Answer to Question #43074 in Inorganic Chemistry for Zeph

Question

You are presented with three identical metal containers of about 5 liters capacity, each about half full of fuel. The containers have short neck and normally closed by a screw cap of about 30mm diameter. The cans contain
1. petrol (gasoline, flashpoint -43)
2. methyl alcohol (methanol, flashpoint 11 C)
3. parrafin (kerosine, flashpoint 50 C)

explain concisely what would happen if the top was taken off and a lit match was immediately dropped into the head space of each cans. Assume ambient temperature and thus that of liquids is 15 C

Answer 1

The flash point of a volatile material is the lowest temperature at which it can vaporize to form an ignitable mixture in air. At the flash point, the vapor may cease to burn when the source of ignition is removed. If flash point is less than ambient temperature, dropping of ignition source into the head space results in ignition (flash). That is why ignition occurs in the first and the second cans (with petrol and methyl alcohol, correspondingly) and does not occur in the third can (with paraffin). In both first and second cans burning stops when ignition source is removed from the can because ambient temperature is much lower than fire point.