Why does soap barely lather in some water?
In some water soap barely lathers; in some, scale builds up. The difference between hard and soft water is not whether the water is clean but how much mineral, like calcium and magnesium, is dissolved in it. In hard water these minerals bind to soap first, making scum instead of lather.
In some places, when you wash your hair, the shampoo hardly lathers and the soap seems to skate over you. In others, the lather is rich, yet your hands stay slippery long after rinsing. The inside of the kettle gathers a white powdery film, while some water leaves none. People call such water hard and soft. Water all looks the same, so how can there be a hard kind and a soft kind? What sets them apart?
We often suppose it is a matter of whether the water is clean or dirty, or tap versus filtered. But hard water can be perfectly clean. This is a different matter from dirtiness. The real divide lies where the eye cannot see: in what, and how much, is dissolved in the water. Whether water is hard or soft is set not by taste, temperature, or cleanliness, but by the amount of mineral dissolved in it.
Let us return to the first question. Why does soap barely lather in some water? Because that water is hard, that is, with much calcium and magnesium dissolved in it, and the soap must deal with that mineral before it can lather. So hard and soft do not tell whether water is clean but how much mineral is dissolved in it. Even the same clear water, depending on what and how much is dissolved, reacts to soap differently, leaves scale differently, and even feels different when rinsed. Lather, white scale, the slipperiness of a rinse: all are signs of the unseen amount of mineral showing on the surface. Because the underground layers differ from place to place, so does the mineral in the water, and that is exactly why washing your hair feels different when you travel.
Top: pick soft or hard water to compare whether the same soap lathers or makes scum. Bottom: move the hardness slider to see lather (left) drop and scale (right) grow as dissolved mineral rises.
As water passes through the ground, it dissolves and carries minerals like calcium and magnesium little by little. The more it has passed through mineral-rich layers like limestone, the more of these it holds. Much of this mineral makes hard water; little of it, soft water. The hardness of water is just the concentration of calcium and magnesium dissolved in it. Though it all looks equally clear and transparent, the amount of mineral dissolved within differs from water to water.
So why does soap barely lather in hard water? Soap works by raising lather and gathering up grime to wash it away. But the calcium and magnesium in hard water bind to the soap first, forming a residue that will not dissolve. So until the dissolved minerals have all bound to soap and settled out, the soap cannot lather. It is not that it will not lather, but that the soap is used up dealing with the minerals first. This is why you must use more soap or shampoo in hard water.
Soft water, by contrast, has little mineral to bind, so even a little soap lathers richly. Yet if the water is too soft, a slippery feel lingers after rinsing. With no mineral to grab and lift the soap away, the soap does not rinse off easily. The slipperiness is not from dirty water but, on the contrary, a feel that comes from too little mineral.
The white scale inside a kettle is a trace of the same minerals. The calcium and magnesium once dissolved in the water are left behind as a solid when the water boils or dries. The more mineral-rich the hard water, the more scale forms. Interestingly, hardness comes in two kinds: the kind that drops out and lessens when boiled, and the kind that stays even after boiling. The white powder left in a kettle is the trail the boiling water left behind.
So when water changes with a move or a trip, how much it lathers and how your hair feels can change. If your usual shampoo suddenly will not lather, the shampoo has not gone bad; the water there may be hard. Where scale gathers easily, mineral is building up in the kettle or showerhead, and you can wipe it away. The words hard and soft tell us that water, that clear liquid, is really many different waters, each holding different minerals. When a lather will not rise, there is a story of unseen mineral inside it. The water we use every day looks all the same, yet is not.