Hardness of water is caused by minerals or more exact, divalent ions in the water. The ions are mostly Ca2+, Mg2+, Fe2+ and Zn2+. There are two kinds of water hardness, permanent hardness and temporary hardness.
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Permanent Hardness is is caused by ions such as nitrates, sulphates, and chlorides reacting with those divalent ions like Ca2+, Mg2+ and etc.

Temporary Hardness or alkalinity of water or buffering depends on hardness derived mainly from carbonate and bicarbonate ions. Therefore, we can have hard water that is lowly buffered. It can br removed by boiling the water. It can affect the pH of the water which is determned by the minerals that were dissolved in water. The general hardness or alkalinity of water is measured in mg /litre of calcium carbonate (CaCO3). See Water Hardness Table.

pH of the water can also be affected by other ativities in the water like plants and fishes respiration or photosynthesis of water plants. The buffering capacity of water can counteract pH fluctuations. Carbon dioxide/ bicarbonate/ carbonate buffering system stabilises pH by taking excess hydrogen ions and then releases them again as levels drop and therefore the pH doesn't change much.

CO2 + H2O   ->   H2CO3   <=>   HCO3- + H+   <=>   CO32- (solid) + 2H+

Carbon dioxide excreted by fish and plants during respiration dissolves in water to form carbonic acid (H2CO3). If pH levels increase, that is the water becomes more alkaline (from plant photosynthesis), then the carbonic acid dissociates to form bicarbonate and hydrogen ions (HCO3- + H+). Hydrogen ions will therefore counteract the alkalinity increase. If the pH continues to increase, the bicarbonate will dissociate to form solid carbonate and release more hydrogen ions (CO32- (solid) + 2H+), to counteract the increased alkalinity. If pH levels falls the process is reversed.

Water Hardness Table Back
Water Calcium carbonate (mg/L) °dH
Soft 0 - 75 0° - 4°
Moderately hard 75 - 150 4° - 8°
Hard 150 - 300 8° - 16°
Very hard > 300 > 16°
To convert °dH to CaCO3, multiply by 17.9

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