How do you extinguish burning magnesium? In this video from the Royal Institution, science presenter Steve Mould demonstrates what happens when you try to extinguish a metal fire with a CO2 fire extinguisher or water—both bad ideas that don’t work and could be very dangerous. Why?
The working principle of a carbon dioxide fire extinguisher is that it starves a fire of oxygen. However, carbon dioxide itself has two oxygen atoms for every one carbon atom, and in the case of metal fires, the burning metal strips CO2 of its oxygen atoms, thereby adding more oxygen to the mix. It’s even worse with water fire extinguishers, as when the oxygen atoms are stripped, only hydrogen remains, which is in itself explosive.
The correct way to put out a metal fire is with a powder fire extinguisher that covers the fire entirely with non-reactive powder.
Bonus: See what it looks like when magnesium burns inside a block of solid carbon dioxide, also known as dry ice.