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For a chemistry scavenger hunt, I need a common household item that fits this description: an equilibrium system containing hydrogencarbonate, bicarbonate ions, and carbonate ions.

Jenkin replies ...
The first thing I would say is that hydrogencarbonate and bicarbonate ions are one and the same thing, HCO₃^– ions.
 
A fizzy drink is one possibility as it is essentially a solution of carbon dioxide in water (disregarding the sugar/sweeteners and flavourings). Some of the dissolved CO₂ will react with water to form hydrogencarbonate ions, and some of these will react with more water to form carbonate ions.
 
To write equations for the equilibria it might be easiest to think first of a reaction between CO₂ and water to form carbonic acid, H₂CO₃. You could then suggest that the H₂CO₃ donates a proton to a molecule of water, setting up an equilibrium involving HCO₃^– ions:
 
H₂O + CO₂ <=> H₂CO₃
 
H₂CO₃ + H₂O <=> HCO₃^– + H₃O⁺
 
And then,
 
HCO₃^– + H₂O <=> CO₃^2– + H₃O⁺
 
This second ionisation happens to a very very slight extent. The extent of ionisation is suppressed (le Chatelier) if an acid is present. Coca-cola, for example, contains quite a lot of phosphoric acid.
 
Traditionally soda water was a solution of carbon dioxide in water containing sodium hydrogencarbonate. This is probably what you need

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updated: 02 July 2003