Water hardness is caused by dissolved calcium and magnesium ions; temporary hardness from hydrogencarbonates can be removed by boiling. Which option best reflects this statement?

• A. Hardness is caused by dissolved calcium and magnesium ions; temporary hardness from hydrogencarbonates can be removed by boiling.
• B. Hardness is caused by dissolved sodium ions; temporary hardness from hydrogencarbonates cannot be removed by boiling.
• C. Hardness is caused by dissolved iron ions; boiling removes chlorides.
• D. Hardness is unrelated to ions; boiling has no effect.

Answer: (A)

Water hardness comes from dissolved calcium and magnesium ions, which make the water feel “hard” because these ions don’t react away easily. Temporary hardness specifically arises from hydrogencarbonates of those ions, and boiling removes it because heating breaks down the hydrogencarbonates into carbonates, CO2, and water. The carbonate ions then form insoluble carbonates with Ca2+ and Mg2+, which precipitate out and remove the hardness-causing ions from solution. What remains after boiling is the permanent hardness, caused by other salts that don’t decompose on heating. So the best statement matches that hardness is due to calcium and magnesium ions, and temporary hardness from hydrogencarbonates can be removed by boiling.