Problems related to the separation of substances from chemistry

Actually, I think this answer is quite unreliable...

Potassium nitrate mixed with calcium hydroxide is to be purified.

Excess K2CO3 was added to filter first, and then excess nitric acid was added to the filtrate, and pure potassium nitrate was obtained after the reaction was completed.

The theoretical basis of the answer is that the solubility of calcium hydroxide decreases with the increase of temperature, and the calcium hydroxide will precipitate during the heating of the solution, and it will be "purified" when it is filtered out while it is hot.

Then you can understand why you can't "heat and cool", right? After cooling, the precipitation does not dissolve back, and how to remove it without filtration.

The method of recrystallization to purify the solution is to be filtered, and there is no "heating and cooling" method.

Calcium hydroxide itself is a microsoluble, its solubility is originally very small, by reducing the solubility precipitation, in fact, there is still calcium hydroxide in the solution, not much is removed at all.

According to the law of solubility, the solubility of potassium nitrate increases rapidly with increasing temperature, but the solubility of calcium hydroxide decreases with increasing temperature (this is a special case, to remember), then after the temperature rises, the calcium hydroxide can be filtered out while it is hot.

The one you are talking about is the separation of potassium nitrate and sodium chloride, the solubility of sodium chloride does not change much with the increase of temperature, but the potassium nitrate can be precipitated after the temperature decreases, and the potassium nitrate can be filtered out.

？Isn't purifying potassium nitrate crystallized by cooling?

The solubility of calcium hydroxide decreases with the increase of temperature, but it will not be completely precipitated in any case, and its solubility is less affected by temperature, so it is impossible to purify calcium hydroxide by heating method.

Thermal filtration after heating is because the solubility of calcium hydroxide decreases with the increase of temperature, and calcium hydroxide precipitates after heating, and after thermal filtration, it becomes purer potassium nitrate. If it is heated and cooled, the calcium hydroxide that was originally precipitated will not have time to dissolve, and although potassium nitrate is precipitated, it will be mixed with calcium hydroxide.

In terms of method, the former is suitable for a substance whose solubility decreases significantly with the increase of temperature, and the latter is suitable for the solubility of a substance that increases significantly with the increase of temperature.

Which answer in the rock is stupid]: c

The test point of this question is the principle of separation of different chromatographic methods. In the above options, polyamide chromatography separates according to the principle of hydrogen bond adsorption; Gel filtration separates molecules according to their size; Ion-exchange chromatography separates according to the degree of dissociation; Silica gel column chromatography belongs to adsorption chromatography, which separates according to the difference in adsorption; Paper chromatography is a partition chromatography method that separates coarse codes according to the different partition ratios of substances in two-phase solvents.

1. Recrystallization method: one substance is easily soluble in water, and the other substance is insoluble in water. A mixture of these two substances can be dissolved in water and then filtered, and the two substances can be separated.

For example, to separate NaCl and BaSo4, BaSo4 is insoluble in water, and NaCl is easily soluble in water, so it can be separated.

2. Cooling hot saturated solution method: there is a big difference according to the solubility of the two substances in water. A mixture of these two substances is formulated into a mixed solution, and then the less soluble substance crystallizes and precipitates the temperature suddenly, so that the two substances are separated.

For example, the solubility of Kno3 varies greatly with temperature, and the cooling hot saturated solution can precipitate Kno3 solids.

3. Gas precipitation method: in the mixture of two substances, one substance can react with other substances to produce precipitation or gas, while the other substance does not react, and use this property to separate the two substances.

For example, to separate Baso4 and BaCO3, Baso4 is insoluble in dilute hydrochloric acid, and BaCO3 is soluble in dilute hydrochloric acid.

It is related to the melting point, solubility, and optical rotation of the substance, so please elaborate.

Extraction: Bromine water is extracted using tetrachloroform Reason: BR2 and CCL4 are non-polar substances and water is a polar solvent, according to the principle of similar miscibility, BR2 has a high solubility in CCL4.

Distillation: When the cadmium in zinc is removed by industry, the mixture is heated to above 765 (Cd boiling point) and below 907 (Zn boiling point), which can remove Cd impurities in Zn.

Different solubility: when K2Cr2O7 is prepared, KCL is added to the hot Na2Cr2O7 aqueous solution and cooled, the solubility of potassium dichromate is lower than that of NaCl at low temperature, and it can be filtered and separated.

Different structure: 5A molecular sieve can separate n-butane and isobutane, because n-butane is a straight chain structure and isobutane has branched chains.

Different magnetism: Copper powder and iron powder can be separated by magnets when mixed. Reason: Iron powder is magnetic.

Noodles are the distillation process, to put it simply:

1. To separate the liquid mixture, only when the boiling point of each component in the mixture is quite different, a more effective separation can be achieved.

2. Determine the boiling point of the pure compound.

3. Purification, by distilling substances containing a small amount of impurities, to improve their purity;

4** solvent, or evaporate part of the solvent to concentrate the solution.

Evaporation is a very common natural phenomenon.

But when the concept of "evaporation" is condensed as: "Evaporation is the evaporation of a solvent by heating to obtain a solid solute." At this time, I felt that the concept of evaporation had lost itself.

Teacher Zixu taught us that an important indicator of the reason why smart people are smart is: association.

Mention: the concept of evaporation, whether it can be associated with a beautiful rainbow after rain; Think of the blue sky, the white clouds; The beautiful illusion of a mirage; It is associated with drying salt with seawater; It is associated with drying clothes, boiling water, cooking rice, ......, has always been associated with a delicate flower that was picked after losing its moisture and evaporating from the world.

Let us enrich the association, let us have flesh and blood, fresh jump, and infinite vitality.

The concepts of evaporation, concentration, crystallization, and recrystallization are not difficult to understand in the specific scenario of coarse salt purification.

Evaporation: The operation of separating the difficult volatile solute from the solution and evaporating the volatile solvent. For example, the water in the solution of the remaining sedan chair in the brine of the food vertical is evaporated and the salt is crystallized.

Premise sodium chloride is difficult to volatile). Bromine iodine cannot be extracted from bromine iodine water by evaporation. (Liquid bromine is volatile, iodine can be sublimated.)

Instrumentation and operation of evaporation.

Note: The liquid poured into the evaporation dish is generally not able to evaporate 2 3 of the volume of the dish, and when it evaporates to near boiling, it is necessary to stir continuously with a glass rod to prevent the droplets from splashing due to excessive local temperature. When a large amount of solids appears in the evaporation dish, the heating is stopped and the residual heat continues to evaporate the remaining small amount of liquid until it is dry.

It is to separate the two substances, and if you want to get pure water, you can use the distillation method. Distillation flasks are used for distillation, and extraction flasks are used for extraction.

To put it simply, what is needed for distillation is what is steamed out (with a distillation flask), and what is needed for evaporation is what is left (with an evaporation dish).

It can be discussed according to one aspect of the chemical separation method. For example:

Extraction. Dispensing. Fractionation. Distillation. What are the conditions for which evaporation applies?

Extraction: suitable for a solute in two immiscible solvents in the solubility of the situation is very different, if you want to extract this solute from the solvent with less solubility, choose the extraction method, such as carbon tetrachloride and water are not miscible with each other, and carbon tetrachloride is easier to dissolve iodine than water, so you can use a separating funnel to extract iodine elements from saturated iodine water, dissolve into carbon tetrachloride, and then separate iodine and carbon tetrachloride by distillation.

In fact, "extraction" and "separation" are often used together, otherwise the solute cannot be separated. If necessary, it is also used in combination with "distillation", as in the case above, to completely separate the solute and solvent.

Dispensing: It is suitable for separating two liquids that are immiscible with large density differences. For example, a mixture of water and vegetable oil will be layered up and down after standing, with water in the lower layer and vegetable oil in the upper layer.

At this time, you can use a separating funnel to open the piston to the lower layer of water just when it runs out, and use a beaker to undertake; Open the glass stopper again and let the vegetable oil pour out of the mouth.

Fractionation: It is suitable for separating liquid mixtures with similar boiling points, such as petroleum is obtained by fractional distillation, which are important chemical products: gasoline, diesel, alcohol, etc.

Distillation: It is suitable for separating liquid mixtures with large differences in boiling points, and separating them in the order of boiling points from low to high by using the different boiling points of each component. or remove volatile, difficult to volatile or non-volatile impurities.

For example, the method of distillation is used to reduce the cl- impurities in tap water. Or separate the solvent in the solution, such as salt water by distillation to obtain purer distilled water.

Evaporation: It is suitable for separating volatile solvents and solutes in a solution, which is similar to the second case of "distillation" I said, but if you want to get pure table salt instead of water, evaporation is generally used instead of distillation. There is also what I call the combination of "extraction" and "evaporation".

In short, there are many chemical separation methods, and it is necessary to choose the appropriate method according to the situation, and some methods are interconnected, and the method should be selected in accordance with the principles of science, safety and simplicity.

a. Ice and dry ice: When exposed, the original substance that produces obvious white mist and no liquid appears is dry ice, while the original substance that melts into liquid is ice can be distinguished

B. Use soap to rent bucket water to identify hard water and soft water: take 50ml of each drop to add the same amount of soapy water, stir, and observe how much foam is produced

c. Use the same volume of water to distinguish quicklime and hydrated lime and measure the temperature: the raw material with obvious temperature increase of the liquid is quicklime, and the raw material without obvious temperature change is hydrated lime

d. Use phenolphthalein to distinguish caustic soda and soda ash: because both are alkaline, they can make phenolphthalein type turn red, so it cannot be identified

Therefore, choose D

1.Crystallization and recrystallization.

Utilizes the solubility of a substance in solution.

It varies greatly with temperature, such as NaCl, Kno3.

2.Distillation cooling method: Envy Yuliang has a large difference in boiling point. Brother B transports alcohol.

Medium (water): Add freshly made CAO to absorb most of the water and then distill.

3.Filtration method: dissolution and dismantling filial piety are insoluble.

4.Sublimation method: SiO2 (I2).

5.Extraction method: For example, Ccl4 is used to extract I2 in water.

6.Dissolution method: Fe powder (A1 powder): dissolved in excess NaOH solution and filtered and separated.

7.Addition method: convert impurities into the desired substance: CO2 (CO): Cuo through heat; CO2 (SO2): by NaHCO3 solution.

8.Absorption method: used to remove gas impurities in the mixed gas, gas impurities must be absorbed by the drug: N2 (O2): The mixed gas absorbs O2 through the copper mesh.

9.Transformation method: it is difficult to separate the two substances directly, and the added drug becomes easy to separate, and then reduced back:

Al(OH)3, Fe(OH)3: Add NaOH solution to dissolve Al(OH)3, filter, remove Fe(OH)3, and then add acid to convert NaALO2 into A1(OH)3