A gas evolution equation describes a chemical reaction that produces carbon dioxide gas (CO₂) as a result of the transformation of certain reactants. In this context, the term "medium product" refers to an intermediate product that forms before it fully converts into the final product. Specifically, in gas evolution reactions involving acids and bicarbonate ions, the medium product is carbonic acid (H₂CO₃).

When hydrogen ions (H⁺) react with bicarbonate ions (HCO₃^-), they form carbonic acid. However, carbonic acid is unstable and cannot exist as a stable product; it quickly decomposes by losing a water molecule (H₂O). This process can be summarized by the equation:

H⁺ + HCO₃^- → H₂CO₃ → CO₂ (gas) + H₂O (liquid)

Similarly, when H⁺ reacts with carbonate ions (CO₃^2-), carbonic acid is again formed as the medium product, which subsequently decomposes to yield carbon dioxide gas and water:

H⁺ + CO₃^2- → H₂CO₃ → CO₂ (gas) + H₂O (liquid)

In essence, the gas evolution equation illustrates how carbonic acid, while initially formed, rapidly breaks down to produce carbon dioxide gas and water, highlighting the dynamic nature of these chemical reactions.

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