What is asphalt? And is it somehow related to gasoline?

Asphalt is a sticky black semi-solid that is found in most crude petroleums (the stuff that people pump out of the ground and sell for lots of money). It is removed by fractional distillation. This process involves heating petroleum so that various "fractions" with different boiling points can be separated.

Asphalt, also sometimes called refined bitumen, is the hardest fraction to extract; you need to heat it to 977 degrees Fahrenheit.

Gasoline is made from lighter fractions of petroleum. Asphalt and gasoline come from the same origin, but you needn't worry about asphalt being a sort of wasted gasoline. If you did try to put asphalt in your gas tank it would be a disaster for your car!

Naturally occurring asphalt has been used since ancient times in the Middle and Far East as mortar, caulking and as a form of waterproofing. The asphalt you know from paving roads is actually more succinctly called asphalt concrete and is a mixture of asphalt and ground rock, or aggregate, and a variety of other substances depending on the application.

Dr. Knowledge is written by physicists Stephen Recruit and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819