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Your Class 11 equilibrium chapter starts with reactions that can run both ways. The first chemist to argue for that clearly was Claude Louis Berthollet, who reached it from two unlikely places: bleaching cloth and a salt lake in Egypt. This post covers his life, the natron lake puzzle, and a worked example of how an excess of one reactant pushes a reaction forward.
Who was Claude Louis Berthollet?
Berthollet was born at Talloires in Savoy on 9 December 1748 and died at Arcueil, near Paris, on 6 November 1822, according to a 2020 review of his life in the journal *Substantia*. He trained in medicine at Turin, graduating in 1770, then moved to Paris and became a French citizen. He was elected to the Academy of Sciences on 21 April 1780 and became a Fellow of the Royal Society of London on 30 April 1789.
In 1785, the review notes, he gave up the phlogiston theory and supported Lavoisier's account of combustion, the subject of our post on Lavoisier and conservation of mass. That year he also worked out the composition of ammonia.
How did Berthollet turn chlorine into a bleach?
The same review says that in 1788 he first prepared pure potassium chlorate, known as Berthollet's salt, by passing chlorine into hot potassium hydroxide:
6KOH + 3Cl₂ → 5KCl + KClO₃ + 3H₂O
Check the balance: 6 K, 6 Cl, 6 O and 6 H on each side. The review adds that in 1788 he also showed that chlorine and potassium hypochlorite were useful bleaching agents. Chlorine is a Group 17 element, part of the p-block listed in Unit 10 of the JEE (Main) 2026 syllabus; check the syllabus for your exam year.
What did Berthollet see at the Natron Lakes?
Berthollet went with Napoleon's expedition to Egypt. On 23 January 1799 he and the mathematician Joseph Fourier joined a five-day trip to the Natron Lakes, about 72 km north-west of Cairo, where a crust of natron (a form of sodium carbonate) lined the shores.
In the laboratory, sodium carbonate and calcium chloride react to give calcium carbonate and salt. At the lakes, a 2020 study by Hamed Ead explains, Berthollet saw the reverse: salty water soaking through a huge bed of limestone produced sodium carbonate, while the calcium chloride drained away into the ground.
CaCO₃ + 2NaCl ⇌ Na₂CO₃ + CaCl₂
He concluded that a very large mass of reactants, plus steady removal of a product, could drive a reaction that affinity alone would not. A 1999 paper by Weller in the *Bulletin for the History of Chemistry* credits him with introducing two ideas: reaction reversibility and chemical equilibrium, and the role of relative masses in setting where equilibrium lies. He set this out in *Essai de statique chimique* (1803).
Worked example: excess reactant and yield
This is a hypothetical reaction A + B ⇌ C + D with Kc = 4, in a 1 L flask.
Equal start, 1 mol each of A and B. If x mol react, Kc = x² ÷ (1 − x)² = 4, so x ÷ (1 − x) = 2 and x = 2/3. About 67% of B is converted.
Triple the A, 3 mol A and 1 mol B. Now x² ÷ [(3 − x)(1 − x)] = 4, which gives 3x² − 16x + 12 = 0.
Solve. x = [16 − √(256 − 144)] ÷ 6 = (16 − 10.58) ÷ 6 = 0.903. (The other root, 4.43, is impossible because only 1 mol of B exists.)
Result. About 90% of B is now converted. Kc did not change; the extra A shifted the position.
The NCERT Class 11 Chemistry chapter "Equilibrium" explains that Guldberg and Waage stated this law of mass action in 1864, about sixty years after Berthollet's book. Our guide to equilibrium constant calculations has more practice.
Where was Berthollet wrong?
He pushed the idea too far. He argued that compounds could have variable composition depending on the amounts mixed. Joseph Proust disagreed, and by about 1810, the *Substantia* review says, most chemists sided with Proust. NCERT's Class 11 chapter "Some Basic Concepts of Chemistry" teaches Proust's law of definite proportions, covered in our post on Joseph Proust.
Common mistakes with equilibrium
Equilibrium means equal amounts. It means equal forward and backward rates; amounts can be very different.
Adding reactant changes K. It changes the position, not Kc, at fixed temperature.
Taking the wrong root. Reject any x larger than the starting amount of a reactant.
If you can set up Kc but get lost solving it, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student take from Berthollet?
Berthollet noticed when nature disagreed with the textbook and asked what was different about the conditions. That question is the heart of every Le Chatelier problem you will solve.
Practice routine for this week:
Repeat the worked example with 2 mol of A and 1 mol of B.
Balance the reaction of chlorine with cold dilute sodium hydroxide and name the products.
In two lines, explain why removing calcium chloride kept the lake reaction going.
Start with step 1, then compare your answer with the 67% and 90% above.
