The furring of kettles and boilers are caused by

Analyze the question: Furring (scale) in kettles and boilers is the hard, chalky deposit that builds up inside them. The question asks which compound causes it.

Concept: Temporary hardness of water is caused by dissolved calcium hydrogen trioxocarbonate (IV), Ca(HCO₃)₂ (and the magnesium equivalent). It forms when rainwater containing CO₂ (carbonic acid) flows over limestone:

CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (soluble)

Solution: When hard water is heated in a kettle or boiler, the soluble calcium hydrogen trioxocarbonate (IV) decomposes:

Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)

The insoluble calcium trioxocarbonate (IV) (CaCO₃) precipitates and sticks to the inside of the vessel. This is the "fur" or scale.

Which option is the right one?

Here there is a subtlety. Strictly speaking:

• The cause of the furring is the dissolved calcium hydrogen trioxocarbonate (IV), which is the temporary hardness in the water (option B).
• The fur itself is the solid calcium trioxocarbonate (IV), CaCO₃ (option C).

The question asks what causes furring. Water containing Ca(HCO₃)₂ is the source of the problem, and heating it produces the deposit.

Checking the other options:

• A: Ca(OH)₂ is not found in natural hard water in any significant amount.
• D: "Calcium tetraoxocarbonate" is not a valid name, and the tetraoxocarbonate (VI) would be a sulphate-like confusion. Calcium tetraoxosulphate (VI), CaSO₄, causes permanent hardness, which does not fur on boiling in the same way.
• E: Magnesium tetraoxocarbonate (VI) is not a real compound name; carbonates are trioxocarbonate (IV).

Tip: Remember the chain: Temporary hardness → hydrogen trioxocarbonate (IV) → boiling → CaCO₃ scale. If a question says "causes" or "temporary hardness," choose the hydrogen trioxocarbonate (IV). If it asks what the "fur/scale is made of," choose CaCO₃.