Hard water is water that contains a high proportion of dissolved minerals, especially calcium and magnesium. Groundwater and seawater contain calcium ions (Ca2+) and magnesium ions (Mg2+), respectively, and the hardness level increases with the concentration of these minerals. The presence of these ions can lead to the formation of insoluble deposits of calcium and magnesium salts, which are visible when hard water is exposed to high temperatures or when soap is added to it.

It's important to note that water is never completely pure in nature and always contains some minerals and salts. The concentration of these minerals determines water hardness, with water containing more than 100 mg/L of minerals considered hard water. Water with less mineral content is considered soft water, and it can be distilled by specialized equipment for use in certain chemical and industrial processes, but it's not suitable for drinking.

As water moves through soil and rocks, it dissolves small amounts of minerals and retains them in solution. Therefore, the mineral concentration varies from lake to lake and from river to river, depending on the soil and rocks present. Groundwater is usually hard water.

While hard water doesn't pose a health risk, it can be a nuisance due to some of its effects, such as making it difficult to create soap lather while bathing and leaving a layer of soap residue on the skin. Mineral deposits remaining on the body can also absorb moisture directly from the skin, leading to dry skin. A study showed that exposure to hard water can increase the risk of eczema in children.

Hard water can also cause mineral buildup on plumbing fixtures and appliances, making them wear out quickly and not function efficiently. It can also cause yellowish stains that tend to be red or brown in some areas. This color may be due to rusted metal or pipes, and it's visible at the base of bathtubs. It can also be visible on white clothes washed in hard water.

High water hardness levels can cause water pollution, making it harmful for use. However, it's easy to distinguish between hard and soft water because hard water has a metallic taste and odor. The World Health Organization also notes that hard water has an additional health benefit because calcium and magnesium are important minerals that the body needs for bone and muscle growth and function. These minerals also regulate blood pressure and enzyme processes and consuming hard water can be a source of these minerals.

Water hardness can be removed by boiling, adding chemicals such as calcium hydroxide (slaked lime) or sodium carbonate (washing soda), using sodium hexametaphosphate, or through ion exchange.

The Vlaby Virtual Science Lab platform can be used to classify the hardness of water.