What are the principal causes of hardness in water?

The principal causes of hardness in water are calcium and magnesium. These two elements are commonly found in natural water sources and contribute significantly to the overall hardness of water. Hard water is characterized by its high mineral content, primarily due to the dissolution of minerals from soils and rocks that water comes into contact with during its natural cycle.

Calcium typically comes from limestone and other calcium-rich mineral deposits, while magnesium is found in minerals like dolomite. When water passes through these geological formations, it can dissolve calcium and magnesium ions, leading to increased hardness. This hardness can impact the effectiveness of soap and detergents and contribute to scaling in pipes and appliances.

In contrast, options such as sodium and potassium do not contribute to water hardness; instead, they result in softening effects. Iron and manganese, while they can affect water quality and taste, are not primary contributors to hardness, and lead and zinc are metals that are typically regarded in the context of water contamination rather than hardness. Overall, calcium and magnesium are the key elements responsible for the hardness of water, making them the correct answer.