What characteristic defines water that requires a large amount of soap to get a lather?

Water that requires a large amount of soap to generate a lather is classified as hard water. This characteristic is primarily due to the presence of dissolved minerals, predominantly calcium and magnesium ions, which interfere with the lathering action of soap. When soap is added to hard water, these minerals react with the soap molecules, forming insoluble compounds that do not produce lather effectively.

In contrast, soft water contains fewer dissolved minerals, allowing soap to work effectively and produce a good lather with less product. Contaminated water can have a variety of characteristics depending on the particular contaminants present but does not specifically correlate to lathering ability with soap. Distilled water, which is purified through the process of distillation, typically has very low mineral content and therefore would require little soap to create a lather, contrary to the focus of this question. Thus, the defining characteristic of hard water is its tendency to require greater amounts of soap to achieve lather due to mineral content.