Carbonic acid and carbonates. Practical work 1

Carbonic acid H₂CO₃ is an unstable, weak, dibasic acid that exists only in solution: CO₂ + H₂O ⇄ H₂CO₃, and it dissociates in two steps: H₂CO₃ ⇄ H⁺ + HCO₃⁻, HCO₃⁻ ⇄ H⁺ + CO₃²⁻. It forms normal salts, carbonates (K₂CO₃, CaCO₃), and acid salts, hydrogencarbonates (KHCO₃, Ca(HCO₃)₂). Only the carbonates of alkali metals and ammonium and all hydrogencarbonates dissolve in water; the other carbonates are insoluble. Strong acids drive CO₂ out of carbonates and hydrogencarbonates: CO₃²⁻ + 2H⁺ → H₂O + CO₂↑, HCO₃⁻ + H⁺ → H₂O + CO₂↑ — this is the qualitative test for the carbonate ion (the gas clouds lime water). On heating, carbonates other than those of the alkali metals decompose: CaCO₃ → CaO + CO₂ (when heated), and hydrogencarbonates become carbonates: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. In nature the equilibrium CaCO₃ + CO₂ + H₂O ⇄ Ca(HCO₃)₂ builds caves, stalactites and stalagmites. Na₂CO₃ + CO₂ + H₂O → 2NaHCO₃ gives baking soda; washing soda is used in glass, soap and paper making, while limestone and marble are building materials. A Na₂CO₃ solution is alkaline because of hydrolysis.