In the case of normal transportation, the solid that is easy to burn quickly or catch fire by friction is defined as flammable solid. Flammable solids may also explode when burning, which is very dangerous.
Knowledge expansion:
Frozen liquefied gas refers to the gas that is partially liquid due to low temperature (critical temperature is generally lower than or equal to -50℃) during transportation.
Liquefaction of gas is usually achieved by lowering the temperature or increasing the pressure. The liquefaction process makes the molecules of gas no longer have the ability to move freely, but are closely arranged and have the characteristics of solid. By freezing and liquefaction, the volume and occupied space of gas are greatly reduced, which is convenient for storage and transportation.
Gaseous state refers to a state of matter, which is highly diffused at normal temperature and pressure and usually does not condense into liquid or solid. Typical gaseous substances include oxygen, nitrogen, carbon dioxide and hydrogen.
On the contrary, liquid state means that a substance has both the density and shape of a solid state and the high diffusivity of a gas state at a certain temperature and pressure. Typical liquid substances include water, oil, alcohol and many other organic substances.
However, when the temperature or pressure changes, liquid substances can be changed into gaseous substances, and vice versa. For example, when water is heated above 100 degrees Celsius, it will become steam and gas.
Similarly, when the pressure decreases, gaseous substances can condense into liquid substances. For example, if you put a bottle of helium in a high-pressure bottle and suddenly release the pressure, the helium will immediately become a liquid.
At normal temperature and pressure, no gas will be a liquid. However, a common man-made product is wrongly called "liquefied gas", that is, liquefied petroleum gas (LPG). Liquefied petroleum gas is made of natural gas or petroleum by pressure cooling. It is a liquid at normal temperature and pressure, but once exposed to air, it will evaporate quickly and become a gas.
Generally speaking, if we know the different states of matter, we can better understand and use them.