All boiler water contains dissolved solids. When the feed water is heated, it evaporates and goes off as distilled steam leaving impurities behind. As more and more water is distilled in the boiler, more feed water is added to replace it. As a result the amount of these solids dissolved in the boiler water gradually increases. After a while there is so much of these highly soluble solids in solution in the boiler water that it does not boil like ordinary water, it boils more like a syrup.