During the sublimation of a substance, what effect does the temperature exceeding the melting point

When the substance is purified by the sublimation method, the temperature exceeds the melting point, which may decompose the molten product (there are some substances that are accompanied by decomposition when melting), and the impurities produced by the decomposition may be mixed into the sublimation product, resulting in a decrease in purity.

Some of the salts composed of organic acids and bases containing sensitive groups are stable in the crystalline state at lower temperatures, but when melting, the crystal lattice is destroyed and the distance between the acid-base parts is shortened, so that the proton exchange and transfer of the acid-base part may destroy some sensitive groups. As a result, impurities are generated. [This paragraph describes why some substances are accompanied by decomposition when they are melted].

Of course, substances that do not decompose at the time of melting (and are not too pure in themselves) are not affected by this problem. Because the vapor pressure is sufficient, it will volatilize even if it is melted, so as not to have a significant impact on the yield. However, if it is purified by sublimation from a mixture with few targets and most impurities, there is a risk that the molten target will leach into the impurities and form a viscous tar, which may affect the yield.

There is no need to look at the substance, sublimation is the process of direct vaporization of solid substances without liquid substances after heating, and vapors are cooled and directly become solids;If the temperature during sublimation exceeds the melting point, it may cause the solid substance to melt, that is, after the liquid substance, it will lead to a decrease in the ** rate, and the purity will not have much impact

Let's take a look at what the substance is, and then analyze it to see what factors are affected.

Impact: The appropriate temperature range can increase the reaction rate, which is conducive to sufficient reaction and purity. Too high or too low is not good. It is a method of separating and identifying a mixture of trace substances under the action of a substance using filter paper or other substances with capillary effect.

Method: The chromatography method uses the differences in the physical and chemical properties of each component in the mixture, such as adsorption force, molecular shape and size, molecular affinity, partition coefficient, etc., to make each component fixed in two Li phases and one phase, which is called stationary phaseThe other phase flows through the stationary phase and is called the mobile phase in a different degree of distribution.

Summary. Hello dear<>

The answer you're looking for: Hello classmates, <>

Substances with high boiling points and easy sublimation can be purified by the following methods:1Crystallization method:

The substance to be purified is dissolved in an appropriate amount of solvent, the dissolved substance is heated to a saturated state, and then slowly cooled to make the solute gradually crystallize. The crystalline substance is obtained by filtration or centrifugation, which is then washed and dried with a pure solvent to finally obtain a pure substance. 2.

Sublimation method: The substance to be purified is heated to its sublimation temperature, so that it is directly converted from a solid state to a gaseous state, and then the gas is collected in a condenser and cooled into a solid state. This removes impurities and obtains a pure substance.

How to purify substances with high boiling point and easy to sublimate?

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The answer you're looking for: Hello classmates, <>

Substances with high boiling points and easy sublimation can be purified by the following methods:1Crystallization Fahuna:

The substance to be purified is dissolved in an appropriate amount of solvent, the dissolved substance is heated to a saturated state, and then slowly cooled to make the solute gradually crystallize. The crystalline substance is obtained by filtration or centrifugation, and then washed and dried with a pure solvent to finally obtain a pure substance. 2.

Sublimation method: The substance to be purified is heated to its sublimation temperature, so that it is directly converted from a solid state to a gaseous state, and then the gas is collected in a condenser and cooled into a solid state. This removes impurities and obtains a pure substance.

Hello dear<>

The answer you're looking for:3Recrystallization method nuclear locust:

If the crystallization solution method cannot achieve sufficient purification effect, the recrystallization method can be used. First, a crystallization is carried out, and then the obtained crystallization substance is dissolved again in an appropriate amount of dissolving eggplant, and the crystallization step is repeated to improve the purity through multiple crystallization. 4.

Vacuum distillation: For substances that are prone to sublimation, vacuum distillation can be used for purification. The substance is sublimated at a lower temperature by heating under reduced pressure conditions, and then the pure sublimated substance is collected through a condenser.

The change of matter directly from a solid state to a gaseous state is called sublimation, and the critical temperature at which sublimation occurs is called the sublimation point. Generally, the sublimation point of a substance that can sublimate is lower than the melting point.

It is because of the rise of iodine.

The bloom point and the melting point are relatively close, and the vapor pressure of iodine increases dramatically with the increase of temperature. The melting point of iodine is , and the triple point of iodine is , and the vapor pressure is kPa. At the three-phase point O, solid, liquid, and gaseous iodine coexist.

Obviously, the higher the vapor pressure at the three-phase point, the easier it is for the solid substance to sublimate. Solids of iodine, which are usually heated in open containers, do not reach 11 due to the constant escape of iodine vapors. With an air pressure of 9 kpa, the solid iodine is sublimated without melting.

To obtain liquid iodine, a vapor pressure of more than 11 is created.

Conditions of 9 kpa. Liquid iodine can be obtained by heating a sufficient amount of iodine in a fine-mouth distillation flask. In our general iodine sublimation experiments, if the heating is relatively fast, the iodine vapor is relatively concentrated.

I've never heard of the conclusion of the first sentence of your question, or your first sentence is completely incomplete

In fact, I blindly guess that the first sentence of your Sencha is to say that the object that can be melted, if it reaches the melting point, it will only observe melting, and not sublimation, in fact, this conclusion is fundamentally wrong, even if it is ice, placed in an environment of 20 degrees, it should also be sublimated and melted at the same time. Of course, we can only clearly observe this waxing process.

Objects that exceed the boiling point are unconditionally sublimated directly.

The reasons why the temperature is constant below the boiling point during purification are as follows.

1.If the heating is too strong and the distillation speed is too fast, the steam will be overheated, and the measured boiling point will be too high (insufficient heating, the distillation speed will be slow, and the measured boiling point will be low or irregular.

2.The internal pressure of the device is stronger than the standard atmospheric pressure, and the higher the pressure is, the higher the boiling point.

3.The distillation flask is not cleaned, dry and sold clean, or the reagent is not pure.

It is related to temperature, pressure, and composition. The melting point is usually at atmospheric pressure. If there is no liquid phase at atmospheric pressure, it does not matter the melting point, or it should be noted that it is under certain pressure.

The melting point can be measured by the change of certain parameters, such as the endothermic and exothermic curve, which can be obtained after mathematical processing where there are mutations.

What are the precautions for measuring the melting point of compounds that are prone to sublimation or moisture absorption?

sublimation substance, the upper end is closed after the sample is loaded; For compounds that are easy to absorb moisture, the loading action should be fast, and the upper end should be heated and closed on a low fire immediately after installation, so as to avoid reducing the melting point of the sample due to moisture absorption when measuring the melting point.

1. The melting point of most covalent compounds is lower than the boiling point 2. Substances that are easy to sublimate, such as aluminum chloride, in order to determine their melting point, are usually determined under pressure conditions, so their melting point is not determined by atmospheric pressure, that is, it is relatively high.

Analysis of the images shows that: (1) the temperature of the substance remains unchanged at 340, and the temperature is unchanged at this time, so the melting point of the substance is 340 from the beginning of the melting of the 6th minute to the end of the 14th minute, because the potato has experienced a total of 8 minutes

2) When the temperature of the substance reaches 360, the state has already melted all of the substance, and its state is liquid

So the answer is: 340; 8；Liquid

A substance sublimates because it is under a specific pressure and temperature. Under a specific pressure (the three-phase point pressure of the substance), its phase transition temperature is called the "sublimation point" ("condensation point"), and at this pressure it has no "melting point" or "boiling point", and can only be converted in this way from "solid" to "gaseous".

When the elevated pressure is higher than the three-phase point pressure of the substance, the phase transition of the substance is the solid-liquid-gas transition, and it has a "melting point" (freezing point) and a "boiling point" (liquefaction point) at this pressure, but no "sublimation point" ("freezing point").

The conditions for sublimation and condensation of a pure substance are: the temperature and pressure of the vapor of the pure substance must be below the temperature and pressure of the three-phase point of the substance. If the vapor pressure of a solid pure substance is lower than its three-phase point pressure, the solid substance will be sublimated directly into a gaseous state when the temperature rises.

Conversely, if the pressure of a gaseous pure substance is lower than its three-phase point pressure. When the temperature is lowered below its three-phase point temperature, the gaseous substance will condense directly into a solid state.

Take a look at the following phase diagram of a substance, which is the best way to describe the change in the state of matter in a complete way:

Three-phase diagram of CO2.

Turn left|Turn right.

Three-phase diagram of water.

Turn left|Turn right.

Three-phase diagram of iodine.