Did you know that in Vancouver you can wash your hair with soap but in Montreal you need shampoo? Why? Because Vancouver water is soft and Montreal water is hard. That makes a big difference when it comes to satisfaction with hair washing. Hard water contains dissolved calcium and magnesium ions which react with soap to form a precipitate. This is the classic “bathtub ring.” The grayish deposit is bad enough on a tub, but you sure don’t want it on the hair. Vancouver has very soft water and soap will suds nicely and will not leave a deposit. In Montreal, on the other hand, the calcium and magnesium in the water will react with soap and form an insoluble scum. The active ingredient in shampoo is a “detergent,” which like soap is a long molecule that has one end that is attracted to water and another that dissolves in fat. Dirt is embedded in the oily layer that naturally coats the hair, and the soap or detergent molecules can remove this by anchoring one end in the oil while the other end binds to water. Rinsing then removes the oily layer and the soil it harbours. The big difference between a soap and a detergent is that the detergent does not form a precipitate with calcium or magnesium. And that’s why you can use soap in Vancouver to wash your hair but not in Montreal.
Why? Because Vancouver water is soft and Montreal water is hard.