Gas is treated before it can be supplied to the marketplace. The extent to which gas needs to be processed will depend on its quality, the amount of associated impurities such as water, carbon dioxide and sulphur compounds, and the ultimate end-use for the gas. Common gaseous impurities found in natural gas are carbon dioxide and sulphur compounds. Both have an acidic reaction and are given the generic name 'acid gases'. These gases can be removed by a number of commercial processes, using either a physical or a chemical solvent. Physical solvent processes tend to be used where gas pressures are high and for gases with lower levels of propane and heavier hydrocarbons.