Which of the following elements contributes to water hardness?

Calcium and magnesium are the primary elements that contribute to water hardness. Hard water contains high concentrations of these minerals, which can originate from various sources such as the dissolution of limestone and other geological formations. When water percolates through soil and rock, it picks up calcium and magnesium ions, leading to the hardness of the water.

Hard water can have significant implications for both household and industrial use. For instance, it may lead to scale buildup in plumbing and appliances, affecting their efficiency and lifespan. Additionally, it can reduce the effectiveness of soaps and detergents, making it more challenging to create lather and clean effectively.

The other options presented relate to different chemical elements that do not contribute to water hardness. Sodium and potassium, for example, are more associated with water salinity than hardness. Nitrogen and sulfur are involved in various chemical processes and can impact water quality but are not related to hardness. Fluoride and oxygen, while important for other aspects of water chemistry, also do not play a role in determining water hardness. Therefore, recognizing the significance of calcium and magnesium in this context is crucial for understanding water quality and its implications.