What is the defining characteristic of hard water?

The defining characteristic of hard water is its high mineral content, particularly calcium and magnesium ions. When water percolates through soil and rocks, it picks up these minerals, which leads to its classification as hard water. Hard water can affect various applications, such as soap effectiveness, where it requires more soap to produce lather, and it can also lead to scale build-up in pipes and appliances.

The other options don't accurately describe hard water. For instance, while hard water can freeze like any other water, it doesn't have unique freezing properties. The clarity of the water does not determine its hardness; water can be crystal clear yet still contain high levels of dissolved minerals. Additionally, treating hard water typically involves processes like water softening, which can sometimes be costly, thus it isn’t correct to say that hard water is inexpensive to treat.