“In chemistry, hardness of water translates to the amount of minerals, mainly calcium and magnesium ions dissolved in water.
The hardness of water is one of the most essential parameters in many parts of the food industries, particularly in water treatment facilities.
Basically, the hardness level determines how well soap will react with water, or soap consuming property. Hard water requires more soap to produce foam or lather. Therefore, cleaning using hard water may require more soap.
Examples of soft water is tap water, or drinking water (equal or below 60 ppm). Interestingly, water from the mountains are also very low in minerals.
One good example of hard water, on the other hand, is seawater, and for obvious reasons. Seawater is rich in dissolved salts and has a typical hardness of at least 6,600 ppm (very hard).”