The scales formed due to hard water containing are:
Easy to remove
Difficult to remove
Soluble in water
Formed due to soap hydrolysis
In a boiler, water is continuously converted into steam which in turn increases the salt concentration.
A stage is reached when these change into a loose and slimy precipitate called sludge and if there are hard adhering coatings called scales.
Thus, scales are hard deposits, which stick very firmly to the inner surface of the boiler.