Qualitative reactions

Such a branch of science, as analytical chemistry, studies the composition and structure of chemicals. It is used to study an unknown compound or mixture. Qualitative analysis allows to determine the presence or absence of elements or radicals in a chemical compound, and the quantitative analysis includes the determination of the chemical content in the sample being analyzed, that is, the number of constituents of the mixture. For qualitative analysis, chemical reactions characteristic of certain elements that are easy to perform are used. Qualitative reactions allow observing the expected effects or their absence.

Analytical chemistry is engaged in developing new and improving existing methods of analysis, their practical application, as well as studying the theoretical foundations of analytical processes. Specialists in this field possess chemical, physical and physicochemical methods of analysis. Many of them are based on qualitative reactions that are used to detect radicals and elements, as well as compounds that make up the samples under study. To study the qualitative and quantitative composition apply:

• Elementary analysis (elemental composition is determined);
• Molecular analysis (the structure of chemical compounds at the molecular level is established);
• Structural analysis (one of the types of molecular analysis, explores the spatial structure of atoms and molecules, their molecular masses and empirical formulas);
• Functional analysis (organic compounds are studied by functional groups).

Thus, both inorganic and organic compounds can be recognized. In the presence of specific elements, the color may appear or disappear, a precipitate may be released or dissolved, gas bubbles and the like can be observed. If the qualitative reactions are selected correctly, that is selective (selective) with respect to a particular cation or anion, and also highly sensitive (that is, the detection threshold allows small amounts to be set), the result will be a reliable result - the conclusion about the presence or absence in the sample of an element or substance . This analysis of aqueous solutions is based on known ionic qualitative reactions.

For inorganic compounds, they often occur in aqueous solutions, but in the case of alkali metal cations, detection is carried out by adding dry salts to the middle (hottest) part of the flame of the alcohol. Lithium cations (Li +) color the flame in a dark pink color. Potassium cations (K +) - in violet, sodium (Na +) - in yellow, rubidium (Rb +) - in red, cesium (Cs +) - in cyan. Qualitative reactions to cations can be carried out for barium salts: the presence of barium cations (Ba2 +) is established by the addition of a reagent with sulfate ions (SO42-), since the resulting barium sulfate precipitates into a white precipitate that does not dissolve in acids: Ba2 + SO42- → BaSO4 ↓. The presence of lead cations (Pb2 +) is detected when the aqueous solution of the salt is exposed to sulfide (S2-), resulting in the formation of lead sulphide, which precipitates as a black precipitate: Pb2 + + S2- → PbS ↓. There are a lot of such known qualitative reactions to cations and anions, and they are described in analytical chemistry.

When choosing qualitative reactions for testing a sample, it is useful to know the general rules for the solubility of chemical compounds:

1. All nitrates are soluble.
2. Virtually all salts of potassium, sodium and ammonium are soluble.
3. All chlorides, bromides and iodides are soluble, with the exception of silver, mercury (I) and lead (II) halides.
4. All sulfates are soluble, with the exception of barium, strontium and lead (II) sulfates , which are insoluble, and calcium and silver sulfates that are sparingly soluble.
5. All carbonates, sulfites and phosphates are not soluble except for carbonates, sulfites and phosphates of potassium, sodium and ammonium.
6. All sulfides are insoluble, except for alkali metal sulfides , alkaline earth metals and ammonium.
7. All hydroxides are insoluble except for alkali metal hydroxides. Strontium hydroxide, calcium and barium are moderately soluble.

Organic substances such as alkanes (saturated hydrocarbons) or alkenes (unsaturated hydrocarbons) can be detected with a solution of potassium permanganate, which in the first case does not change its color, since paraffinic hydrocarbons with permanganate do not react in the cold. In the second case, the solution becomes discolored due to the Wagner reaction (for example, with ethylene): 2KMnO4 + 3C2H4 + 4H2O - → 2KOH + 3CH2OH-CH2OH + 2MnO2 ↓. As a result, a precipitate of manganese dioxide is obtained, which has a brown color. Proteins are complex organic compounds that ensure the vital activity of any living organism. Their vast number, their definition is of great practical importance. For these purposes, qualitative reactions to proteins are used, they are divided into color and name. With their help, not the proteins themselves are determined, but the amino acids that make up their composition.