A standard for the amount of these substances dissolved in water is set, and if more than the standard is dissolved, it is classified as 'hard water (hard water)', and if it is less than that standard, it is called 'soft water (sweet water)'.

Generally, 0-75 mg/L is divided into sweet water, 75-150 mg/L is relatively weak water, 150-300 mg/L is hard water, and 300 mg/L or more is very strong water.

Pure rainwater has a hardness of about 5 ppm. 17ppm is classified as soft water, but it is judged that such a phenomenon occurs because it is closer to hard water than rainwater. thank you.