Bottled gas is a term used for substances which are gaseous at standard temperature and pressure (STP) and have been compressed and stored in carbon steel, stainless steel, aluminum, or composite bottles known as gas cylinders. Gas state in cylinders There are four cases: either the substance remains a gas at standard temperature but increased pressure, the substance liquefies at standard temperature but increased pressure, the substance is dissolved in a solvent, or the substance is liquefied at reduced temperature and increased pressure. In the last case the bottle is constructed with an inner and outer shell separated by a vacuum (dewar flask) so that the low temperature can be maintained by evaporative cooling. Case I The substance remains a gas at standard temperature and increased pressure, its critical temperature being below standard temperature. Examples include: air argon fluorine helium hydrogen krypton nitrogen oxygen Case II The substance liquefies at standard temperature but increased pressure. Examples include: ammonia butane carbon dioxide (also packaged as a cryogenic gas, Case IV) chlorine nitrous oxide propane sulfur dioxide Case III The substance is dissolved at standard temperature in a solvent. Examples include: carbon dioxide in the form of a soft drink sulfur trioxide in the form of fuming sulfuric acid nitrogen dioxide in the form of red fuming nitric acid hydrogen chloride in the form of muriatic acid Note: these four are most often found in containers other than metal bottles, and at low pressure, e.g. . acetylene Note: Acetylene cylinders contain an inert packing material, which may be agamassan, and are filled with a solvent such as acetone or dimethylformamide. The acetylene is pumped into the cylinder and it dissolves in the solvent. When the cylinder is opened the acetylene comes back out of solution, much like a carbonated beverage bubbles when opened. This is a workaround to acetylene's propensity to explode when pressurized above 200 kPa or liquified. Case IV The substance is liquefied at reduced temperature and increased pressure. These are also referred to as cryogenic gases. Examples include: liquid nitrogen (LN2) liquid hydrogen (LH2) liquid oxygen (LOX) carbon dioxide (also packaged as a liquefied gas, Case II) Note: cryogenic gases are typically equipped with some type of 'bleed' device to prevent overpressure from rupturing the bottle and to allow evaporative cooling to continue. Expansion and volume The general rule is that one unit volume of liquid will expand to