molecular weight hydrocarbons. Those that are liquid at ambient conditions (i.e., pentane and heavier) are called natural-gas condensate (sometimes also called natural gasoline or simply condensate). Natural gas is called sweet gas when relatively free of hydrogen sulfide; gas that does contain hydrogen sulfide is called sour gas. Natural gas, or any other gas mixture, containing significant quantities of hydrogen sulfide, carbon dioxide or similar acidic gases, is called acid gas. Raw natural gas can also come from methane deposits in the pores of coal seams, often existing underground in a more concentrated state of adsorption onto the surface of the coal itself. Such gas is referred to as coalbed gas or coalbed methane (coal seam gas in Australia). Coalbed gas has become an important source of energy in recent decades. Contaminants in raw natural gas Raw natural gas typically consists primarily of methane (CH4) and ethane (C2H6), the shortest and lightest hydrocarbon molecules. It often also contains varying amounts of: Heavier gaseous hydrocarbons: propane (C3H8), normal butane (n-C4H10), isobutane (i-C4H10) and pentanes. All of these are collectively referred to as Natural Gas Liquids or NGL and can be processed into finished by-products. Liquid hydrocarbons (also referred to as casinghead gasoline or natural gasoline) and/or crude oil. Acid gases: carbon dioxide (CO2), hydrogen sulfide (H2S) and mercaptans such as methanethiol (CH3SH) and ethanethiol (C2H5SH). Other gases: nitrogen (N2) and helium (He). Water: water vapor and liquid water. Also dissolved salts and dissolved gases (acids). Mercury: very small amounts of mercury primarily in elemental form, but chlorides and other species are possibly present. Naturally occurring radioactive material (NORM): natural gas may contain radon, and the produced water may contain dissolved traces of radium, which can accumulate within piping and processing equipment. This can render piping and equipment radioactive over time. The raw natural gas must be purified to meet the quality standards specified by the major pipeline transmission and distribution companies. Those quality standards vary from pipeline to pipeline and are usually a function of a pipeline system's design and the markets that it serves. In general, the standards specify that the natural gas: Be within a specific range of heating value (caloric value). For example, in the United States, it should be about 1035 ± 5% BTU per cubic foot of gas at 1 atmosphere and 60 °F (41 MJ ± 5% per cubic metre of gas at 1 atmosphere