Hard water has absorbed high quantities of minerals from the ground. This absorption occurs when rainwater percolates through the ground as it travels downward.

Areas with high chalk, gypsum, or limestone content typically have hard water. In addition, agricultural areas that use lime as a soil supplement may have hard water. By contrast, regions with low-calcium stones, such as granite, tend to have naturally soft water.

Hard water that contains high levels of dissolved minerals such as calcium and magnesium is not a health concern. However, it can build up as limescale in pipes, water heaters, and faucets and gradually impair their function. In addition, water with high mineral content may cause itching after bathing, reduce the lathering of soap, and create a sticky scum that is challenging to rinse from hair, clothes, sinks, and tubs.

Water softeners typically function through an ion exchange process that uses a negatively charged resin bed. As hard water travels through the ion exchange system, positively charged calcium, sodium chloride, and magnesium ions attract and stick to the resin bed. At the same time, sodium ions balance the electrical charge in the water. The water does not appear softer or taste different; it just has a substitute for the invisible elements that make it "hard," such as calcium and magnesium, in place.