Gasoline, also known (in the UK) as petrol, is a mixture of volatile hydrocarbons with 4 to 12 carbon atoms per molecule, which is used as a fuel for internal-combustion engines, and as a solvent. Although gasoline can be derived from oil, coal, and tar, or synthesized from carbon monoxide and hydrogen, almost all is produced from petroleum by refining, cracking, and alkylation, the fractions being blended to produce fuels with desired characteristics. Motor gasoline boils between 30°C and 200°C, with more of the low-boiling components in cold weather for easy starting. If, however, the fuel is too volatile, vapor lock can occur – i.e., vapor bubbles form and hinder the flow of fuel. Aviation gasoline contains less of both low- and high-boiling components. The structure of gasoline components is also carefully controlled for maximum power and efficiency, as reflected by the octane rating; this may be further improved by antiknock additives. Other additives include lead scavengers (ethylene dibromide and dichloride), antioxidants, metal deactivators (which remove metal ions that catalyze oxidation), anti-icing agents, and detergents.