Sulfuric acid

Sulfuric acid is made in greater quantity than any other chemical, and almost none of it is sold to the public. It is an intermediate: something consumed inside industrial processes to make other things, which is why its production statistics track industrial activity so closely that economists have used them as an index.

Its dangerous reputation is deserved but often misattributed. Concentrated sulfuric acid harms mainly by dehydration rather than by acidity — it strips water out of organic matter, which is why it chars sugar and paper and why the burns it causes are severe. Diluting it releases a great deal of heat, which is why acid is added to water and never the reverse.

Made by the contact process. Sulfur — increasingly recovered from oil and gas processing rather than mined — is burned to sulfur dioxide, which is then oxidised to sulfur trioxide over a vanadium pentoxide catalyst.

The last step is counter-intuitive: sulfur trioxide is not added to water, because the reaction is so violent it produces an unmanageable mist. It is absorbed into existing concentrated sulfuric acid to make oleum, and the oleum is then diluted. A plant is built around handling that exotherm.

Fertiliser dominates. Most sulfuric acid is used to treat phosphate rock to make phosphoric acid and superphosphate, which is to say most of it ends up growing food.

Beyond that: leaching copper, zinc and uranium from low-grade ores; pickling steel to remove scale before rolling or coating; refining petroleum; making detergents, dyes and explosives; and serving as the electrolyte in lead-acid batteries, which is the one form most people ever handle.