Detection of Gases, Cations and Anions
Air around us consists of various gases. Most of these gases are colourless and odourless. These gases can be identified by various chemical tests based on their characteristic properties. These chemical tests include preliminary and confirmatory tests. Oxygen is produced on heating metal nitrates, oxides and potassium chlorate. When a glowing splinter is brought in contact of the colourless and odourless gas and the splinter glows more brightly, it shows the presence of oxygen gas. Carbon dioxide is liberated on heating bicarbonates and carbonates. Presence of carbon dioxide gas is detected by passing it in lime water. If the lime water turns milky, then carbon dioxide gas is present. Chlorine is liberated by the action of concentrated hydrochloric acid on metal oxides and the action of dilute acids on bleaching powder. Presence of chlorine gas is detected by bringing moist starch iodide paper in contact with the evolved gas. If the paper turns blue black, then chlorine gas is present. Sulphur dioxide gas is liberated by the action of dilute acids on sulphites and bisulphites. On passing the gas through potassium permanganate solution, if it turns pink potassium permanganate solution colourless, then sulphur dioxide gas is present. Hydrogen sulphide is liberated by the action of dilute acids on metal sulphides. On bringing a paper dipped in lead acetate solution in contact with the gas, if the paper turns black, then hydrogen sulphide gas is present. Similarly other components of air can also be detected by specific tests. Generally, the presence of cations like Ca2+ , Pb2+ , Zn2+ , Cu2+ , Fe2+ (except ammonium ion) etc. can be detected in the unknown mixture by using original solution which is the concentrated solution of the salt and distilled water or dilute hydrochloric acid or concentrated hydrochloric acid. Their characteristic precipitates are formed on addition of sodium hydroxide or ammonium hydroxide solutions. Sodium ion (Na+) and potassium ion (K+ ) can be detected by the flame test. The presence of anions such as carbonate, sulphite, sulphate, chloride sulphide, and nitrate can be detected in the unknown mixture by their specific chemical tests Different substances behave differently on heating. Plumbous nitrate decomposes on heating and forms lead oxide, nitrogen dioxide and oxygen. When white amorphous powder of zinc carbonate is heated strongly, its colour changes to pale yellow. It forms zinc oxide and carbon dioxide.
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