Metal ions, flame test

Another qualitative analysis technique is a flame test. A dissolved ionic compound is placed in a flame. Table 9.4 lists the flame colours associated with several ions. Notice that all the ions are metallic. The flame test is only useful for identifying metallic ions in aqueous solution. 

Qualitative chemistry is an area of chemistry concerned with identifying substances. In Activity 9.1 you will perform a qualitative analysis to detect the presence of certain ions that, in turn, may reveal an art forgery. The ions could come from paints that were not available at the time of the artwork. In this qualitative analysis, metal ions (cations) and nonmetal ions (anions) are reacted with solvents and with each other. Then the cations and anions present are identified by the products produced. In addition, flame tests and pH determinations are used to identify ions. Qualitative analysis is an engaging opportunity for you to develop experience with chemical change and review solubility principles. Nowadays, however, most of the time a chemist analyzes a substance to detect ion content using quantitative analytical computerized instruments. 

The flame of an alcohol lamp looks almost colourless. Vhen a length of platinum v/ire which is dipped into a metal salt solutioh is put into the flame, the flame is coloured in the upper part by the wire. The colour is peculiar to the kind of the metal strontium colours the flame red, sodium yellow, barium pale green and copper blue. This is applied to the qualitative analysis of metal ions as the colour flame test. The emission of the coloured light is caused by atomic metal gas or a gas consisting of molecules of metal compound, and the process may be set out as follows ... 

Identify a set of flame-test color standards for selected metal ions. 

Relate the colors of a flame test to the behavior of excited electrons in a metal ion. 

Organizing data Examine your data table, and create a summary of the flame test for each metal ion. 

Analyzing data Account for any differences in the individual trials for the flame tests for the metal ions. 

Identifying patterns For three of the metal ions tested, explain how the flame color you saw relates to the lines of color you saw when you looked through the spectroscope. 

The ammonia gas is detected either by noting its characteristic odor or by observing a piece of wet red litmus paper turning blue when placed above (not in contact with) the solution. To confirm the presence of Na and ions, we usually use a flame test, as follows A piece of platinum wire (chosen because platinum is inert) is moistened with the solution and is then held over a Bunsen burner flame. Each type of metal ion gives a characteristic color when heated in this manner. For example, the color emitted by Na ions is yellow, that of ions is violet, and that of Cu ions is green (Figure 16.11). 

Excitation of the outer ns electron of the M atom occurs easily and emission spectra are readily observed. We have aheady described the use of the sodium D-line in the emission spectrum of atomic Na for specific rotation measurements (see Section 3.8). When the salt of an alkali metal is treated with concentrated HCl (giving a volatile metal chloride) and is heated strongly in the non-luminous Bunsen flame, a characteristic flame colour is observed (Li, crimson Na, yellow K, lilac Rb, red-violet Cs, blue) and this flame test is used in qualitative analysis to identify the M ion. In quantitative analysis, use is made of the characteristic atomic spectrum in flame photometry or atomic absorption spectroscopy. 

Group 5. The alkali metal ions and The ions that remain after removing the insoluble phosphates are tested for individually. A flame test can be used to determine the presence of KT, for example, because the flame turns a characteristic violet color if is present. 

These flame colours can be observed in the laboratory if flame tests are carried out on volatile metal salts. At the high temperature of the flame, electrons are promoted to higher energy levels. The colours observed arise from these excited metal atoms or ions. When the electrons return to the gtoimd state, the atoms emit light of characteristic frequencies. 

Qualitative analysis involves identifying or classifying substances based on their chemical or physical properties. Examples include flame tests (Chapter 2) to identify metal ions and precipitation reactions to identify cations or anions, such as the halides (Chapter 3). The melting and boiling points of substances are commonly used to identify organic substances and detect the presence of impurities. 

Flame tests are not commonly used to identify transition metals. The presence of a certain transition-metal ion in a solution is sometimes obvious from the solution s color. Some transition-metal ions can be more accurately identified using a procedure called qualitative analysis. Qualitative analysis is the identification of ions by their characteristic reactions. The transition-metal ions most often identified through qualitative analysis include copper, nickel, zinc, chromium, iron cobalt, cadmium, manganese, and tin. Most tests to identify the presence of an ion in a mixture involve causing the ion to precipitate out of solution. Some of the more dramatic precipitation reactions for transition metals are shown. 

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