When water high in calcium bicarbonate is softened by ion exchange, what remains in the product water?

When water high in calcium bicarbonate is softened through the ion exchange process, the primary purpose is to replace the calcium ions (Ca²⁺) with sodium ions (Na⁺). During this process, calcium bicarbonate dissociates, and calcium ions are removed from the water, while sodium ions are exchanged. As a result, the sodium bicarbonate essentially remains in the softened water when this ion exchange occurs.

Sodium bicarbonate (NaHCO₃) does not precipitate out of solution like calcium carbonate would, and it is a common byproduct of the ion exchange process when water is treated in this manner. This is particularly relevant for water that initially contained high levels of calcium bicarbonate because the sodium ions introduced in the exchange process combine with the bicarbonate ions (HCO₃⁻) present in the water, leading to the formation of sodium bicarbonate.

Thus, the presence of sodium bicarbonate in the product water after the ion exchange is a direct result of the replacement of calcium ions with sodium ions, making this option the correct answer to the question.