Blue fire is a rare phenomenon that occurs when certain chemicals burn and produce flames that are blue in color. The blue color of the flames is due to the presence of ionized particles in the air, which emit light in the blue part of the spectrum.

One of the most well-known examples of blue fire is the blue flames that can be seen at the Kawah Ijen volcano in East Java, Indonesia. The blue flames at Kawah Ijen are caused by the combustion of sulfuric gases that emerge from the volcano's vents. The gases ignite when they come into contact with oxygen, producing intense blue flames that can reach up to 5 meters in height.

Blue flames can also be seen in other natural phenomena, such as lightning, and are sometimes produced intentionally in science experiments or pyrotechnic displays. The color of the flames can be used to identify the chemical elements that are being burned, with some elements producing distinct blue flames, such as copper or lead.