Hard water is formed when the water passes through deposits of chalk and limestone, which are mainly comprised of magnesium and calcium carbonates. Hard water can cause problems with cooling towers, boilers, and other equipment in industrial settings. If hard water is present, water softening (removal of magnesium and calcium ions) is commonly used to reduce the effects of hard water. A few examples of hard water include: rings in the bathtub and mineral deposits on dishes. Most people assume that the scum is from poor housekeeping, but these two scenarios are examples of hard water. The ions found in hard water usually do not pose any health threat, but they do leave mineral deposits, which can make water unsuitable for many different uses.