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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.

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