Before propane is used, it exists in one of two forms, liquid or gas (or vapor). Both liquid propane and gas are usable but cannot be used interchangeably. In other words, a propane system designed to use gas can’t utilize propane in its liquid form and vice-versa.
Liquefied Petroleum Gas (LPG), is a fuel in liquid form at or below its boiling point (-44 degrees F) and when it is stored under pressure, otherwise it turns to gas (or vapor). It is used mainly as a vehicle fuel.
Propane becomes a vapor at temperatures above -44 degrees F. Similar to water when it boils and gives off steam, propane gives off vapor when it boils. It is mainly used for engines, barbeques, portable stoves and home heating.
See full article at: http://www.amerigreen.com/what-is-the-difference-between-lpand-propane-gas/.
Source – Propane 101