Water is described as being 'hard' if it forms an insoluble scum before it forms a lather with soap.
The hardness of natural water is usually caused by the sulphates and hydrogencarbonates of calcium and magnesium, for example
Water is regarded as being 'hard' if it has a concentration of dissolved calcium and magnesium ions of 0.2 moldm-3 or greater.
Hard water causes 'furring' in kettles, boilers and pipes leading to reduced heat transfer efficiency and in extreme cases blockage.
Hard water can be a source of the calcium ions needed to form strong bones and teeth.
There is circumstantial evidence that people who live in soft water areas have a slightly higher risk of certain heart diseases, however the evidence for this claim has been disputed and there are several other possible explanations of the apparent correlation.