What is "Hard" water? Water hardness is the traditional measure of the capacity of water to react with soap, hard water requiring considerably more soap to produce a lather. Hard water has a high mineral content, this is caused by a variety of dissolved minerals in water. The minerals most associated with hard water are calcium and magnesium, although other minerals such as aluminium, barium, iron, manganese, strontium and zinc also contribute (World Health Organization Hardness in Drinking Water, 2003). These minerals accumulate in your water during the water cycle when water journeys from rain to underground aquifers. As rain falls to the earth it collects impurities in the atmosphere such as carbon dioxide, or C02, which is a weak acid. According to the Environmental Protection Agency in addition to CO2, sulfur dioxide and nitrogen oxide also contribute to the acidity of rain (Balun, 2021). As this slightly acidic water percolates through soil and rocks into underground aquifers it passes though layers of limestone and gypsum. These rocks are rich in calcium and magnesium, which the water absorbs as it gets filtered through them. These minerals cause hard water, and the more minerals there are the harder the water becomes. This is why certain cities and counties within the same state can have varying degrees of water hardness.