What is hard water primarily defined by?

Hard water is primarily defined by the presence of high levels of dissolved metal ions, particularly calcium and magnesium ions. These metal ions originate from the dissolution of minerals in the water, such as limestone and chalk, which contain calcium carbonate. When water percolates through these geological formations, it picks up these ions, leading to what we recognize as hard water.

The significance of measuring hardness in water comes from the impact these metal ions can have on soap's effectiveness, reducing lather and leading to scale formation in pipes and appliances. Thus, the characteristic that defines hard water is its elevated concentration of specific dissolved metal ions, rather than simply dissolved salts, pH levels, or gases. Understanding these properties is essential because they affect water use in households and industries.