Liquid gas is any gas that has been cooled or compressed enough to turn into a liquid. The most familiar examples are liquefied natural gas (LNG), liquefied petroleum gas (LPG), and industrial cryogenic liquids like liquid nitrogen and liquid oxygen. Converting a gas into a liquid makes it dramatically more compact, sometimes taking up less than one-thousandth of its original volume, which makes it practical to store, transport, and use as fuel or in industrial processes.
How a Gas Becomes a Liquid
Every gas has a temperature below which it can be squeezed into liquid form by applying enough pressure. The process works by running warm gas through refrigeration cycles that cool it to extremely low, or “cryogenic,” temperatures while compressors raise the pressure. Heat is pulled out of the gas and rejected through air or water coolers until the gas condenses into liquid.
Different gases liquefy at different temperatures. Methane, the main ingredient in natural gas, turns liquid at about negative 161°C (negative 259°F). Nitrogen liquefies at negative 196°C, oxygen at negative 183°C, and hydrogen requires an extraordinary negative 253°C. These temperatures are so far below freezing that specialized equipment is needed at every stage.
LNG vs. LPG: The Two Main Fuel Types
When most people encounter the term “liquid gas,” they’re thinking of one of two fuels: LNG or LPG. They sound similar but are chemically distinct.
Liquefied natural gas (LNG) is predominantly methane. It’s created by chilling natural gas to roughly negative 161°C, which shrinks its volume by about 600 times. LNG is used to ship natural gas across oceans in tanker ships and to fuel heavy-duty trucks and some power plants. One pound of LNG contains about 21,240 BTU of energy. To match the energy in a single gallon of diesel, you need about 6 pounds of LNG.
Liquefied petroleum gas (LPG) is a mix of propane and butane, with small traces of propylene and butylene. It’s a byproduct of crude oil refining and natural gas processing. LPG stays liquid under relatively modest pressure at normal temperatures, so it doesn’t need the extreme cooling that LNG does. A gallon of propane contains about 84,250 BTU, roughly 73% of the energy in a gallon of gasoline. LPG powers barbecue grills, portable heaters, vehicles in some countries, and provides heating and cooking fuel for homes not connected to natural gas pipelines.
Industrial Liquid Gases
Beyond fuels, several gases are routinely liquefied for industrial, medical, and scientific use. Liquid nitrogen is one of the most common. It’s inert, extremely cold, and widely used for flash-freezing food, preserving biological samples, and cooling equipment. Liquid oxygen supports welding, steelmaking, and medical oxygen supplies in hospitals.
Liquid hydrogen plays a critical role in the space industry as a primary rocket fuel, combusted with oxygen to produce enormous thrust. It’s also a raw material in chemical manufacturing, from producing plastics like polyethylene and polypropylene to hydrogenating food-grade oils. Semiconductor manufacturers and metallurgical operations use hydrogen to reduce metal oxides and prevent oxidation during heat treatment of metals and alloys.
What all these liquids share is a massive expansion ratio. When liquid nitrogen warms back to a gas, it expands roughly 710 times in volume. Liquid oxygen expands about 875 times. This property is both what makes them useful (you can store a huge amount of gas in a small tank) and what makes them dangerous.
How Liquid Gases Are Stored
Keeping a substance hundreds of degrees below zero requires serious insulation. Cryogenic storage tanks typically use a double-wall design with a vacuum between the inner and outer shells, sometimes filled with insulating materials like perlite (a lightweight volcanic mineral) or multilayer insulation made of reflective foil sheets. The vacuum minimizes heat transfer, while the insulation layers block radiant heat from warming the liquid inside.
Insulation quality matters enormously. Research on cryogenic tank safety shows that non-combustible multilayer insulation limits heat leaking into the tank to around 3 kilowatts, while combustible versions that degrade under extreme conditions can allow heat ingress more than twice as high. Even small insulation failures create measurable temperature gradients, approaching 10°C per 100 millimeters of tank height during sustained heat exposure. Tank operators monitor vacuum pressure as an early warning sign that insulation is breaking down.
LPG storage is simpler. Because propane and butane liquefy under moderate pressure at room temperature, they’re stored in standard pressurized steel cylinders, from the small tank on a backyard grill to large above-ground or underground tanks at homes and businesses.
Safety Risks
Liquid gases carry two main categories of risk: extreme cold and rapid expansion.
Contact with cryogenic liquids causes frostbite almost instantly. Skin can freeze on contact, and clothing soaked in a cryogenic liquid can freeze to the body. If that happens, the fabric should be thawed before removal, not pulled away. Only medical personnel should attempt to thaw frostbitten tissue. Anyone handling refrigerated or cryogenic liquids should wear thermal protective clothing.
The expansion risk is equally serious. If a pressurized container of liquid gas is heated (by fire, for example), the liquid inside rapidly boils and expands. When the pressure exceeds what the container can hold, the result can be a boiling liquid expanding vapor explosion, often called a BLEVE. This type of event is particularly dangerous with LPG fuels like propane and butane because they’re flammable, so the rapidly expanding vapor ignites in a fireball. Emergency response guidelines call for significant evacuation distances around LPG containers involved in fires.
Energy and Efficiency Tradeoffs
Liquid gas fuels are popular partly because they burn cleaner than gasoline or diesel, producing fewer hydrocarbons and particulates. LPG in particular is known for extending engine life because it leaves fewer deposits. LNG produces lower carbon emissions per unit of energy than coal or oil, making it a transitional fuel for power generation and shipping.
The tradeoff is energy density. Gasoline packs about 112,000 to 116,000 BTU per gallon, and diesel about 128,500 BTU per gallon. Propane delivers only about 84,250 BTU per gallon, so a vehicle running on LPG needs a larger fuel tank to travel the same distance. LNG is measured by weight rather than volume since it’s stored at cryogenic temperatures, and matching one gallon of gasoline requires about 5.4 pounds of LNG. For applications where clean combustion, lower emissions, or abundant supply outweigh the need for compact fuel storage, liquid gas fuels make practical sense. For long-range vehicles or weight-sensitive applications, the lower energy density is a real limitation.