Qualitative reactions can detect a particular ion, chemical or functional group.
To carry out qualitative reactions, appropriate reagents, indicators, in some cases - the flame of the burner are needed.
Qualitative reactions to cations and anions
To determine the silver cation, it is necessary to carry outReaction with some chloride. The interaction of Ag (+) and Cl (-) results in a white precipitate of AgCl ?. Barium cations Ba2 + are detected in the reaction with sulfates: Ba (2 +) + SO4 (2 -) = BaSO4? (White precipitate). Equally the reverse is true: in order to detect chloride ions or sulfate ions in the solution, it is necessary to carry out a reaction, respectively, with silver and barium salts.
To determine Fe (2+) cations,Hexacyanoferrate (III) potassium K3 [Fe (CN) 6], more precisely, the complex ion [Fe (CN) 6] (3-). The resulting dark blue precipitate of Fe3 [Fe (CN) 6] 2 is called "turbulence blue". For the identification of iron (III) cations, hexacyanoferrate (II) of potassium K4 [Fe (CN) 6] is already taken, which gives a dark blue precipitate Fe4 [Fe (CN) 6] 3 - "Berlin azure" when interacting with Fe (3+) . It is also possible to detect Fe (3+) in the reaction with ammonium thiocyanate NH4CNS. As a result, malodisociating iron (III) thiocyanide - Fe (CNS) 3 - is formed and the solution acquires a blood-red color.
Excess hydrogen cations H + creates acidicMedium, in which the colors of the indicators accordingly change: orange methyl orange and purple litmus become red. In excess of hydroxide ions OH- (alkaline medium), the litmus becomes blue, methylorange-yellow, and phenolphthalein, colorless in neutral and acidic environments, acquires crimson staining.
To understand whether there is in the solution an ammonium cationNH4 +, you need to add alkali. When reversibly reacted with hydroxide ions, NH4 + gives ammonia NH3? And water. Ammonia has a characteristic smell, and the moist litmus paper in such a solution turns blue.
A qualitative reaction to ammonia uses the HCl reagent. In the process of formation from ammonia and hydrogen chloride of ammonium chloride HN4Cl, white smoke can be observed.
Carbonate and hydrocarbonate ions CO3 (2-) and HCO3 (-)Can be detected by the addition of acid. As a result of the interaction of these ions with hydrogen cations, carbon dioxide is released and water is formed. When the gas is passed through the lime water Ca (OH) 2, the solution becomes turbid, since an insoluble compound, calcium carbonate CaCO3, is formed. With further transmission of carbon dioxide, an acid salt is formed - soluble calcium carbonate Ca (HCO3) 2.
Reagent for the detection of sulfide ions S (2-) - soluble lead salts, which give a black precipitate PbS? In reaction with S (2-).
Detection of ions with a burner
Salts of some metals when introduced into the flameBurners paint it. This property is used in qualitative analysis to detect the cations of these elements. So, Ca (2+) stains the flame in brick-red color, Ba (2+) - in yellow-green. Combustion of potassium salts is accompanied by a violet flame, lithium - bright red, sodium-yellow, strontium-carmine-red.
Qualitative reactions in organic chemistry
Compounds with double and triple bonds (alkenes,Alkadienes, alkynes) decolorize red-brown bromine water Br2 and a pink solution of potassium permanganate KMnO4. Substances with two or more hydroxy groups -OH (polyhydric alcohols, monosaccharides, disaccharides) dissolve in the alkaline medium a freshly prepared blue precipitate of Cu (OH) 2, forming a solution of bright blue. Copper (II) hydroxide also reacts with aldehydes, aldoses and reducing disaccharides (aldehyde group), but here already precipitates Cu2O? Brick red color.
Phenol in a solution of iron (III) chloride formsComplex compound with FeCl3 and gives a violet stain. Substances containing an aldehyde group give reactions of a "silver mirror" with an ammoniacal solution of silver oxide. The solution of iodine, when starch is introduced into it, becomes violet, and the peptide bonds of proteins are detected in the reaction with a saturated solution of copper sulfate and concentrated sodium hydroxide.