Calcium hydrogen carbonate is a weathering product that is formed when calcium carbonate dissolves in water, releasing CO2 gas. As it partially dissolves in water, carbon dioxide forms carbonic acid (or, in simpler terms, H2CO3). The carbonic acid further dissociates into hydrogen carbonate (HCO4-), producing free protons.

Calcium hydrogen carbonate is a compound that forms when calcium ions are dissolved in water. It has the molecular formula Ca(HCO3)2 and a theoretical molar mass of 162.11 g-mol-1.

Calcium hydrogen carbonate is not a pure substance, but only exists as an impure solution. This means that it cannot be represented as a pure substance under normal conditions and has no properties such as melting point or boiling point. Its solubility at 20 °C and 1 atm is 1.66 g per 100 g H2O and is therefore considerably higher than that of calcium carbonate (1.4 mg per 100 g H2O at 20 °C).

Calcium carbonate is a mineral that is formed when calcium and carbon dioxide dissolve in water. It is the main component of limestone, also known as calcite. Calcium carbonate occurs on the earth's surface in many different forms, e.g. in sandstone and chalk, but also in shells and coral reefs.

Calcium bicarbonate is formed during the weathering of limestone by the action of water and carbon dioxide. The carbon dioxide partially dissolves with water to form carbonic acid, which further dissociates to form hydrogen carbonate with the formation of protons. At the same time, small amounts of calcium carbonate dissolve in water, releasing carbonate ions. These take over the hydrogen ions offered by the carbonic acid and also become hydrogen carbonate ions.[3] In the carbonic acid dissolution of limestone, one equivalent part of the hydrogen carbonate ions therefore comes from the carbonic acid, the other from the stone.

In order to keep calcium hydrogen carbonate in solution, a certain concentration of so-called "associated carbonic acid" is required. This is chemically identical to any other type of carbonic acid; it is just a question of quantity.

In dissociation equilibrium with the hydrogen carbonate ions present, this associated carbonic acid sets the pH value of the water just low enough so that the proportion of carbonate ions, which in turn depends on this pH value, together with the calcium concentration present, does not yet exceed the solubility product of calcium carbonate.

If more than the corresponding amount of free carbonic acid is present in the solution, this amount of carbonic acid is referred to as "excess" and can react with further limestone and dissolve it. The amount that dissolves further limestone and turns into additional calcium hydrogen carbonate is referred to as "calcium-aggressive carbonic acid". The remainder increases the associated carbonic acid to a new, higher level.

When water evaporates or is heated, carbon dioxide escapes from the solution; it can also be reduced by photosynthesis. This shifts the dissociation equilibrium of the carbonic acid back towards the carbonate ions, i.e. to the left-hand side of the above reaction equation. As a result, the solubility product of calcium carbonate is again exceeded and insoluble limestone is formed again. This process underlies the formation of calcareous sinter, travertine or calcareous tuff, but also the formation of calcareous sediments (lake chalk) in lakes and oceans. The Nagelfluh conglomerate found in the northern foothills of the Alps (deposits of molasse or glacial debris) can also be formed in this way, as gravel or crushed stone is caked by the binding agent lime to form a natural concrete.

You may not know it, but your tap water is actually a little acidic.

This is true! The waterworks add carbonic acid to your drinking water to regulate the pH value so that a thin layer of limescale forms in the iron pipes to prevent rusting. In order not to reduce the pipe cross-section too much, the carbon dioxide content must be constantly adjusted. The speed at which this balance is achieved depends on the ions present: Pure calcium carbonate takes an exceptionally long time, whereas in the presence of foreign ions (e.g. Mg2+ or SO42-) it is established very quickly; dolomite is therefore used to bind excess carbonic acid.

The calcium hydrogen carbonate content of tap water is the main contributor to the hardness of tap water, which we call "carbonate hardness".