Hard water is water that has high mineral content (in contrast with soft water). Hard water minerals primarily consist of calcium (Ca2+), and magnesium (Mg2+) metal cations, and sometimes other dissolved compounds such as bicarbonates and sulfates. Calcium usually enters the water as either calcium carbonate (CaCO3), in the form of limestone and chalk, or calcium sulfate (CaSO4), in the form of other mineral deposits. The predominant source of magnesium is dolomite (CaMg(CO3)2). Hard water is generally not harmful to one's health.
The simplest way to determine the hardness of water is the lather/froth test: soap or toothpaste, when agitated, lathers easily in soft water but not in hard water. More exact measurements of hardness can be obtained through a wet titration. The total water 'hardness' (including both Ca2+ and Mg2+ ions) is read as parts per million (ppm) or weight/volume (mg/L) of calcium carbonate (CaCO3) in the water. Although water hardness usually measures only the total concentrations of calcium and magnesium (the two most prevalent, divalent metal ions), iron, aluminium, and manganese may also be present at elevated levels in some geographical locations. Iron in this case is important for, if present, it will be in its tervalent form, causing the calcification to be brownish (the color of rust) instead of white (the color of most of the other compounds).
Soft water used to describe types of water that contain few or no calcium or magnesium metal. The term is usually related to hard water, which does contain significant amounts of these ions.
Soft water usually comes from peat or igneous rock sources, such as granite but may also derive from sandstone sources, since such sedimentary rocks are usually low in calcium and magnesium. Water softened by sodium ion exchange will have a higher sodium ion content than the natural water it was derived from.
Water treated through a reverse osmosis unit will be very "soft" because most of the salts are removed. Typically this water will need some hardness correction or blending with hard water before it can be used in normal reticulation systems. Adding Epsom salt (magnesium sulfate) replaces magnesium ions. Soft water does not contain dissolved substances that produce scum and scale.
| 
