Which of the following is a component of total hardness?

Total hardness in water primarily refers to the concentration of divalent metal ions, specifically calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions contribute to the scaling and precipitation processes that can occur in water systems, affecting water quality and treatment processes.

In this scenario, magnesium ions (Mg²⁺) are indeed a direct contributor to total hardness, making it the correct choice. They are commonly found in natural waters as a result of the leaching of minerals, and their presence is critical in determining the hardness levels necessary for various applications, including in industrial processes and ecological health.

The other components listed do not contribute to total hardness: water (H2O) is a solvent and does not affect hardness levels, sodium ions (Na⁺) are typically associated with salinity or alkalinity rather than hardness, and chloride ions (Cl⁻) are similarly unrelated to hardness measurements. Understanding the importance of magnesium and calcium in defining total hardness is essential for water quality assessment and management.