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Dry point (KK): The point at which distillation of the oil continues until the highest vapor temperature is reached is known as the final boiling point or dry point. Example: The dry point is determined by placing 100 milliliters of gasoline in a small flask equipped with a side tube, inserting a thermometer, and heating it for distillation. The temperature indicated by the thermometer at the moment the first drop of oil is distilled is called the initial boiling point. The temperature when the volume of the distilled liquid reaches 10 milliliters is called the 10% point; similarly, the 20% point, 30% point, and so on can be determined, until the temperature at which the last drop of oil is distilled is reached – this is the dry point. 2. Boiling point: The temperature at which a liquid mixture begins to boil under a certain pressure is known as the boiling point at that pressure. 　　Unless the pressure value is specified, it usually refers to the bubble point at 0.101325 MPa. The boiling point changes depending on the composition of the liquid. For pure compounds, the bubble point is the boiling point at a given pressure. 　　It is the temperature at which the first bubble appears when a liquid of fixed composition is heated under constant pressure; in other words, it is the temperature at which such a liquid reaches vapor-liquid equilibrium with steam at a given pressure. The bubble point varies with the liquid composition and pressure. When a minimum or maximum value appears in the relationship between the boiling point and the composition of the liquid phase, this extreme temperature is referred to as the minimum azeotropic temperature or maximum azeotropic temperature, respectively. At this point, the compositions of the vapor phase and the liquid phase are identical; such mixtures are known as azeotropic mixtures. At vapor-liquid equilibrium, the bubble point of the liquid phase is the dew point of the vapor phase. 3. Flash point. The flash point is the lowest temperature at which, in a stable air environment, the vapor generated on the surface of a flammable liquid or solid ignites when exposed to a test flame. Flaming occurs when the vapor emitted by a flammable liquid mixes with air to reach a certain concentration and then catches fire momentarily; or when a flammable solid is heated to a certain temperature and then ignites briefly upon contact with a flame. This phenomenon is known as flashing. The lowest temperature of a solid at which flash ignition occurs is called the flash point. 　　The flash point is the lowest temperature at which a flammable liquid or solid can release enough vapor to form a flammable mixture with air at the surface of the liquid or solid within its container. The flash point of flammable liquids changes with their concentration. 　　As the temperature rises, more vaporized oil and gas form on the surface of the fuel. When the mixture of this vaporized oil and gas with air reaches a certain concentration, it will produce a brief flash of light when exposed to an open flame; this temperature is known as the flash point. There are two methods for determining the flash point: the open cup method and the closed cup method. The flash point determined by the open cup method is 15–25°C lower than that obtained by the closed cup method. The level of the flash point is related to the molecular composition of the oil and the pressure above its surface; higher pressure results in a higher flash point. The flash point is an important indicator for preventing fires involving oil. In an open container, the heating temperature of the oil should be 10 degrees below its flash point℃ ； There is no such limitation when heating in a pressure vessel. 　　The lowest temperature at which a mixture of vapor and air above the oil surface can sustain continuous combustion when exposed to an open flame (for a duration of no less than 5 seconds) is known as the ignition point. The ignition point is higher than the flash point. 　　From a fire prevention perspective, it is desirable for the flash point and ignition point of oil to be higher, with a larger difference between the two. From a combustion perspective, it is desirable for the flash point and ignition point to be low, with the difference between the two also being as small as possible. 　　The flash point refers to the lowest temperature at which a petroleum product, under specified conditions, will ignite instantly when its vapor comes into contact with a flame upon heating. The lighter the oil, the lower its flash point. The hazard level of oils is determined based on their flash point. The flash point can be used to determine the density of an oil’s composition and assess the risk of fire involving that oil. The flash point determined using a closed-cup flash point tester is called the closed-cup flash point; it is generally used to determine the flash point of light petroleum products. The higher the flash point, the safer it is. The flash point is a parameter indicating the tendency of petroleum products to evaporate and their safety properties. The hazard classification of oils is determined by their flash point; those with a flash point below 45°C are considered flammable substances ； Substances with a temperature above 45°C are flammable. During storage and use, it is prohibited to heat the oil to its flash point; the maximum heating temperature should generally be 20–30°C below the flash point. During the use of oils, the flash point is also of great significance. For example, if the flash point of the engine oil in use drops significantly, it indicates that the engine oil has been diluted by fuel; the engine should be inspected and the oil replaced. 　　Under specified conditions, lubricating oil is heated. When the oil temperature reaches a certain level, the mixture of lubricating oil vapors and surrounding air ignites upon contact with a flame. The lowest temperature at which this ignition occurs is known as the flash point of the lubricating oil. When selecting a lubricant, one should consider its flash point relative to the operating temperature. Generally, the flash point should be 20 to 30 degrees higher than the operating temperature, so as to ensure safe usage and minimize evaporation losses. The flash points of Mobil’s automotive engine oil series are all very high, generally above 200 degrees. (This paragraph can be found on page 30 of “Lubricant Materials and Lubrication Technology”). Open flash point: The value obtained using a specified open flash point tester is called the open flash point, and it is expressed in °C. It is commonly used to determine lubricating oils. 4. Boiling point: The temperature at which a liquid mixture begins to boil under a certain pressure is called the boiling point at that pressure. 　　Unless the pressure value is specified, it usually refers to the bubble point at 0.101325 MPa. The boiling point changes depending on the composition of the liquid. For pure compounds, the bubble point is the boiling point at a given pressure. 　　It is the temperature at which the first bubble appears when a liquid of fixed composition is heated under constant pressure; in other words, it is the temperature at which such a liquid reaches vapor-liquid equilibrium with steam at a given pressure. The bubble point varies with the liquid composition and pressure. When a minimum or maximum value appears in the relationship between the boiling point and the composition of the liquid phase, this extreme temperature is referred to as the minimum azeotropic temperature or maximum azeotropic temperature, respectively. At this point, the compositions of the vapor phase and the liquid phase are identical; such mixtures are known as azeotropic mixtures. At vapor-liquid equilibrium, the bubble point of the liquid phase is the dew point of the vapor phase. 5. Dew point　　Image of a dew point sensor The lower the air temperature, the lower the saturated water vapor pressure. Therefore, for air containing a certain amount of water vapor, the temperature at which the pressure is kept constant and the saturated vapor pressure drops to equal the actual vapor pressure at that time is called the dew point. Additional note: When this temperature is below zero degrees Celsius, it is also referred to as the frost point. 　　Dew point temperature refers to the temperature at which air becomes saturated when cooled, with neither its water vapor content nor pressure changing. Put simply, the temperature at which water vapor in the air turns into dew drops is called the dew point temperature. The dew point temperature is originally a temperature value, so why is it used to represent humidity? This is because when the water vapor in the air reaches saturation, the temperature is equal to the dew point ； When the water vapor is not saturated, the air temperature must be higher than the dew point temperature. Therefore, the difference between the dew point and the air temperature can indicate the degree to which the water vapor in the air is close to saturation. At 100% relative humidity, the temperature of the surrounding environment is the dew point temperature. The lower the dew point is compared to the ambient temperature, the less likely condensation will occur; this also means that the air is drier. The dew point is not affected by temperature, but it is affected by pressure. 　　The wet-bulb temperature is defined as the adiabatic saturation temperature reached when air is in direct contact with water under constant-pressure adiabatic conditions, at which a stable thermohygric equilibrium is attained. 　　Dew point: The dew point. The temperature at which condensation begins to occur and moisture is formed. If the temperature of the outside air is lower than the temperature inside enclosed spaces such as ship cabins or containers, moisture forms on the metal surfaces inside the ship or container. On the other hand, if the temperature of the outside air is higher than the temperature inside the ship or container, moisture forms directly on the surface of the cargo. In some cases, it is necessary to ventilate the cabin to change the dew point temperature and prevent condensation from occurring. 　　In a single-phase gas system, it is the temperature or pressure at which the first liquid droplet appears as a result of changes in temperature and pressure. This temperature or pressure is called the dew point. 6. English: Boiling Point 　　Boiling point: The temperature at which, under a certain pressure, the saturated vapor pressure of a substance equals that pressure. 　　Saturated vapor pressure: The pressure of the vapor that is in phase equilibrium with a liquid or solid at a certain temperature is called the saturated vapor pressure. 　　Boiling is a violent vaporization process that occurs simultaneously inside and on the surface of a liquid at a certain temperature. The temperature at which a liquid boils is called the boiling point. The higher the concentration, the higher the boiling point. Different liquids have different boiling points; the boiling point refers to the temperature at which various liquid substances boil. The boiling point changes with external pressure; lower pressure results in a lower boiling point. 　　The temperature at which a liquid boils ； That is, the temperature at which a substance changes from a liquid state to a gas state. When a liquid is boiling, the saturated vapor pressure within the bubbles formed inside it must be equal to the external pressure; only then can these bubbles grow and rise. Therefore, the boiling point is the temperature at which the saturated vapor pressure of the liquid equals the external pressure. The boiling point of a liquid is related to the external pressure. When the pressure exerted on a liquid increases, its boiling point rises ； As the pressure decreases ； Boiling point depression. For example, the steam pressure in a steam boiler is several dozen atmospheres; consequently, the boiling point of water in the boiler can exceed 200°C. For example, when cooking on a high mountain, the water boils easily, but the rice does not cook well. This is because atmospheric pressure decreases with increasing altitude, and the boiling point of water also gradually decreases at higher elevations. (At an altitude of 1900 meters, the atmospheric pressure is approximately 79,800 Pascals (600 mmHg), and the boiling point of water is 93.5°C.) 　　At the same atmospheric pressure, liquids have different boiling points. This is because the saturated vapor pressure is related to the type of liquid. At a certain temperature, the saturated vapor pressure of various liquids is also fixed. For example, the saturated vapor pressure of diethyl ether at 20°C is 5865.2 Pascals (44 cm of mercury), which is lower than atmospheric pressure. By raising the temperature slightly so that the saturated vapor pressure of diethyl ether equals the atmospheric pressure, it can be boiled when heated to 35°C. If a liquid contains impurities, it also affects the boiling point of the liquid. When a solute is present in a liquid, its boiling point becomes higher than that of the pure liquid. This is because the presence of the solute increases the attractive forces between liquid molecules; as a result, the liquid is less likely to vaporize, and its saturated vapor pressure is also lower. To make the saturated vapor pressure equal to the atmospheric pressure, the boiling point must be increased. Different liquids have different boiling points at the same external pressure. The relationship between boiling point and pressure can be determined using Clausius’ equation. Boiling points of several substances: Boiling points of various liquids in degrees Celsius (at standard atmospheric pressure): Liquid iron: 2750; Liquid lead: 1740; Mercury: 357; Flaxseed oil: 287; Cooking oil: approximately 250; Naphthalene: 218; Kerosene: 150; Toluene: 111; Water: 99.974; Alcohol: 78; Ether: 35; Liquid ammonia: -33.4; Liquid oxygen: -183; Liquid nitrogen: -196; Liquid hydrogen: -253; Liquid helium: -268.9. The boiling point of water: It is commonly believed that the boiling point of water is 100°C, but this is inaccurate; the correct value is 99.974°C. 　　In 1988, the International Committee for Weights and Measures made a recommendation; the 18th General Conference on Weights and Measures and the 77th session of the International Committee for Weights and Measures resolved that the revised International Temperature Scale should be adopted worldwide starting from January 1, 1990. This version was named the 1990 International Temperature Scale, with the code ITS-90. The term “practical” was removed, as advances in science and technology had rendered this scale quite close to the thermodynamic temperature scale. Compared to IPTS-68, it is 0.026°C lower at 100°C; in other words, the boiling point of water under standard conditions is no longer 100°C, but rather 99.974°C. Word pronunciation: fèi diǎn. Definition: Basic explanation – The temperature at which a liquid begins to boil; specifically, it is the temperature at which the vapor pressure of the liquid equals the external pressure. Therefore, the boiling point decreases as pressure decreases. 7. Melting point: The melting point is the temperature at which a solid changes from a solid state to a liquid state (melts). The temperature at which the opposite process occurs (that is, the transition from a liquid state to a solid state) is called the freezing point. Unlike boiling point, melting point is little affected by pressure. In most cases, the melting point of a substance is equal to its freezing point. 　　The temperature at which a crystal begins to melt is called the melting point. Substances can be crystalline or amorphous; crystals have a melting point, while amorphous substances do not. Crystals also have different melting points depending on their type. Generally, the melting points of crystals, from highest to lowest, are: atomic crystals > ionic crystals > metallic crystals > molecular crystals. Among molecular crystals, there are some particularly special ones, such as water and ammonia. Their molecules do not follow the rule of \"regular variation in the melting points of hydrides of elements in the same group\" due to the presence of hydrogen bonds. 　　Melting point is a physical property of a substance. The melting point of a substance is not fixed; two factors have a significant impact on it. The first is pressure; the melting point of a substance, as it is commonly referred to, usually refers to the condition at one atmosphere of pressure ； If the pressure changes, the melting point also changes. There are two different scenarios regarding the change in melting point with pressure. For most substances, the melting process involves an increase in volume; as pressure increases, the melting point of these substances rises ； For substances like water, unlike most materials, the volume decreases as ice melts into water (the same is true for metals such as bismuth and antimony). The melting point of ice decreases as pressure increases. Another factor is the impurities in a substance; when we talk about the melting point of a substance, we are usually referring to a pure substance. But in real life, most substances contain other substances within them; for example, pure liquid substances may have small amounts of other substances dissolved in them, known as impurities. Even in small quantities, these impurities can cause a significant change in the melting point of the substance. For instance, when salt is dissolved in water, its melting point drops significantly. Seawater is water in which salt is dissolved, and this is why seawater freezes at a lower temperature than river water in winter. The melting point of saturated salt water can drop to around -22°C. In northern cities, when heavy snow falls in winter, salt is often spread on the snow-covered roads; as long as the temperature is above -22°C, enough salt can melt the ice and snow. This is another example of the application of the melting point in everyday life. 　　The melting point is essentially the temperature at which the solid and liquid phases of a substance can coexist in equilibrium. Taking the melting of ice into water as an example, the melting point of ice at 1 atmosphere of pressure is 0°C; at this temperature, ice and water can coexist. If there is no heat exchange with the outside environment, this state of coexistence between ice and water can remain stable for a long time. In various crystals, the interparticle forces differ, which is why their melting points vary. For the same crystal, the melting point is related to pressure; generally, the melting point of a substance at 1 atmosphere is considered the normal melting point. At a certain pressure, crystalline substances have the same melting point and freezing point. Substances that expand in volume when melted have a higher melting point when the pressure increases. 　　In the field of organic chemistry, pure organic compounds generally have fixed melting points. That is, under a certain pressure, the changes between the solid and liquid phases are very sharp; the temperature range from initial melting to complete melting does not exceed 0.5–1°C (the melting range, also known as the melting interval or melting span). However, if impurities are present, its melting point decreases and the melting distance increases as well. Therefore, melting point determination is a fundamental method for identifying the nature of a substance, and it is also one of the important methods for determining purity. 　　The common methods for determination are capillary method and micro melting point determination. In practical applications, we use professional melting point apparatus to determine the melting point of a substance. (The image on the right shows a microimage melting point apparatus.) Tungsten (W) is the metal with the highest melting point; even at high temperatures of 2000°C to 2500°C, its vapor pressure remains very low. Tungsten has high hardness, high density, and good strength at high temperatures. 　　Below are the melting points in degrees Celsius (°C) and the pressure (at standard atmospheric pressure) for several substances: Carbon (diamond): 3550; Tungsten: 3410 ± 22; Platinum: 1769; Iron: 1535; Steel: 1300–1400; Gray cast iron: 1200; Copper: 1083; Gold: 1064; Aluminum: 660; Lead: 327; Tin: 232; Naphthalene: 80.5; Sodium thiosulfate: 48; Water (ice): 0; Solid mercury: -39; Solid toluene: -95; Solid alcohol: -117; Solid nitrogen: -210; Solid oxygen: -218; Solid hydrogen: -259. The melting point of a substance is the temperature at which a pure substance is in equilibrium between its solid and liquid states under a certain pressure. In other words, at that pressure and melting point, the chemical potential of the pure substance in its solid state is equal to that in its liquid state. For pure substances in a highly dispersed solid state (such as nanoscale systems), the surface layer cannot be ignored; the chemical potential in such cases depends not only on temperature and pressure but also on the particle size of the solid particles. 8. Autoignition point: The autoignition point refers to the lowest temperature at which a combustible material can ignite spontaneously under specified conditions. 　　The main ways in which combustible materials catch fire spontaneously are: (1) heat generation through oxidation ； (2) Dissociation heat ； (3) Polymerization exotherm ； (4) Adsorption exotherm ； (5) Heat release during fermentation ； (6) The active substance in contact with water ； (7) Mixing of combustibles with strong oxidizers. 　　Main factors affecting the autoignition temperature of liquid and gaseous combustibles: Pressure: The higher the pressure, the lower the autoignition temperature ； Oxygen concentration: The higher the oxygen concentration in the mixture, the lower the auto-ignition temperature ； Catalysis: Active catalysts can lower the autoignition point, while inert catalysts can raise it ； Material and inner diameter of the container: Different materials for the container walls have different catalytic effects ； The smaller the container diameter, the higher the auto-ignition temperature. 　　Main factors affecting the auto-ignition temperature of solid combustibles: Heating and melting: After melting, it can exist in the form of liquids or gases ； Amount of volatiles: The more combustible substances that evaporate, the lower its auto-ignition point ； Particle size of solids: The finer the solid particles, the greater their specific surface area, and the lower their auto-ignition temperature ； Heating time: When combustible solids are exposed to heat for an extended period, their autoignition temperature decreases. There are interfaces between phases in a multiphase system. *It is common for people to refer only to gas-liquid and gas-solid interfaces as surfaces. 　　Generally, due to differences in the environment, molecules at the interface experience different forces compared to those within the bulk phase. The net force acting on a water molecule inside the water is 0, but this is not the case for a water molecule at the surface. Since the attraction exerted on it by the gas phase molecules in the upper layer is less than that exerted by the liquid phase molecules inside, the net force acting on this molecule is not zero; the direction of this net force is perpendicular and points inward toward the liquid. As a result, the liquid surface tends to contract automatically, and this contracting force is known as surface tension. To disperse water into mist droplets, that is, to increase their surface area, many internal water molecules must move to the surface, and work must be done on the system to overcome this force—surface energy. Obviously, such a dispersed system contains more surface energy. 　　Surface tension is a property of substances, and its magnitude depends on temperature and the properties of the materials in the two phases at the interface. 　　At 293 K, the surface tension of water is 72.75×10-3 N·m-1, that of ethanol is 22.32×10-3 N·m-1, and that of n-butanol is 24.6×10-3 N·m-1, whereas the interfacial tension of water-n-butanol (4.1‰) is 34×10-3 N·m-1. 　　There are various methods for measuring surface tension. In laboratories and textbooks, the method most commonly used is the maximum bubble pressure method. Since the equipment required for this method is readily available and its procedures are relatively easy for students to understand in terms of the principles behind surface tension, it has long been a standard method for teaching. As methods for testing surface tension using instruments, there are typically the platinum plate method, platinum ring method, hanging drop method, drop volume method, and maximum bubble pressure method, among others. Definition and related concepts: (1) Definition or explanation: ① The force that causes the surface of a liquid to contract is called surface tension. 　　②The force of mutual attraction per unit length between adjacent parts on the surface of a liquid. 　　(2) Unit The unit of surface tension in the SI system is Newton per meter (N/m), but dynes per centimeter (dyn/cm) is also commonly used; 1 dyn/cm = 1 mN/m. 　　(3) Explanation 　① The direction of surface tension is tangent to the liquid surface and perpendicular to the boundary line between the two portions; if the liquid surface is flat, then the surface tension lies on that plane. If the liquid surface is curved, the surface tension lies on the tangent plane of this curve. 　　②Surface tension is a manifestation of intermolecular forces. It occurs at the boundary where a liquid and a gas come into contact. It is determined by the special conditions of the liquid molecules in the surface layer. The molecules within a liquid are almost in close contact with one another; they maintain a balanced distance between them. When they are slightly farther apart, they attract each other, while when they are slightly closer together, they repel each other. This is why liquid molecules cannot spread infinitely like gas molecules, but can only vibrate and rotate around their equilibrium positions. Molecules near the liquid surface are subjected to uneven forces, as they are only significantly affected by the molecules inside the liquid; this allows molecules with higher speeds to easily escape to the surface and turn into vapor. As a result, the molecular distribution in the surface layer of the liquid (the thin layer in contact with the gas) is sparser than that in the interior parts of the liquid. Compared to the distribution of molecules within the liquid, they are in a special situation. The repulsive force between molecules in the surface layer decreases as the distance between them increases; in this special layer, the attractive forces between molecules become dominant. Therefore, if a dividing line MN is drawn arbitrarily on the liquid surface to divide it into two parts, a and b, as shown in the figure. F represents the attraction between the molecules in the surface layer of part a and those in part b, while F6 represents the attraction between the molecules in the surface layer of the right-hand part and those in part a. The forces exerted by these two pairs of molecules are always equal in magnitude but opposite in direction. The mutual attraction between any two parts of this surface layer leads to a tendency for the liquid surface layer to contract. Due to surface tension, the liquid surface always strives to become as small as possible; as a result, small droplets in the air tend to be spherical in shape. 　　③The magnitude of the surface tension F is proportional to the length of the dividing line MN. It can be written as F=σL or σ=F/L. 　　The ratio σ is called the surface tension coefficient, and its unit is commonly dyn/cm. Numerically, the surface tension coefficient is equal to the force per unit length of attraction between adjacent parts of a liquid surface. 　　Surface tension coefficient of the liquid film = Surface energy of the liquid film / Area of the liquid film = F_surface tension / (2 * Length of the selected line segment). 　　The surface tension coefficient is related to the properties of the liquid, and not to the size of the liquid surface. Surface tension in nature In nature, we can observe many phenomena related to surface tension as well as its applications. For example, dew always forms spheres as much as possible (as shown in the image), while certain insects can float on the surface of water thanks to surface tension. Experiments on Surface Tension Experiment 1: This experiment can also be considered a small game related to surface tension. First, find a friend to play this game with you, then prepare a glass of water (fill it up to the edge of the glass, so that the water level is at the same height as the rim), and 16 one-yuan coins (or more). Then there are the rules of this game: friends take turns putting coins into the cup, one at a time. There is no limit on the number of coins placed each time – you can put in 1 coin, 2 or 3 coins, or even more, until someone causes the water to overflow from the cup. When inserting the coin, use your thumb and index finger to gently place it into the cup filled with water. I have placed as many as 82 1-yuan coins at one time. 　　Experiment 2: Holding Water with a Punched Paper Sheet Materials: One bottle, one paper clip, one piece of paper, and a full cup of colored water. Procedure: 1. Fill the empty bottle with colored water. 　　2. Pierce many holes in the white paper with pushpins. 　　3. Cover the bottle opening with a perforated sheet of paper. 　　4. Press the paper piece with your hand and turn the bottle upside down, with the opening facing downward. 　　5. Gently remove your hand, and the piece of paper remains firmly in place over the bottle’s opening, with no water leaking out through the hole. 　　Explanation: A thin piece of paper can hold up the water in a bottle because atmospheric pressure acts on the paper, creating an upward lifting force. Water does not leak out of the small holes because of surface tension; a thin film of water forms on the surface of the paper, preventing the water from leaking out. It’s like a rain umbrella made of cloth; even though the cloth has many small holes, it still doesn’t let water leak through.