Name: flame test metal color clues
Uploaded: 2020-03-26T07:22:25.000Z
Duration: 2 min 47 s
Channel: JoVE

K12 Standards

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Chemistry and Physics

IPC.7

IPC.7.E

Metal Flame Emission

Le test de flamme est une technique analytique dans laquelle un échantillon est appliqué sur une flamme et le spectre d’émission caractéristique est utilisé pour identifier différents éléments. Lorsque cette technique est utilisée pour identifier des espèces métalliques, on parle d’essai d’émission de flamme métallique. Lorsqu’une énergie suffisante est appliquée à certains métaux par le biais d’une flamme chaude, ils émettent une lumière d’une longueur d’onde ou d’une couleur caractéristique. Vous pouvez clairement voir ce phénomène lorsque vous regardez un feu d’artifice.
Différents métaux émettent des couleurs différentes. Ainsi, nous pouvons utiliser la couleur de la lumière émise pour déterminer le métal présent. Mais avant de plonger plus loin dans la technique, prenons un peu de recul. Les atomes ont plusieurs niveaux d’énergie pour leurs électrons, qui sont différents pour chaque élément.
Lorsqu’un électron est à son niveau d’énergie le plus bas possible, on dit qu’il est à son état fondamental. Lorsqu’un atome absorbe de l’énergie, il est excité et les électrons de l’atome passent de l’état fondamental à un niveau d’énergie supérieur. Lorsque les électrons se détendent jusqu’à un état inférieur, ou à l’état fondamental, l’énergie excédentaire gagnée est libérée sous forme de lumière émise.
La longueur d’onde de la lumière émise dépend du niveau d’énergie auquel l’électron a été excité et du niveau auquel il se détend. Cette longueur d’onde émise, ou couleur de la lumière, est spécifique à l’atome présent et est utilisée pour identifier un échantillon métallique dans le test d’émission de flamme métallique.
Dans le test d’émission de flamme métallique, nous appliquons un échantillon de métal à une flamme chaude et observons la couleur de la lumière émise. Par exemple, le baryum émettra une couleur vert jaunâtre dans la flamme, tandis que le cuivre émettra un bleu-vert et le potassium un violet rosé.
Maintenant, un échantillon se compose en fait de métal, d’ions métalliques, d’oxydes métalliques, d’hydroxydes métalliques et de sels métalliques. Et comme les atomes et les molécules absorbent et libèrent de l’énergie différemment, la lumière émise contient en fait une gamme de longueurs d’onde et d’intensités formant un spectre d’émission.
Un spectrophotomètre est utilisé pour mesurer la gamme de longueurs d’onde de la lumière émise. 
Dans cet atelier, vous utiliserez le test d'émission de flammes métalliques pour identifier divers métaux, puis analyserez la lumière émise et absorbée à l'aide d'un spectrophotomètre.

Émission de flammes métalliques

<h4>Émission de flammes métalliques</h4>
Le test à la flamme est une technique analytique dans laquelle un échantillon est placé dans une flamme et la couleur caractéristique de la flamme est utilisée pour identifier la substance. Chaque élément a une émission lumineuse caractéristique lorsqu’il est placé dans une flamme, ce qui signifie que chaque élément produit une couleur unique. Ce phénomène est utilisé dans les feux d’artifice, où la couleur des feux d’artifice correspond à une caractéristique spécifique d’un métal. L’émission de lumière colorée à partir d’un échantillon en combustion est le résultat direct de l’absorption par le composant métallique de l’énergie due à l’excitation de la flamme et de l’émission de lumière en conséquence.
<h4>L’excitation des électrons</h4>
Lorsqu’une espèce atomique ou moléculaire absorbe de l’énergie, celle-ci est utilisée de quatre manières différentes. Tout d’abord, l’énergie est utilisée dans la traduction, ce qui accélère le déplacement des molécules. Deuxièmement, l’énergie est utilisée dans les vibrations, ce qui fait que la distance entre les espèces change rapidement. Troisièmement, l’énergie provoque la rotation, qui induit la rotation des atomes autour des liaisons de la molécule. Enfin, l’énergie absorbée entraîne une excitation des électrons, ce qui fait que les électrons se déplacent vers un niveau d’énergie plus élevé à partir de l’état le plus stable ou fondamental.
Selon le modèle de Bohr de l’hydrogène, les électrons d’un atome existent dans des états discrets, qui correspondent à des couches individuelles ou à des orbitales autour du noyau. L’état d’énergie le plus bas est appelé état fondamental et est représenté par la notation n = 1. Les états d’énergie excités ont des énergies plus élevées et sont représentés par la notation n = 2, 3, 4, etc.
Pour que les électrons passent à un état d’énergie supérieur, ils doivent absorber une quantité d’énergie égale à la différence entre leur état fondamental et l’état d’énergie supérieur. Par exemple, si un électron absorbe une quantité d’énergie égale à la différence entre l’état fondamental et le niveau d’énergie n=3, l’électron se déplacera vers le niveau d’énergie n=3. Un électron peut se détendre spontanément jusqu’à l’état fondamental ou à tout autre niveau d’énergie inférieur. Lorsque cela se produit, un photon est émis, libérant l’énergie absorbée.
L’énergie libérée est émise sous forme de lumière. La lumière émise a une énergie caractéristique, et donc une longueur d’onde, qui est en corrélation avec les niveaux d’énergie de l’atome. La lumière visible, c’est-à-dire la lumière que les humains peuvent voir avec leurs yeux, varie d’environ 400 nm à 700 nm sur le spectre électromagnétique.
Les absorptions et les émissions atomiques sont des longueurs d’onde discrètes, appelées lignes. Ces lignes sont des caractéristiques uniques d’un élément, comme un code-barres, et peuvent être utilisées pour identifier l’élément.
<h4>Test d’émission de flamme métallique</h4>
Dans l’essai d’émission de flamme métallique, un échantillon de métal est placé dans une flamme. La flamme fournit l’énergie nécessaire pour exciter les électrons à un niveau d’énergie plus élevé. Lorsque les électrons se détendent pour revenir à l’état fondamental, la lumière est émise avec une énergie spécifique par rapport aux niveaux d’énergie des atomes de l’échantillon. Étant donné que différents atomes ont des niveaux d’énergie différents, l’énergie absorbée et émise par un échantillon, et donc la longueur d’onde, est spécifique à l’échantillon.
Les métaux ont des longueurs d’onde d’émission atomique caractéristiques dans le domaine visible qui sont facilement déterminées par inspection visuelle. Par exemple, le lithium émet une couleur rouge, le sodium émet une couleur jaune, le potassium émet une couleur rose-violet et le baryum émet une couleur jaune-vert.
Alors que les émissions atomiques sont des longueurs d’onde ou des lignes discrètes, la plupart des échantillons de métaux contiennent non seulement le métal, mais aussi divers ions métalliques, oxydes et sels. Étant donné que chaque atome absorbe et émet une longueur d’onde caractéristique de la lumière, la lumière absorbée et émise par le test de flamme contient une gamme de longueurs d’onde. Ainsi, les spectres d’absorption et d’émission atomiques peuvent être mesurés pour l’échantillon à l’aide d’un spectrophotomètre.
Les longueurs d’onde et les formes des spectres sont uniques pour chaque substance. Par exemple, l’intensité relative des caractéristiques du spectre dépend de la concentration de l’espèce. Les intensités absolues dépendent de la distance entre l’échantillon et le spectrophotomètre.
<h4>références</h4>
<ol>
	<li style="font-size :17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimie et réactivité chimique. Belmont, Californie : Brooks/Cole, Cengage Learning.</li>
	<li style="font-size :17px">Silderberg, M.S. (2009). Chimie : la nature moléculaire de la matière et du changement. Boston, MA : McGraw Hill.</li>
	<li style="font-size :17px">Harris, D.C. (2015). Analyse chimique quantitative. New York, NY : W.H. Freeman et compagnie.</li>
</ol>

Émission de flammes métalliques
Le test à la flamme est une technique analytique où un échantillon est placé dans une flamme, et la couleur caractéristique de la flamme est utilisée pour identifier ...

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flame test metal color clues

Flame testing uses a metal sample and a flame to identify a substance by its color. Each element gives off a characteristic light emission in the flame, so the color can help distinguish one element from another. This same idea appears in fireworks, where different metal components create different colors.

The color comes from energy absorbed by the metallic component. When a species absorbs energy, that energy can go into translation, vibration, rotation, or electron excitement. Electron excitement is the key step in flame emission because it moves electrons from the ground state, the lowest energy state, to a higher energy level.

In the Bohr model of hydrogen, these energy states are separate, or discrete. The ground state is written as n = 1, and higher states are written as n = 2, 3, 4, and so on. An electron moves up only if it absorbs the exact energy difference between its current level and the higher one.

The electron can then relax back to the ground state or another lower level. When that happens, a photon is released as light. The emitted light has a specific energy and wavelength, and visible light ranges from about 400 nm to 700 nm. These atomic absorption and emission wavelengths are discrete lines, and they act like a barcode for identifying an element.

In a metal flame emission test, the flame supplies the energy that excites the electrons. As the electrons fall back down, the sample gives off light with a wavelength tied to its atomic energy levels. Because different atoms have different energy levels, the emitted wavelength is specific to the sample.

Many metals show visible colors that are easy to spot by eye. Lithium gives a red color, sodium gives yellow, potassium gives pink-purple, and barium gives yellow-green. Real samples often contain metal ions, oxides, and salts as well as the metal itself, so the light from the flame test can include a range of wavelengths.

That range can also be measured with a spectrophotometer as atomic absorption and emission spectra. The shapes and wavelengths of the spectra are unique for each substance. The relative intensity of spectral features depends on concentration, while the absolute intensity depends on the distance from the sample to the spectrophotometer.

Flame Test Metal Color Clues

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Learn how flame testing identifies metals by color, why electrons emit light in the flame, and how spectra reveal each element's unique wavelengths.

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Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

lewis-structures-of-molecular-compounds-and-polyatomic-ions

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Bonding in Metals

Metallic bonds are formed between two metal atoms. A simplified model to describe metallic bonding has been developed by Paul Dr&#xFC;de called the ...

Model basic atomic structure and relate an element's atomic structure to its bonding, reactivity, and placement on the Periodic Table.

IPC.7.A

physical-and-chemical-properties-of-matter

Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Alkali Metals

Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the ...

Halogens

Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. ...

Noble Gases


The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name &ldquo;noble&rdquo; because they were ...

Use patterns within the Periodic Table to predict the relative physical and chemical properties of elements.

IPC.7.B

Science concepts. The student knows that relationships exist between the structure and properties of matter. The student is expected to:

Flame Test Metal Color Clues

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