What is the cause of sublimation of matter? Why can matter be sublimated or condensed?

The process in which a solid substance evaporates directly into steam without going through a liquid process is called "sublimation".Sublimation is an endothermic process, generally at room temperature and pressure, sublimation will occur on any solid surface. For example, potassium iodide, dry ice, sulfur, phosphorus, camphor and other substances have significant sublimation phenomena.

From a microscopic point of view, the process by which the molecules on the surface of the crystal break free from the attraction of other molecules and run outside the crystal to become vapor molecules is sublimation. When heated below the pressure of the three-phase point, the solid-phase substance can be turned into the gas-phase without going through the liquid phase. For example, the gradual reduction of mothballs and the drying of clothes that have been frozen outside in winter will dry out, which is the result of sublimation.

We know that from macro, sublimation is the process of direct transformation of matter from solid to gaseous state. From the micro**, if the solid substance is placed in a closed container, when sublimated, the molecule with greater kinetic energy on the solid surface overcomes the binding force between neighboring molecules and escapes from the solid surface and directly becomes vapor molecules; On the other hand, when vapor molecules approach the solid surface due to irregular motion, they may be attracted to the solid surface molecules and return to the solid. At the beginning, more molecules escape from the solid surface than the number of molecules returned to the solid surface at the same time.

As the sublimation process progresses, the density of the vapors in the container increases, and so does the number of molecules returning to the solid. Until the average of the number of molecules escaping from the solid surface per unit time is equal to the average of the number of molecules returning to the solid surface per unit time, the vapor in the closed container reaches dynamic equilibrium with the solid.

If the sublimation and condensation of a pure substance is carried out in the air on the ground. It is only necessary to specify the "pressure of the vapor of the pure substance" in the above condition as "the partial pressure of the vapor of the substance in the air".For example; The sanitary ball is made of naphthalene (C10H8), the temperature of its three-phase point is about open, and the corresponding pressure is megapascals; The temperature of the three-phase point of iodine is on, and the corresponding pressure is megapascals.

Their sublimation and condensation are easy to observe at room temperature and in the air on the ground, so the junior high school textbook describes the sublimation phenomenon of the sanitary ball, the sublimation of solid iodine and the condensation experiment of iodine vapor.

On the one hand, it is necessary to overcome the binding force between the solid particles to do the work, and on the other hand, it is necessary to overcome the external pressure to do the work due to the volume expansion, so the substance needs to absorb a large amount of heat during the sublimation.

The influence of the environment, the cause of the substance itself.

1. Sublimation: The direct conversion of matter from the solid state to the gaseous state is called sublimation.

Causes: In physics, sublimation refers to the phase transition process of a substance from a solid state to a gaseous state without going through a liquid state due to a large temperature difference.

Examples of practical phenomena of sublimation: iodine turns into iodine vapor and ice becomes water vapor.

The camphor brain pill is gone.

2. Condensation: the phenomenon that a substance skips the liquid state and changes directly from the gaseous state to the solid state. It is the temperature and pressure of the substance below the triple point.

A change in the state of matter.

The condensation process material is exothermic.

Causes: The conditions for the formation of condensation are relatively special, generally requiring the concentration of gas to reach a certain requirement, and the temperature should be lower than the temperature of the condensation point, such as water vapor when it is below 0 degrees Celsius, etc., and the cause of formation is generally caused by rapid cooling or sublimation.

An example of the actual phenomenon of Ninghua: a light bulb that has been used for a long time.

It turns from transparent to black, and it heats up when the light bulb is working.

The tungsten vapor formed by the heating sublimation of the tungsten filament is condensed on the wall of the light bubble into a very thin layer of solid tungsten.

The main reason for the sublimation of substances is: the intermolecular force is small, and the vapor pressure is relatively large.

For example, iodine elemental I2 is a non-polar molecule.

The intermolecular force is only dispersion, and the vapor and pressure of iodine at different temperatures are relatively large, so when the iodine solid is heated, it gradually reaches the external pressure, and it can be directly gasified without going through the liquefied state of the tarnel trap and reaching the boiling point.

1. Reason: Sublimation is one of the methods for purifying solid organic compounds. Some substances have a fairly high vapor pressure when they are solid, and when heated, they are directly vaporized without going through the liquid state, and the vapor is cooled and directly condensed into a solid, this process is called sublimation.

2. If the substance that is easy to sublimate contains non-volatile impurities, or separates the solid mixture with significantly different volatility, it can be purified by the ascending Li Zhihua method. The samples obtained by this method are of high purity, but the operation is time-consuming and the loss is large. Sublimation can only be used in solid substances that have a sufficiently high vapor pressure at not too high temperatures, so there are certain limitations.

Physics] [Liquefaction, Sublimation and Condensation] - How are clouds formed?

Sublimation. Sublimation one.

The phenomenon in which a solid substance does not undergo a transformation into a liquid state but directly becomes a gaseous state is called sublimation.

Sublimation II. Sublimation: refers to the phenomenon that solid substances are directly transformed into gaseous states without going through the liquid state, and can be used as a method for separation by applying solid-gas equilibrium.

Some substances (such as iodine) have a higher vapor pressure when they are solid, so they can be turned into vapor without melting after being heated, and then re-become a solid when condensed. The temperature at which the vapor pressure of a solid substance is equal to the external pressure is called the sublimation point of the substance. At the sublimation point, sublimation occurs not only on the surface of the crystal, but also inside it, and the effect is very violent, and it is easy to bring impurities into the sublimation product.

In order for sublimation to occur only on solid surfaces, usually always.

It is carried out at a temperature below the sublimation point, when the vapor pressure of the solid is lower than the external pressure.

Ammonium halide also "sublimates", but its mechanism is different from general sublimation. When heating, because ammonium halide decomposes into gaseous ammonia and hydrogen halide and gasifies, it is recombined into ammonium halide and deposited when cooled, and the apparent phenomenon is the same as sublimation, so it is often attributed to sublimation, but its essence is different.

A brief history of human beings to understand the phenomenon of sublimation very early, the Eastern Jin Dynasty (4th century AD) Ge Hong in the "Bao Puzi Inner Chapter" recorded that: "take the female yellow, male yellow burned, among which the copper cast as the instrument to restore the ......."For 100 days, this vessel is full of red milk, and it is several minutes long. This passage describes the sublimation of arsenic trisulfide and tetraarsenic tetrasulfide.

The Compendium of Materia Medica (1596) written by Li Shizhen of the Ming Dynasty contained a method of heating and sublimating a mixture of mercury, alum, and salt to make light powder (mercurous chloride).

Methods In addition to atmospheric pressure sublimation, there are:

Vacuum sublimation Since sublimation is related to the relative magnitude of solid vapor pressure and external pressure, reducing the external pressure can reduce the sublimation temperature, and substances that cannot be sublimated or sublimated very slowly at normal pressure can be sublimated by vacuum. Vacuum sublimation also prevents the sublimated substance from decomposing due to high temperature or being oxidized during sublimation. Magnesium and samarium, titanium trichloride, benzoic acid, saccharin, etc. can be purified by this method.

Low temperature sublimation In 1976, J W Mitchell proposed the technology of low temperature sublimation, that is, the temperature and pressure are maintained below the three-phase point of the sublimated substance, so that it is sublimated at a very low pressure (a few millimeters of mercury), and after condensation, it is trapped in the cold trap and separated from impurities. This method is simple to operate, the product purity is very high, for example, it is difficult to purify hydrogen peroxide into high-purity reagents by general methods, and purification by this method can reduce cobalt, chromium, copper, iron, manganese, nickel and other impurities from 1000ng ml at one time.

3. Do not store it in the container open for a long time. If iodized salt is in contact with sunlight and air for a long time, iodine is easy to volatilize. It is best to put it in a colored glass bottle, and after use, the cap should be tightly closed and stored.

Fourth, avoid adding vinegar. Iodine is destroyed when it binds to acids. According to the test, if vinegar is added at the same time when stir-frying, the consumption rate of iodine will drop by 40% to 60%.In addition, the consumption rate of iodized salt will also decrease when it encounters sour vegetables (such as sauerkraut).

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When the air pressure is strong, the temperature return degree of the solid substance when the DAO vapor pressure is equal to the external pressure, the sublimation occurs not only on the surface of the solid, but also inside it, and the effect is very violent.

Sublimation is an endothermic process, and the heat of sublimation of the same substance is always greater than the value of the heat of evaporation. The phase transition of a substance from a solid state to a gaseous state without going through a liquid state.

Principle: Sublimation is mention.

One of the methods of pure solid DU organic compounds.

Some substances have a fairly high level of vapor in the solid state.

Internal pressure, when heated, is directly vaporized without going through the liquid state, and the vapor is cooled and directly condensed into a solid, this process is called sublimation.

Sublimation can be purified by sublimation if the substance contains non-volatile impurities, or when separating a mixture of solids with significantly different volatility. The samples obtained by this method are of high purity, but the operation is time-consuming and the loss is large.

Sublimation can only be used with solid substances that have a sufficiently high vapor pressure (higher than the vapor pressure before the melting point) at not too high temperatures, so they have certain limitations.