hydroxide dangerous to handle, and why does the chapter mention it feels soapy?
Yes — NaOH is corrosive to skin and eyes, so it must be handled with gloves and care, never touched directly with bare hands in a lab. The "soapy, slippery" feel you may have read about is actually a sign of a mild chemical reaction: NaOH reacts with the oils and fatty acids on skin, and this same reaction, done deliberately with fats and oils in a controlled way, is exactly how soap is manufactured. The feeling is a warning, not a comfort.
Why is it called caustic soda if the substance itself is a base and not an acid?
"Caustic" describes a chemical's ability to burn or corrode living tissue, not whether it is acidic or basic — both strong acids and strong bases can be caustic. Sodium hydroxide (chemically a base/alkali since it releases hydroxide ions in water) earns the everyday name "caustic soda" because concentrated NaOH is severely corrosive to skin, similar to how a strong acid would burn. So "caustic" is about corrosive strength; "soda" simply refers to its sodium content, the same root as washing soda and baking soda.