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Flames are incandescent gases produced by combustion. A flame is the light emitted by the products of combustion reactions, that is, of chemical reactions with oxygen in the air, which are strongly exothermic. The characteristics of the chemical reaction, such as the type of products that are generated, determine the colors produced by the flame of each combustion reaction. This basic principle is supported by the flame test, a fun and useful analytical technique that helps identify the chemical composition of a sample based on the way a flame changes color, as shown in the figure below. However, interpreting your results can be tricky if you don’t have a reference. There are many shades of green, red, and blue, often described by names that you wouldn’t find even on a box of colored paints.
It must be taken into account that the color that is identified in the flame when inserting the sample will depend on the fuel that is used to generate the flame and if the result is being seen with the naked eye or through a filter. It is also possible to take photos to compare color changes when testing different samples. It must be taken into account that the color changes may vary depending on the way the test is carried out and the purity of the sample. Let’s see below references of flame color tests of some elements.
Sodium and iron: yellow
Most fuels, such as candles and wood, contain sodium, which is why we are familiar with the yellow color this metal gives a flame. This color changes when the sodium salts are placed in a blue flame, such as a Bunsen burner or spirit lamp. It should be noted that sodium yellow hides other colors. If in another test the sample has some contamination with sodium, the observed color may include an unexpected contribution of yellow. Iron can also produce a golden flame, although it is sometimes orange.
Calcium: orange
Calcium salts produce an orange flame. However, the color can appear dull, dim, so it can be difficult to distinguish between sodium yellow or iron gold. The most common calcium sample to be tested in the laboratory is calcium carbonate. If the sample is not contaminated with sodium, a nice orange color should be obtained; in the following figure, orange is combined with a yellow of another origin.
Potassium; violet
Potassium salts produce a characteristic purple or violet flame. If the burner flame is blue, it may be difficult to see much of a color change. Also, the color can be rather pale, closer to lilac. In the following figure, only a faint purplish arc can be distinguished.
Cesium: purple or blue
The cesium flame test gives a purplish or blue color, just like potassium, so it is likely to be confused with cesium. Cesium compounds are less frequent in laboratories, so if this coloration is observed in a flame test, the sample can be associated with potassium, but it is still not a conclusive result.
Lithium and rubidium: hot pink
Lithium produces a red to purple coloration in a flame test. It is possible to obtain a strong pink color or also more muted tones. The color obtained is more degraded than strontium red, the next element to consider. Another element that can produce a similar flame coloration is rubidium, and also radium, but it is very rare to find this element in a laboratory.
strontium: red
The coloration of the flame that strontium gives is red; it is the red of emergency flares and fireworks. It is a coloration that goes from deep crimson to brick red.
Barium, manganese(II) and molybdenum: green
Barium salts produce a green flame that is described as a yellow-green, apple-green, or lime-green color, although it sometimes produces a yellow flame with no green enhancement. Oxidation state II manganese and molybdenum can also produce greenish-yellow flames.
Boron: green
Boron gives the flame a bright green coloration. It is the typical test in a laboratory to determine the presence of borax.
Copper: blue or green
The coloration that copper gives to the flame depends on its oxidation state; copper salts with oxidation state I give a blue coloration, while oxidation state II colors the flames green.
It should be noted that the color of the blue flame is misleading, since it is the usual color of a methanol flame or a burner or stove, as shown in the article presentation figure. Other elements that can give a blue color in a flame test are zinc, selenium, antimony, arsenic, lead, and indium. It must also be considered that a large number of elements do not produce color changes in a flame. Therefore, if the result of the flame test is blue, very relevant information will not be obtained.
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Flame test . Accessed November 2021.
