Hard water is water that contains dissolved chemicals that act on soap to form a scum. If water comes from limestone areas, some rock is dissolved in the water, and this makes it hard.
There are several disadvantages in hard water. More soap or soap powder must be used to obtain a suitable lather. Also, the scum clings to the object being washed. Hard water leaves a scaly deposit in kettles and boilers, which reduces the efficiency of both.
But hard water can be treated to remove the unwanted chemicals. In the home small amounts of washing soda or borax can be added. At large water softening plants which serve a community, the water is filtered through a mineral called zeolite ceases to be effective, but it can be restored by washing it with salt water.