Propane is a byproduct of natural gas processing and petroleum refining. 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. Additionally, the characteristics of propane liquid and propane gas are so different that the primary properties we are concerned with are as different as night and day. With propane liquid, temperature is the primary factor whereas weight is the main concern regarding propane vapor. Think of it this way, water is liquid and steam is water vapor. The same holds true for propane and is explained in detail below.
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). A propane and butane mixture has been successfully used for over 95 years in more than 270,000 American vehicles and over 13 million worldwide used mainly as a vehicle fuel or to easily identify a leak (due to the smell).
Propane Gas (or Vapor)
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. Propane gas is heavier than air and will settle in the lowest place possible. If there is a leak, the level will continue to rise and may ignite if it finds a source. It is mainly used for engines, barbeques, portable stoves and home heating.
See the video for the full explanation: http://www.amerigreen.com/difference-liquid-propane-propane-gas/.
Source – Propane 101