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Why does borax soften water?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — boron group element Modern version of the periodic table of the elements (printable).

Borax, or sodium tetraborate decahydrate, functions as a water softener primarily due to its chemical properties in aqueous solutions.12

Water hardness is caused by the presence of dissolved calcium and magnesium salts. Borax softens water through two main mechanisms:34

Formation of Insoluble Precipitates: Borax reacts with the calcium and magnesium ions present in hard water to form insoluble borates. By converting these dissolved minerals into an insoluble form, they are effectively removed from the water, preventing them from interfering with soap or forming scale in pipes and boilers.342
Alkaline Nature: Aqueous solutions of borax are basic (alkaline). This alkalinity assists in the softening process by helping to precipitate the hardness-causing ions out of the solution.32

When used for this purpose, borax is often employed in conjunction with other agents, such as sodium carbonate (washing soda), to ensure the effective removal of these mineral salts.34