In this explainer, we will learn how to identify the uses of flame tests and the colors produced by alkali, alkaline earth, and other metal atoms during a flame test.
Fireworks are an impressive sight for onlookers because they illuminate the night sky with brightly colored light, as shown in the image below.
The colors are emitted when the gunpowder in the fireworks is ignited. The ignition of the gunpowder causes the temperature to increase, and this high temperature can excite certain chemical elements and make them emit colorful light.
Many chemical elements can also be made to emit colored light when they are heated in a Bunsen burner flame. Ordinary hot Bunsen burner flames emit a characteristic pale blue colored flame that is usually called a nonluminous flame. Nonluminous flames can be transformed into different colored flames when they come into contact with different types of compounds and samples of pure metals.
Definition: Nonluminous flame
Nonluminous flames are pale-blue flames that are made when the temperature of a flame is very high because the flame is being powered by complete combustion reactions.
Sodium chloride emits an intense yellow color when it is heated with a strong Bunsen burner flame. Streetlamps produce a similar bright-yellow color when they are switched on because they also contain sodium atoms. In either case, heat energy from the flame, or electrical energy in the lamp, has caused the sodium atoms to emit a bright-yellow light. The emission of light from any one element can be understood by considering how electrons move between discrete energy levels in atoms.
Electrons in atoms can only exist at certain energy levels. Electrons always fill the lowest energy level in any one atom first, and then they fill the other higher energy levels. Electrons can be excited from one low energy level to a higher energy level if they absorb discrete packets of energy.
Excited electrons are not stable, and they almost always fall back down to a lower energy level. This can happen in stages as bursts of energy are emitted one after the other, or it can happen in one short instant as one high energy photon is emitted by the excited electron.
Energy is released in the form of electromagnetic radiation when the excited electron falls from one high energy level to a lower energy level. The energy of the emitted electromagnetic energy can be calculated with the following equation: In this equation, is the energy, is Planck’s constant, and is the frequency of the wave. The energy is usually measured in joules (J), and the wave frequency is usually measured in hertz (Hz).
We can see that the energy of the emitted electromagnetic radiation is proportional to the frequency of the electromagnetic waves.
The electromagnetic spectrum is a continuous spectrum that is made up of lots of different types of electromagnetic radiation. Visible light makes up one relatively small part of the electromagnetic spectrum. The other parts of the spectrum are made up of low frequency forms of energy, such as radio waves, and high frequency forms of energy, such as gamma rays. The diagram below shows the electromagnetic spectrum.
The chemical elements can produce visible light radiation if the energy difference between two energy levels is equal to energy of a packet of visible light radiation. Each element has its own unique set of energy levels, and it will tend to produce its own unique forms of visible light radiation. Some chemical elements tend to produce high-energy-visible-light radiation, and others tend to produce lower energy forms of visible-light radiation.
Blue light has a higher frequency and energy value than green light. Green light has a higher frequency and energy value than red light. Blue light is produced when excited electrons move between states that have significantly different energy values. Red light is produced when electrons move between energy levels that have more similar energy values.
The energy-level diagram below shows the electronic configuration for an atom of sodium. The ground state electronic configuration of sodium is . Sodium atoms therefore have a single valence electron. When these atoms are heated, there is enough energy to excite an electron from the 3s subshell to the 3p subshell, as shown in the right-hand diagram.
Heated sodium atoms emit a bright yellow light when their excited electrons drop down from a high energy 3p subshell to a lower energy 3s subshell. This gives rise to the characteristic flame color of sodium atoms.
Example 1: Understanding Which Atom Property Determines a Flame Color Change
When a salt is placed in the flame of a Bunsen burner, the flame changes color. What feature of the metal ions in the salt determines the color of the flame in this experiment?
- Spacing of electron energy levels
- Atomization energy
- Reactivity with oxygen
- Bond energy
- Spacing of nuclear energy levels
Answer
Metal atoms contain electrons in their ground state energy levels under normal conditions. When the metal atom is placed into a hot flame, energy is provided to the electrons in the atom so that they become excited to higher energy levels. When the electrons fall back to their original energy level, the energy may be released as visible light. The color produced is dependent on the energy spacing of the energy levels concerned. This has little to do with the reactivity of the metal concerned. The metal atoms are not being “burned” or oxidized.
When this phenomenon occurs, the atoms in the salt are in the vapor phase. The color emitted by the atom does not depend on the atomization energy of the metal atom concerned. Atomization energy for a compound is the energy required to produce gaseous atoms from a mole of the compound in its standard state under standard conditions. This would involve bond breaking and is therefore an endothermic process. This enthalpy change is separate from the energy absorbed by valence electrons in atoms when they are excited to higher energy levels in the vapor phase. Bond energies are not a consideration here either, as the metal atoms have already been atomized in the flame. Atomization energies and bond energies are enthalpy changes that are not responsible for the flame color.
Since the flame color is a phenomenon associated with valence electrons in atoms, the energy of the nucleus is irrelevant.
We can use these statements to determine that option A is the correct answer for this question.
Flame tests are qualitative tests that are used to detect the presence of metals from their emission spectra. Flame tests are relatively easy to set up and perform, and this explains why they are used by so many different types of chemists.
It is important to stress here that not all metals produce colored flames and flame tests cannot be used to test for the presence of all metals. Some metals do not produce a colored flame because the thermal energy of a Bunsen burner is not sufficient to excite the electrons of these elements enough to release energy in the visible range.
Definition: Flame Test
The flame test is a qualitative test used in chemistry to determine the identity or possible identity of a metal or metalloid from its emission spectrum.
Flame tests are performed by first dipping a nichrome wire in a solution of concentrated hydrochloric acid. Nichrome is an alloy of nickel and chromium that does not react with hydrochloric acid. A platinum wire can be used instead because platinum is a highly inert metal, but it tends to be the more expensive option. Hydrochloric acid is used to clean the wire, and it is used to remove any deposits of metal ions. Hydrochloric acid also tends to be used because it makes metals transform into metal chloride salts. The metal chloride salts are usually very volatile, and they can be vaporized relatively easily. The wire is then placed into a hot, or pale-blue, nonluminous Bunsen burner flame. The yellow safety flame, or luminous flame, should not be used to conduct a flame test.
Definition: Luminous Flame
Luminous flames are yellow-colored flames that are made when the temperature of a flame is relatively low because the flame is powered by incomplete combustion reactions.
This cleaning process must be repeated until the wire does not impart any color to the Bunsen burner flame. The wire is then dipped into a solution of the metal compound. Alternatively, the wire may be dampened with fresh acid to pick up a few solid crystals of the metal compound. The end of the wire is then returned to the flame. If a color is observed in the flame, this is noted and the wire is then cleaned again and ready for another flame test. The flame colors can be quite pale, and they sometimes last for just a few seconds. The test is best performed in a dark room or a darkened laboratory.
It is important to explain here that the flame colors are usually produced by atoms and not ions. Metal ions usually absorb electrons and turn into neutrally charged atoms before they emit any visible-light radiation. Most excited state ions tend to emit packets of energy that cannot be seen by the human eye. Ions tend to release a type of nonvisible-light radiation when their electrons drop from one high and excited energy level to a lower energy level.
Example 2: Explaining Why Hydrochloric Acid Is Used in a Flame Test
Before a compound is analyzed using a flame test, it is typically dissolved in hydrochloric acid. What is the main reason for preparing the sample in this way?
- The resulting salts are more flammable.
- Ions in the resulting salts produce more intense colors due to their greater charge.
- Treatment with hydrochloric acid removes impurities.
- Treatment with hydrochloric acid displaces anions that could alter the flame color.
- The resulting salts are more easily vaporized.
Answer
When a flame test is performed, the metal ions in the sample must be atomized into the hot Bunsen burner flame. This is made easier if the sample is made into a metal chloride salt. Metal salts that are treated with concentrated hydrochloric acid are converted into more volatile metal chloride salts. This means that the ions are vaporized more easily.
When doing a flame test, the salts are not burned, so flammability is not an issue. The charge on a metal ion in a metal chloride salt remains unaltered from its charge in the original sample salt. Hydrochloric acid is not an oxidizing agent.
Although hydrochloric acid, which helps to clean the nichrome or platinum wire, converts compounds into soluble chloride salts, it will not remove impurities from the salt sample. If impurities are present, the sample may be recrystallized to remove them.
Most anions do not produce colored flames. They either contain excitable electrons that emit light that is not in the visible light section of the electromagnetic spectrum, or the excitation efficiency of these anions is low. This means that, compared to metal cations, most anions give little or no flame color.
Using these statements, we can deduce that the correct answer is option E.
The alkali metals are group one elements that make up the leftmost column of the periodic table. Alkali metal atoms produce very interesting and unusual flame colors when they come into contact with a hot Bunsen burner flame. The colors of five different alkali metal flames are shown in the following image.
Lithium atoms produce a bright-red or a crimson flame. Sodium atoms produce a flame that has a golden-yellow color. Potassium atoms produce a flame that has a lilac, pink, or pale-violet color. Rubidium atoms produce a red-to-violet flame, and cesium atoms produce a blue-to-violet flame.
The group two metals are collectively known as the alkaline earth metals, and they can also produce interesting flame colors when they are heated with a Bunsen burner. The flame colors for calcium, strontium, and barium are shown in the following image.
Calcium atoms produce a flame that is often described as brick red, while strontium atoms produce a bright-red flame. Barium atoms produce a flame that is a pale-yellow-green color.
The chloride salts of metal elements are usually used to produce flame colors because they are volatile and the chloride ions do not produce any flame color of their own that would interfere with the flame test. It is easier to test for the presence of calcium or barium metals if we are using calcium or barium chloride ( and ) rather than some other calcium or barium compound.
Many other metal elements can be excited with Bunsen burner flames to produce characteristic flame colors, such as atoms of lead that give a grey-white flame color. Copper is probably the most interesting example because it produces a distinctive green flame. The flame tests for these two elements are shown in the image below.
Example 3: Recalling the Color Produced by an Alkali Metal in a Flame Test
Which of the following terms best describes the color of the flame produced by rubidium salts?
- Blue
- Yellow
- Violet red
- Orange red
- Blue green
Answer
Rubidium is an alkali metal, found in group one of the periodic table. The alkali metals all produce colored flames in a flame test. Lithium is described as red or crimson. Sodium gives a strong yellow-to-orange color. Potassium is described as pale violet, lilac, or pink. Rubidium is seen as a red-to-violet flame, and cesium is described as blue to violet.
The best description provided here for rubidium is violet red.
The correct answer must be option C.
Example 4: Recalling the Color Produced by s-Block Metals in a Flame Test
Which s-block metal does not produce a colored flame in a flame test?
- Calcium
- Magnesium
- Rubidium
- Barium
- Cesium
Answer
The metals found within group one and group two of the periodic table are collectively described as s-block elements. This is because their valence electrons occupy s subshells. In group one, rubidium and cesium give red-violet and blue-violet flame colors respectively. From group two, barium gives a pale-green flame color and calcium gives a brick-red flame color. These colors are caused by excited electrons emitting visible light as they return to their ground state in the atoms.
Magnesium metal burns with an intensely bright-white light when heated in a Bunsen flame. This light is produced due to the high temperature reached by the burning magnesium metal. The chemical reaction, between magnesium metal and oxygen, is a highly exothermic reaction. A common misconception among students that have observed this is that magnesium compounds give a bright-white flame color. This is not the case; magnesium atoms in a flame do not emit visible light, and no flame color is observed.
The correct answer is option B.
Flame tests are usually not used to analyze mixtures of compounds because the flames of different elements can interfere with each other. This can be particularly confusing when a mixture contains different types of metal elements that yield similar flame test results. The lithium, calcium, and strontium metal elements all yield red flames, and it can be quite difficult to distinguish between them. It would be difficult to try and detect the presence of two or three of these elements if we were to analyze the color of just one flame.
The barium and copper metal elements produce similar flame colors as well. The barium and copper elements both yield a green-colored flame although it is interesting to note that barium tends to make paler green flames that do not persist for a very long time. A flame test is therefore not a precise test for identifying metal elements, and other qualitative tests may be needed to confirm the identity of a metal element.
Example 5: Explaining the Limitations of a Flame Test
When an alkali metal is dissolved in hydrochloric acid and analyzed using a flame test, an intense orange flame is observed. Why might more information be needed to confirm the identity of the metal in the sample?
- The orange color is difficult to differentiate from the color of the Bunsen flame.
- The orange color of the flame is produced by chloride ions.
- The alkali metal producing the orange flame may be present as a contaminant.
- The orange color of the flame is produced by the solvent.
- More than one alkali metal produces an orange flame in a flame test.
Answer
Alkali metals all give characteristic flame colors in a flame test. The Bunsen flame should be nonluminous and will have a pale-blue color. This is achieved by having the air hole on the Bunsen burner fully open. The colors produced by the alkali metals are quite distinct, in comparison to the nonluminous Bunsen flame. Lithium is described as red or crimson. Sodium gives a strong yellow or a golden-yellow color. Potassium is described as pale violet, lilac, or pink. Rubidium is seen as a red-to-violet flame, and cesium is described as blue to violet. We would have no problem differentiating these colors from the Bunsen flame color.
The solvent, hydrochloric acid, is chosen so that it provides volatile ionic chlorides. Neither the solvent nor the chloride ions emit visible light in the flame. This is an important consideration because we do not want the flame color of interest to be masked by other colors.
The only alkali metal that produces an intense orange flame is sodium. If small amounts of sodium ions are present, either in the sample or on the platinum wire, the characteristic sodium flame color is often all that is observed. This is true when sodium is present as a contaminant. The intense golden-yellow-to-orange color can mask other colors that may be produced by other metal ions present. Further tests may be needed to determine whether other metal ions are present in the sample.
The correct statement is option C.
Key Points
- Flame tests are performed to identify metals atoms that may be present in a sample of an unknown compound.
- Hydrochloric acid is used to clean an inert wire and form volatile metal chloride salts before samples are introduced into the flame.
- The process of determining a flame test is described as a series of steps involving some metal wire.
- The flame color is produced by energetically excited electrons releasing visible light energy when they return to lower energy levels.
- The possible identity of the metal atom can be established by matching the color produced by the metal atom in a nonluminous Bunsen burner flame to the colors produced by known metal salts.
- Only a select number of metal atoms produce flame colors. Most are s-block metals found in group 1 and group 2 of the periodic table.
- It is not possible to analyze mixtures of compounds using this method, as the flame colors produced by several metals may interfere with each other, giving a misleading result.
