What does the term "hardness" in water indicate?

The term "hardness" in water specifically refers to the concentration of calcium and magnesium ions present in the water. When water passes through limestone and other mineral deposits, it can dissolve these minerals, leading to increased levels of calcium (Ca²⁺) and magnesium (Mg²⁺) ions. This phenomenon directly affects many practical aspects of water use, including the efficiency of soaps and detergents, the potential for scaling in pipes and boilers, and overall water quality.

The presence of dissolved gases, temperature of the water, and mineral impurities pertain to different aspects of water chemistry and quality, but do not define the specific characteristic known as hardness. Thus, the focus on calcium and magnesium ions maps directly to the industry's understanding and management of water hardness, making it the correct interpretation in this context.