What are flash point, ignition point, and autoignition?

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What are flash point, ignition point, and spontaneous combustion? 1. Flash point: A flammable liquid can evaporate to form vapors that mix with the air. As the temperature rises, evaporation accelerates. When a mixture of volatile vapor and air comes into contact with a fire source and sparks are generated, this brief combustion process is called flash ignition, and the lowest temperature at which flash ignition occurs is known as the flash point. From a fire safety perspective, the flash point of a liquid is the lowest temperature at which it can cause a fire. The lower the flash point, the greater the risk of fire. 2. Ignition point: Whether in solid, liquid, or gas form, combustible substances will burn when in contact with an ignition source once they reach a certain temperature, and they will continue to burn even after the ignition source is removed. At this point, the lowest temperature at which a combustible material can burn is called the ignition point, or burning point. Generally, the ignition point of liquids is higher than their flash point, and the ignition point of flammable liquids is 1–5°C higher than their flash point. 　　3. Spontaneous combustion: Under normal conditions, when common combustible materials come into contact with air, a slow oxidation process occurs; however, this process is very slow, producing little heat, and the heat is continuously dissipated into the surrounding environment, so no light is emitted as in the case of combustion. If the temperature rises or other conditions change, the oxidation process speeds up, generating more heat. If this heat cannot be dissipated entirely, it accumulates, causing the temperature to rise gradually. When it reaches the temperature at which this substance burns on its own, it will ignite spontaneously; this is known as spontaneous combustion. The lowest temperature at which a substance catches fire on its own when heated is known as its autoignition point, or autoignition temperature. 　　At the autoignition temperature, a combustible material in contact with air can burn without the need for an open flame. The value of naturalness is not a fixed one; it mainly depends on the heat released during oxidation and the way in which heat is conducted outward. It can be seen that for the same combustible material, different oxidation conditions and various influencing factors result in different autoignition points. 　　Spontaneous combustion can be divided into two cases. Spontaneous combustion that occurs due to the effect of an external heat source is called heat-induced spontaneous combustion ； Certain combustible substances generate heat on their own due to biological, physical, or chemical processes occurring within them, without the need for an external heat source; when the conditions are right, this heat is sufficient to cause the substance to ignite automatically, a phenomenon known as spontaneous combustion. 　　The essence of spontaneous combustion and combustion induced by heat is the same; it’s just that the source of heat differs – in the former case it’s the thermal effect of the substance itself, while in the latter case it’s due to external heating. Spontaneous combustion of materials occurs under certain conditions; some can happen at room temperature, while others can occur at low temperatures. The phenomenon of spontaneous combustion itself indicates that this substance poses a greater latent fire hazard than other substances. Under normal circumstances, common substances that can cause spontaneous combustion include plant products, oils and fats, coal, and other chemical substances. Substances such as phosphorus and phosphine have a low auto-ignition point.