What defines hard water?

Hard water is primarily defined by its mineral content, particularly the presence of high levels of calcium and magnesium ions. When water has a significant concentration of these minerals, it often results in difficulty producing suds when soap is added, which is a characteristic of hard water. This is because the minerals react with soap to form insoluble compounds, leading to reduced lathering and increased soap usage.

The other options, while related to water quality, do not accurately define hard water. High chlorine levels pertain more to water disinfection and safety rather than hardness. The presence of bacteria is an entirely different concern that relates to water safety and potability, not mineral content. Thus, the hallmark of hard water is indeed its impact on soap's effectiveness, making the definition that focuses on difficulty in producing suds the most accurate representation of hard water.