Balls of fire!

Students should know that sulfuric acid can act in three main ways: as an acid, an oxidising agent and a dehydrating agent.

Demonstrations of the dehydrating properties of sulfuric acid are commonly given to audiences of 16–18 year olds (and to younger audiences for their visual appeal). Previously we looked at the most popular of these¹ in which sulfuric acid dehydrates sucrose, causing a black carbon ‘snake’ to rise from the reaction vessel. The production of sulfur dioxide fumes indicates that oxidation processes also occur.

A less common demonstration involves the dehydration of permanganic acid (HMnO₄). This is produced in the acid-base reaction of potassium manganate(VII) (KMnO₄) with sulfuric acid. The resulting manganese(VII) oxide (Mn₂O₇) reacts violently with organic compounds and this can be shown through its action on cotton wool.

With an accompanying video, Declan Fleming takes you through the preparation and teaching of a demonstration on the dehydration of permanganic acid, and how to teach students about the action of the acid involved.