Hardness of Water
Background
Water is an excellent solvent and even very pure natural waters contain some dissolved minerals. The ground water which is used to supply water to the tap will often contain enough minerals that it is called “hard water”. Hard water is water that has metal ions dissolved in it such as magnesium (Mg2+), calcium (Ca2+), and/or iron (Fe3+). These ions are generally harmless and can be consumed without ill effects. Even so, hard water has the disadvantage of making washing up harder to do. In hard water soap forms an insoluble salt with the metal ions and is not available to do the work of washing. Instead it leaves an unsightly scum behind both in the sink and on the things being washed. Hard water also leaves a white flaky substance called scale inside pots and appliances used to heat water (such as coffee machines and tea kettles).
Soft water is water with much lower amounts of dissolved minerals. Soft water is ideal for washing since soap works best when there are only small amounts of dissolved minerals. Soap can create far more bubbles in soft water than it can in hard water.
By using more soap it is possible to get the same cleaning power and sudsing ability in hard water that much smaller amounts of soap provide in soft water. This is possible because all of the minerals in the hard water are used up in combining with the extra soap and more soap is available for cleaning. This means that it costs more to do the washing and that more soap scum clings to the things being washed. This results in dingy clothes and dull dishes. Not to mention dull and lifeless hair.
Washing Soda (sodium carbonate, Na2CO3) is a water softener. When dissolved in hard water it makes soaps work better by reacting with the metal ions. It reacts to make insoluble white solids such as magnesium carbonate and calcium carbonate. Since the metal ions are busy reacting with the washing soda they are not available to react