4 Cations in water

Sodium, potassium, magnesium and calcium are found in natural water (Table 1). The most important sources of calcium are the mineral deposits of calcium carbonate, CaCO₃, which are formed from the fossilised remains of long-dead marine organisms. Examples include the minerals limestone and chalk.

Dissolved carbon dioxide makes rainwater slightly acid.

This accounts for the levels of both calcium and bicarbonate (HCO₃⁻) ions seen in natural water samples (Table 1). Furthermore, this dissolution of calcium carbonate contributes to the hardness of water.

Water hardness is often due to the presence of dissolved calcium and magnesium salts. For instance, calcium hydrogen carbonate upon heating is converted to calcium carbonate. This calcium carbonate is insoluble and deposits in appliances, such as kettles.

Human derived acid rain can weather minerals by, for instance, converting calcium carbonate (Figure 15) into calcium sulfate, CaSO₄, leading to harder water. The resulting volume change leads to surface cracking and new conduits for water to percolate. Freezing of this water leads to further damage. Such acid rain can arise from the combustion of sulfur-containing fuels because this yields sulfur dioxide, SO₂, which upon oxidation can yield sulfuric acid. Consequently sulfur compounds are removed from fuels before combustion where possible, often using zeolites.

Figure 15 (a) Eroded stonework and (b) Limestone blocks at Wells cathedral, covered with a calcium sulfate crust, which flakes away exposing a friable surface that weathers rapidly.

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Figure 15 Figure (a) is a photograph of a stone statue in front of a stone wall. The wall is a smooth light brown. Most of the statue is a light grey, but the face is almost black. There are pieces of metal around the arms and neck which look as if they attach the statue to the wall. Although the statue is still recognisably a person, much of the detail is missing due to weathering, especially around the hand, which might possibly be holding a crook.

Figure (b) is a photograph of a wall constructed from rectangular blocks, each of which is taller than it is wide. Most of the blocks are a dark brown colour, but one is badly weathered exposing a fresh surface that is almost completely white. Similarly, a narrow band along the join between some of the blocks is coloured white where some of the stone has fallen off.

Figure 15 (a) Eroded stonework and (b) Limestone blocks at Wells cathedral, covered with a calcium sulfate crust, which flakes away ...