What is the hardness level that classifies water as hard?

Water is classified as hard based on its mineral content, particularly the levels of calcium and magnesium ions present. The classification generally stipulates that water with a hardness level between 7 and 10 grains per gallon is considered moderately hard. This range signifies the onset of noticeable effects on appliances and plumbing systems due to mineral buildup, which can impact the efficiency and longevity of steam equipment and other appliances in the food service industry.

Understanding hardness levels is crucial for maintenance and operational efficiency, as hard water can lead to scale formation, which affects heating efficiency and can cause equipment failure over time if not managed properly. The other ranges, while they indicate varying degrees of hardness, do not fall within this commonly accepted classification for moderate hard water, making the range of 7-10 grains per gallon the correct answer in this context.