The furring of kettles is caused by the presence of what compound in water?

Step 1: Analyze the question "Furring" is the hard scale that forms inside kettles and boilers. We need the compound in water that is responsible for it.

Step 2: The concept Water hardness comes in two types:

• Temporary hardness is caused by dissolved calcium hydrogentrioxocarbonate (IV), Ca(HCO₃)₂ (calcium hydrogencarbonate). It can be removed by boiling.
• Permanent hardness is caused by calcium or magnesium sulphates and chlorides. Boiling does not remove it.

Step 3: What happens when you boil the water Heating decomposes the soluble calcium hydrogencarbonate into insoluble calcium carbonate:

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

The insoluble CaCO₃ (calcium trioxocarbonate (IV)) deposits as a hard, chalky layer on the kettle walls and heating element. This layer is the "fur".

Step 4: Evaluate the options

• A. Calcium tetraoxosulphate (IV): CaSO₄ causes permanent hardness and does not precipitate on boiling. Wrong.
• B. Calcium hydrogentrioxocarbonate (IV): This is the compound present in the water that decomposes on heating to form the scale. Correct.
• C. Calcium hydroxide: This is not the cause of hardness in natural water. Wrong.
• D. Calcium trioxocarbonate (IV): This is the scale itself, but it is the product of the reaction, not the compound present in the water (it is almost insoluble). Wrong.

Tip: Remember "Temporary hardness = hydrogencarbonate = removed by boiling = furring." The question asks what is in the water, so choose the soluble hydrogencarbonate (B), not the carbonate scale (D) that is left behind.