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Industrial gases are, in the **standard \"Classification and Labelling of Commonly Used Hazardous Chemicals\" (GB13690-1992), generally classified as Category 2 compressed gases and liquefied gases. Such chemicals refer to gases that are compressed, liquefied, or dissolved under pressure. By pressurizing the gas or reducing its temperature, the distance between gas molecules can be **decreased**, allowing the gas to be compressed into a steel cylinder. Such gases are known as compressed gases (also referred to as permanent gases), such as oxygen, nitrogen, argon, hydrogen, etc. By continuing to apply pressure to the compressed gas and cooling it appropriately, the gas will turn into a liquid, which is known as a liquefied gas (such as liquid chlorine, liquid ammonia, liquid carbon dioxide, etc.). In addition, there is another type of gas that is extremely unstable; when compressed, it needs to be dissolved in a solvent and stored in steel cylinders. Such gases are known as dissolved gases (such as dissolved acetylene). Industrial gases can be divided into 4 categories based on their chemical properties: (1) Highly toxic gases, which are extremely poisonous and can cause poisoning or even death upon entering the human body. Such as chlorine, ammonia, etc. ⑵Flammable gases are highly combustible and pose a risk of chemical explosion; they also have certain toxicity. Such as hydrogen, acetylene, etc. ⑶Oxidizing gases, which have the ability to support combustion but do not burn themselves, and pose a risk of exacerbating fires, such as oxygen. ⑷Non-flammable gases that are asphyxiating to humans, stable in nature, and do not burn, such as nitrogen, carbon dioxide, and argon. **In the standard GB13690-1992, the above four types of gases are divided into 3 subcategories: Category 2.1 flammable gases, Category 2.2 non-flammable gases (including oxidizing gases), and Category 2.3 toxic gases. Industrial gases can be classified by composition into industrial pure gases, which are single-component gases, and industrial mixed gases, which are binary or multi-component gases. **In the standard \"Classification of bottled compressed gases\" (GB16163-1996), industrial pure gases are classified based on their physical state in gas cylinders and critical temperature, and are further grouped according to their chemical properties, flammability, toxicity, and corrosivity. Category 1 consists of inert gases, whose critical temperature is < -10°C; they remain in gaseous state during filling as well as during storage, transportation, and use at the permitted operating temperatures. These gases are divided into two subgroups: Group A includes non-flammable, non-toxic and non-flammable, toxic gases (such as oxygen, nitrogen, argon, etc.), while Group B includes flammable, non-toxic and flammable, toxic gases (such as hydrogen, etc.). Category 2 consists of liquefied gases with a critical temperature of ≥ -10°C, including high-pressure liquefied gases and low-pressure liquefied gases. Among them, high-pressure liquefied gases have a critical temperature of ≥-10°C and ≤70°C; they are in a liquid state during filling, but as the temperature rises to the critical value during storage, transportation, and use at the permitted operating temperatures, they evaporate into a gaseous state. These gases are divided into three groups: Group a consists of non-flammable, non-toxic and non-flammable, toxic gases (including carbon dioxide) ； Group B consists of flammable non-toxic and self-igniting toxic gases ； Group C consists of flammable gases that are prone to decomposition or polymerization. Low-pressure liquefied gases with a critical temperature greater than 70°C remain in a liquid state during filling as well as during storage, transportation, and use at permitted operating temperatures. They are also divided into three groups: Group A includes non-flammable, non-toxic gases, as well as non-flammable, toxic gases and acidic corrosive gases (including chlorine) ； Group b consists of flammable non-toxic gases, flammable toxic gases, and alkaline corrosive gases (including ammonia) ； Group C consists of flammable gases that are prone to decomposition or polymerization. Category 3 consists of dissolved acetylene, which is a gas that is dissolved under pressure in a solvent within a cylinder; it includes only Group a: flammable gases that are prone to decomposition or polymerization (including acetylene). This classification is the basis for preparing the air-fuel mixture. Industrial mixtures are a new type that have emerged over the past two decades; they have a wide range of applications, but there is yet no unified standard for their classification. Industrial mixtures include two categories: naturally synthesized and pure-component formulated. Based on their state, they are divided into gaseous mixtures and liquid mixtures. Based on the main hazardous components they contain, they can generally be classified into flammable mixtures, self-igniting mixtures, highly toxic mixtures, and corrosive mixtures. The common physical properties of industrial gases can be summarized as compressibility and expansibility. When a certain amount of gas is kept at a basically constant temperature, the greater the pressure applied, the smaller its volume becomes; if the pressure is increased further, the gas will be compressed into a liquid. This is the compressibility of gases. Industrial gases are usually stored in steel cylinders in a compressed or liquefied state. When a gas is exposed to light or heat, its temperature rises, the thermal motion of its molecules intensifies, and its volume increases. If it is contained within a fixed container, the higher the temperature of the gas due to heating, the greater the pressure generated as a result of its expansion. This is what is known as the expansibility of gases when heated. Compressed gases and liquefied gases are stored in containers; when exposed to high temperatures or direct sunlight, these gases expand easily, generating high pressure. When this pressure exceeds the container’s tolerance limit, an explosion can occur. Therefore, all industrial gases pose a great risk of explosion.