Flame color reaction, also known as flame color test and flame color test, is the reaction that some metals or their compounds burn in a colorless flame to make the flame show a special color.The principle is that each element has its own spectrum.The sample is usually in the form of powder or small pieces.Use a clean and less active metal wire (such as platinum or nickel chromium alloy) to hold the sample, and then put it into a flameless flame (blue flame).In chemistry, it is often used to test whether a certain metal exists in a compound.At the same time, using flame reaction, peopleFireworksConsciously add specificmetallic element, makeFireworksMore colorful.
Flame reaction is a physical change.It does not generate new substances. The flame reaction is the change of the electronic energy level in the atom of the substance, which is generally the change of the electronic energy in the atom, and does not involve the change of the structure and chemical properties of the substance.Flame color reaction is the reaction that some metals or their volatile compounds burn in a colorless flame to make the flame show a characteristic color.Some metals or their compounds can give the flame a special color when burning.
For flame reactionPlatinum wire(Nickel wire, iron wire).Embed inGlass rodThe wire on theDilute hydrochloric acidinDip washingLater, (this is becausemetallic oxideThe chloride generated by reaction with hydrochloric acid is easy to vaporize and volatilize when burning;If sulfuric acid is used, due to the high boiling point of the generated sulfate, a small amount of impurities are difficult to be removed and interfere with the color of the flame)Alcohol lampFlame (preferablyGas lamp, because its flame color is light and its temperature is high, if not, it can be usedAlcohol blowtorchIt can also be burned in) until the color of the original flame is the same, then dip the metal wire in the tested solution and put it on the flame, then you can see the characteristic flame color of the elements contained in the tested solution.
Flame reaction is a very ancientQualitative analysis methodAs early as the Southern and Northern Dynasties in China, famous alchemists and medical mastersTao Hongjing(456-536)《Collection of Materia Medica》There is such a record that "fire burns it, purple smoke rises, and clouds are real saltpeter (potassium nitrate)".Due to the low level of productivity at that time and for many years after, this method has not been widely applied and developed.
In the modern chemical period in Europe after the 18th century, due to the great development of metallurgy and machinery industry, it was required to provide more ores in quantity and variety;At the same time, in order to reduce production costs, reasonably use raw materials and improve product quality, new requirements are put forward for analytical chemistry.a germanMagraf(1709~1782) was a famous qualitative analytical chemist of this period.One of his important research achievements is that he has observed alkaloids（Vegetation ash, namely potassium carbonate) and mineral alkali (soda, namely sodium carbonate).In 1762, he systematically compared the various types of alkalisylviteAnd sodium saltDeliquescenceAnd solubility, and found that sodium salt and potassium salt can respectively make the flame color with their own characteristics.Since then, it has become a common method to identify potassium and sodium salts by flame reaction.Later, many people also noticed that many salts and oxides can also show different colors in the flame. For example, in 1818, Gemelin found that lithium salt was dark red and copper salt was blue-green, but he did not understand the reason.Both lithium salt and strontium salt make the flame red, which affects the reliability of the flame reaction test material.
Flame reaction
In the middle of the 19th century, the famous German chemist Bunsen (1811-1899) designed and manufactured the Bunsen burner, which made the gas produce almost colorless flame when burning, with a temperature of more than 1000 degrees.Bunsen used this lamp to study the phenomenon that various salts present different flame colors in the flame, and tried to detect various elements according to the color signals in the flame.He lit three gas lamps at the same time, and dropped salt solution into each flame.
One drop is saturated salt solution, the other is mixed with lithium salt, and the third is mixed with potassium salt.Results The three flames were all yellow, and no difference could be seen.Obviously, the yellow of the sodium flame obscures the other colors.Bunsen also observed the flame through blue glass or indigo solution as a color filter, and found that the yellow was filtered out, the flame with saturated salt solution dripping became colorless, the flame of salt solution mixed with lithium salt was dark red, and the flame mixed with potassium salt was light purple.Later, he collected many glasses of different colors and prepared many solutions of different colors as color filtering materials to try to improve the selectivity of the flame reaction to distinguish the dark red color of lithium salt and strontium salt in the fire, but failed.Obviously, the identification of elements by naked eye observation of flame color is greatly limited.We can only identify a few metals such as potassium and sodium by flame reaction, and the blue cobalt glass used to observe the flame color of potassium also comes from Bunsen's experiment.
In addition to using gas flame, Bunsen also uses coal flame, hydrogen oxygen flame, hydrogen flame, etc.After a detailed study of the flame reaction, he also found that even if an element is in a different compound, even if it occurs in the flameChemical changeEven if the temperature of the flame is different and the type of flame used is different, these factors have no effect on the characteristic flame color of an element.
Later, I was a good physicistKirchhoffUnder the suggestion of, we realized the qualitative inspection of elements by observing spectra, and created an important branch of analytical chemistry: spectral analysis.
reason
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Flame color reaction is a chemical experiment method to test these metal ions according to the fact that some metals or their volatile compounds will present different colors of flame when burning in a colorless flame, so as to judge whether these metals or metal compounds are contained in the substance.[1]
Whenalkali metalWhen its salt is burned on the flame, the electrons in the atom absorb energy from the lower energy orbittransitionTo the orbit with higher energy, but the electron in the orbit with higher energy is unstable, and soon jumps back to the orbit with lower energy. At this time, the excess energy will be released in the form of light.The wavelength of the emitted light isvisible lightRange (wavelength: 400nm~760nm), so the flame can be colored.In the flame reaction experiment, different metals or their compounds will emit a variety of light with different wavelengths when burning. Within the visible light range that can be perceived by the naked eye, due to the different wavelengths of different light, the colors presented will be different.[2]However, due to the different atomic structures of alkali metals,Electron transitionWhen the energy changes differently, it emits light of different wavelengths. The special flame color seen from the flame reaction experiment is the color of spectral lines.The spectrum of each element has some characteristic spectral lines, which emit characteristic colors to color the flame. The existence of an element can be judged according to the flame color.Such as flame magenta containsstrontiumElement: the flame color blue-green contains copper, the flame color yellow contains sodium, the flame color purple contains potassium, and the brick red contains calcium.
① Dip the platinum wire in dilute hydrochloric acid and burn it on a colorless flame until it is colorless; ②Dip the sample (the solid can also be dipped directly), burn it on the colorless flame, and observe the flame color (if the potassium is tested, it needs to penetrateBlue cobalt glassIt is observed that since sodium is required for potassium production in most cases, the potassium ion solution often contains sodium ions, and the flame reaction of sodium is yellow, and yellow and a small amount of purple cannot be distinguished). ③Dip the platinum wire in dilute hydrochloric acid and burn it until colorless, and then continue to do new experiments.
If the flame is yellow when reacting with flame color, it means that the color of sodium combustion in the glass obscures the color of metal combustion.
application
(1) Flame reaction can be used to test some metal elements that cannot be identified by conventional chemical methods.
(2) Different metals and their compounds correspond to different flame color reactions and are colorful, so they can be used to make fireworks for festivals[3]。
First prepare a platinum or nickel chromium wire, iron wire, cobalt blue glass and salt or its solution.
Immerse the coil in concentrated hydrochloric acid to remove the remaining substances before, and then place the coil in a flameless flame (blue flame) until there is no color change.
Use distilled water orDeionized waterOr pure water washing line.
Use the coil to contact the salt or solution, and heat it through the flameless (blue flame).
When sodium ion exists in the tested solution, filter the flame color of sodium ion with cobalt blue glass.
Finally, the flame color will be observed.The flame color of sodium is bright golden yellow flame.
The flame reaction of sodium should not be difficult to do, but actually it is the most troublesome.Because the flame color of sodium is yellow, while the flame of alcohol lamp is mostly yellow due to unclean lamp holder and impure alcohol.Even if it is a nearly colorless (light blue) flame, a new iron wire (or nickel wire, platinum wire) is placed on the external flame to burn, and the flame is yellow at the beginning, it is difficult to explain whether the flame color is sodium ion or the flame color of the original alcohol lamp.To clearly see the yellow flame of sodium, the following methods can be used. ⑴Method 1 (tweezers cotton alcohol method): take a small ball of cotton (degreased cotton, the same below) with tweezers and suck a little alcohol (95% ethanol, the same below), squeeze the alcohol on the cotton dry, dip the cotton with some sodium chloride or anhydrous sodium carbonate powder (finely ground) and ignite it.
(2) Method 2 (iron wire method): ① Take a thin iron wire, wipe one end with sandpaper, and burn it on the external flame of the alcohol lamp until there is no yellow flame; ② Dip the iron wire at this end in water, and then dip some sodium chloride orAnhydrous sodium carbonatePowder, ③ light a new alcohol lamp (the lamp cap and wick are clean and alcohol is pure), ④ put the iron wire stained with sodium salt powder on the outer flame tip and burn it, then there is a small yellow flame on the outer flame tip, that is, sodium flame.The above method is easy for teachers to demonstrate the experiment, but it is difficult to see the flame tip in students' experiment because most of the alcohol lamps are not clean. It can be changed to the following method: put the iron wire stained with sodium salt on any part of the outer flame with a blue flame and burn it. If the yellow flame covers the blue flame, it can be considered that the yellow flame is the sodium flame.
Potassium ion
⑴ Method 1 (beaker alcohol method):
Flame reaction
Take a small spoon of anhydrous potassium carbonate powder (fully ground) and put it on an inverted small beaker. Add 5~6 drops of alcohol and ignite it. You can see an obvious light purple flame. If you observe it across a cobalt glass slice, you can see a more obvious purple flame.
(2) Method 2 (wire method):
Same as the student experiment method in Method 2 of sodium.The effect of this method is not as good as that of methods 1, 2 and 3, but it is close to that of textbooks.
When observing the color of potassium flame, the indoor light should not be too strong, otherwise the light purple potassium flame is not obvious. If you want to avoid the impact of sodium salt on the observation of potassium salt, you should passBlue cobalt glassObserve the flame color.[4]
Lithium ion
Flame reaction
⑴ Method 1 (tweezers cotton alcohol method):
Take a ball of cotton with tweezers, suck up alcohol, squeeze the alcohol dry, and coat the cotton with LitwoCOthreePowder, ignited.
(2) Method 2 (wire method): the same as method 2 of sodium.
calcium ion
⑴ Method 1 (tweezers cotton alcohol method):
Same as method I of potassium.
(2) Method 2 (beaker alcohol method):
Take a spoonful of finely ground anhydrous calcium chloride powder (absorb a small amount of water, if there is really no water, let it absorb moisture in the air for a while), put it on an inverted small beaker, add 7-8 drops of alcohol, and ignite it.
⑶ Method 3 (spoon method): use a stainless steel spoon to put a little anhydrous calcium chloride (as above) on the external flame of the alcohol lamp for burning.
Strontium ion
Same method as lithium carbonate.
Barium ion
(1) Method 1 (iron wire cotton water method):
Put a small amount of finely ground barium chloride powder into a small evaporating dish, add one or two drops of water to make a paste, take a small iron wire, wipe one end with sandpaper, bend a small circle, and clip a small ball of cotton in the circle. After the cotton is saturated with water, squeeze it dry again, dip the ball of cotton with the above paste barium chloride, and put it on the outer flame below the alcohol lamp flame to burn, and you can see the obvious yellow green barium flame.
(2) Method 2 (cotton water beaker method):
Similar to method 1, a small ball of cotton is soaked with water and squeezed dry, covered with paste barium chloride, placed on an inverted beaker, and seven or eight drops of alcohol are added to ignite.It can be compared with cotton+alcohol combustion.
Copper ion
Flame reaction
(1) Method 1 (metal wire cotton water method): the same as method 1 of barium ion.
⑵ Method 2 (tweezers cotton alcohol method): the same as sodium ion method.
⑶ Method 3 (beaker alcohol method): the same as method 1 of potassium ion.
⑷ Method 4 (spoon method): the same as method 3 of calcium ion.
summary
The phenomenon of flame reaction should be obvious, and the flame color should look like the comet tail before it can be seen clearly. The reason why some salt's flame reaction needs to be dissolved in a small amount of water is that the ions will volatilize into the comet tail shape with the evaporation of water when burning, which is obvious;Some ions are easy to volatilize into comet tail when burning, so it is unnecessary to add water to dissolve them.
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Windscreen clip can prevent flame interference
For common modelsAlcohol lamp, take a piece of 14 × 7.5cmtwoThe size of thin iron sheet is folded twice according to 2.5cm, becoming 2.5cm in length and width, and 14cm in heightWindproof clip。Bend the lower end slightly and clamp it on the alcohol lamp when using.Then take a few rustless iron wires and fix them on the glass rods respectively, and affix the marks.Each wire is specially used to dip a sample.As a substitute for platinum wire (due to limited conditions, many schools do not have so many platinum wires, especially in group experiments).[5]
Simplify operating procedures
When demonstrating the experiment in class, turn the empty side of the windshield clip on the alcohol lamp towards the students.Dip special iron wire into correspondingsample。Instead of "burn first... wash with dilute hydrochloric acid every time you use it...", place it directly on the external flame for burning.When demonstrating the flame reaction of potassium, first dip a small amount of synthetic glue with iron wire, then "stick" a small amount of potassium chlorate solid to burn directly, place the cobalt glass slide on the empty part of the windshield clip for observation, and repeat the experiment after the iron wire is slightly cold.This method can also be used for other objects that are easily interfered by external conditions and whose flame display is not obvious.After the experiment, remove the windshield clamp and turn off the alcohol lamp.[3]
Improved advantages
Flame reaction
The addition of wind clips can not only avoid the interference of the alcohol lamp flame from the wind during the experiment, but also effectively increase the length of the flame without blocking the students' vision, and can also form a background color, which is conducive to comparative observation;Using special iron wire to dip the sample solves the problem of lack of platinum wire and simplifies the operation.The solid sample "glued" with synthetic glue for burning will not affect the flame color observation, but also can effectively prevent the sample from splashing and extend the flame color time, which is simple and practical.
other
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The flame reaction of alkali metals with other metals and their corresponding ions can be used to analyze the composition of substances and identify related substances.E.g. sodium or containing Na+The flame color reaction of the compound is yellow;Potassium or K+The flame color reaction of the compound is light purple (through cobalt glass).
Magnesium, aluminum, iron, platinum, nickel and other metals are flameless.
All kinds of visible light released in the flame reaction are essentially due to the high temperature of the heat source. When heated, the metal ions in the flame reaction will increase in energy, which will stimulate the electronic transition. The energy difference between energy levels will be released in the form of visible light. This process does not generate new substances, but only the electronic transition, which should belong to physical changes.Therefore, although it is called flame reaction, it is actually not a chemical change, but a physical change.
The flame reaction can only be used to wash the platinum wire with hydrochloric acid
Cause: metal chlorides are generated, and general metal chlorides are volatile at high temperatures.
matters needing attention
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During the experiment, for unknown liquids, flame reaction is used to detect ions, because the solution may contain other toxic substances, which may volatilize after heating, or toxic substances may be generated during heating, which may cause harm to the experimenters.
