"White deposits on water taps and scale in kettles are the result of hard water. Water hardness is a measure of the concentration of calcium and magnesium ions in water. Water hardness occurs when water filters through soil and rock layers. Carbon dioxide from the air and soil dissolves in the water. The carbon dioxide in the water dissolves minerals on its way through the soil that enter the water and make it ""hard"". The hardly soluble chalk in the water reacts with the carbon dioxide to form calcium hydrogen carbonate. If the carbon dioxide escapes, for example through heating, the dissolved calcium ions react with the hydroxide ions to form calcium carbonate again. This deposits as scale on pipes and heating coils. The harder the water, the greater the limescale deposits. The hardness of the water varies from region to region because it depends on the type of soil and rock layers. Water hardness is high in regions with particularly high chalk levels.
This film shows how water hardness is measured and which consequences the hardness of water has. Also methods for softening water are shown."