What compound is often responsible for the deposits found inside boilers and hot-water heaters?

The correct answer, calcium carbonate (CaCO3), is commonly associated with the formation of deposits, or scale, in boilers and hot-water heaters. This phenomenon occurs due to the precipitation of dissolved minerals that find their way into the water supply. When water is heated, its ability to hold certain minerals decreases, leading to the crystallization of calcium carbonate, especially in higher temperature environments like those found in boilers.

Calcium carbonate can originate from the natural hardness of the water, where calcium ions are prevalent. Over time, as water is heated repeatedly, the accumulated salts can form thick layers of scale that can insulate heating elements, reduce efficiency, and potentially lead to greater risks of overheating or even equipment failure.

Other compounds listed, such as sodium chloride (NaCl), magnesium sulfate (MgSO4), and iron(III) oxide (Fe2O3), do not typically contribute to deposit formation in the same significant way as calcium carbonate does. Sodium chloride might dissolve well in water but does not precipitate as scale upon heating in the same manner. Magnesium sulfate does have its own solubility characteristics, but it’s less likely to form the solid deposits that are characteristic of scale in this context. Iron(III) oxide is associated with