Chemical analysis and the dilute hydrochloric acid group

We find anions by adding a reagent and watching, not by looking at the salt. They fall into three groups, each shown by one reagent: dilute hydrochloric acid, concentrated sulphuric acid, and barium chloride solution. The first two groups use one idea. The added acid is stronger and does not break down, while the acid of the anion is weak and unstable, so the added acid pushes it out and it leaves as a gas that we can test. For a carbonate, the weak acid is carbonic acid, H₂CO₃, which sits in balance with water and carbon dioxide; the gas CO₂ escapes with effervescence. The gas turns lime water milky because calcium carbonate forms, and more gas clears it again because soluble calcium hydrogencarbonate forms. Hydrogencarbonate gives the same gas, so a second test separates the two: magnesium sulphate solution gives a white precipitate at once with a carbonate, but with a hydrogencarbonate only after heating. These are paper tests here. Most metal carbonates other than those of sodium, potassium and ammonium are insoluble in water, and all of them dissolve in acids.