What is the difference between crystallization and recrystallization in high school chemistry

Crystallization is a process: it refers to the process by which crystals are separated from a solvent (only the liquid phase, but also the gas phase in a broad sense) in the form of a solid to form a single compound.

Recrystallization is often an operation method, which refers to the crystallization of crystals, and then dissolved in solvents, by changing other physical conditions (most of which refers to temperature) to dissolve it all, and then changing the physical conditions back to the original state, then the crystals will crystallize again: this process is recrystallization, mainly for the purification of crystals.

"Crystallization" refers to the first crystallization, that is, when a compound is prepared from a solution, the solvent is heated and evaporated, and then cooled, so that the target substance is first crystallized and precipitated from the solution, and then filtered to prepare the compound.

"Recrystallization" means that the compound prepared by the first crystallization is not pure, continues to dissolve and recrystallizes out of the solution, at this time, because the amount of impurities is much smaller than that of the target substance compared with the first preparation, the impurities will not precipitate, so it plays a role in purifying the target compound.

However, recrystallization will cause the loss of the target substance, especially the solubility is larger, so it is necessary to evaporate the solution as much as possible to reduce this loss, but the problem is that the impurities may reach saturation again, even if it is not saturated, the concentration will become larger, so that when filtering, the solution containing very concentrated impurities will be adsorbed on the crystallization of the target substance, so a recrystallization can not make the target compound very pure, so it is necessary to continue to recrystallize twice, tertiorily, or even more times, once per recrystallization, It was purified once.

The recrystallization method is suitable for purifying substances with small solubility at low temperature and significantly increased with increasing temperature. When the temperature of the hot solution gradually decreases, the solubility of the target substances dissolved in the solution drops sharply and a large amount of precipitation is released, while the impurities are generally substances that still have a large solubility at low temperatures (even if they are not, they will not reach saturation and precipitate due to the small amount in the recrystallization process).

Recrystallization is the removal of impurities by cooling hot saturated solution method in junior high school.

Whereas, crystallization is the method of separating the solute from the solvent.

There is an essential difference between the two, but the operation is similar.

1. Crystallization method: solubility.

For solutions that are not greatly affected by temperature, use evaporative crystallization (e.g., NaCl). Solutions whose solubility is highly affected by temperature use the cooling crystallization method, also known as the cooling saturated solution method (e.g., calcium hydroxide).

2. Recrystallization.

Method: evaporation crystallization, heating and concentration crystallization, which is also the same ion effect of evaporation or water absorption.

Crystallization, salt is added to crystallize by changing the concentration of ions, and the crystalline substance can be dissolved with solvent and crystallized again.

3. Graded crystallization method: after the crystallization is recrystallized, each part of the mother liquor can be obtained after treatment, and the second and third batches of crystallization can be obtained respectively.

4. Distributed crystallization method: crystalline substances are crystallized repeatedly.

In the process, the precipitation of crystals is always faster and faster, and the purity is getting higher and higher. The purity of the crystals obtained by the step-by-step crystallization method is often quite different, but more than one crystalline component can often be obtained, and it should not be mixed together rashly without inspection.

1. The structure is different. Crystallization refers to the first crystallization, that is, when preparing a compound from a solution, the solvent is heated and evaporated, and then cooled, so that the target substance is first crystallized and precipitated from the solution, and then filtered, so as to prepare the compound; Recrystallization refers to the process of dissolving crystals in a solvent or melting, and then recrystallizing them from a solution or melt.

2. The process is different. Crystallization is equal to the transformation of melted crystals into solid crystals, i.e., liquid to solid state; Recrystallization is equal to the crystals first dissolving into a liquid state and then crystallization, i.e., solid to liquid to solid state.

1. The structure is different. Crystallization refers to the first crystallization, that is, when the hidden compound is made from the solution, the solvent is heated and evaporated, and then cooled, so that the target substance is first crystallized and precipitated from the solution, and then filtered, so as to prepare the compound; Recrystallization refers to the process of dissolving crystals in a solvent or melting, and then recrystallizing them from a solution or melt.

2. The process is different. Crystallization is equal to the transformation of melted crystals into solid crystals, that is, the liquid state travels to the solid state; Re-knotting is equivalent to the crystals first dissolving into a liquid state and then crystallizing, that is, solid to liquid and then to solid state.

The methods of crystallization include removing the crystallization of the solvent and not removing the solvent crystal. The removal of solvent crystallinity is to evaporate the solvent and internalize the steam, the solution is concentrated to reach supersaturation, and the solubility decreases with the decrease of temperature, and the crystallization is precipitated. Crystallization without removing solvent is to crystallize the solution by cooling it to supersaturation.

There is no way to return to which method is used to crystallize, and the purity still does not meet the requirements, so the crystallization is carried out again in order to improve the purity, which is called recrystallization or recrystallization.

Crystallization is the precipitation of solute from the solvent under certain conditions, and recrystallization, as the name suggests, is to add the solute to the saturated solution of the same solute, and then change the conditions to make the solution become an unsaturated solution, so as to dissolve the added solute, and then restore the condition to recrystallize the solute.

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Later, the solute is precipitated in the form of crystals.

This DAO process is called crystallization.

Recrystallization is the process of dissolving the inner crystal body in a solvent or melting, and then recrystallizing it from the solution or melt. Also known as recrystallization. Recrystallization allows impurities to be purified or salts that are mixed together separate from each other.

Crystallization is the separation of a substance from a mixed system in the form of crystals. Recrystallization is a means of improving purity and reducing impurities.

To put it simply, crystallization is the original solution, allowing the solute to precipitate in solid form;

Recrystallization, that is, it is originally a solid (crystal), which may contain impurities, dissolved in water to form a solution, and then the solute is precipitated to re-obtain the crystal.

Crystallization and recrystallization result from the difference in purity and type of crystal.