A 50% (v/v) alcohol solution is composed of 50% alcohol and 50% water in which water acts as a solvent. By igniting a paper banknote completely soaked with 50% alcohol solution, the alcohol (which is the fuel in the fire triangle) is combusted into carbon dioxide and water vapour. On the other hand, water is heated up with some being evaporated as it absorbs energy from the combustion of alcohol. The evaporation of water helps cool down the system, so not all water is evaporated and the paper banknote is not burnt. The water to alcohol ratio should be 50% or higher; a lower ratio leads to the banknote being slightly burnt because there is not enough water to absorb the combustion energy and cool down the system.
CHOH + (3n/2) O → n CO + (n+1) HO
Common alcohol fuels for this experiment can be methanol (n=1), ethanol (n=2) and both isomers of propanol (n=3). The fire lit in this scenario is categorized as a class B fire (fire from flammable liquids), while the fire from burning paper (banknote) is categorized as class A. The alcohol-water mixture flame can be hard to detect, so sodium chloride can be added to give the flames an orange-yellow color. For safety purpose, a water tray should be prepared for emergency use in case a paper banknote caught a fire, and flammable and combustible materials should not be kept or put near the flame.