The furring of kettles is caused by the presence in water of

Analysis

The question asks for the chemical substance responsible for the "furring" (the formation of white, crusty deposits) inside kettles when water is boiled. This phenomenon is directly linked to the concept of temporary hardness of water.

Concept Explanation

• Hardness of water: This is caused by the presence of dissolved calcium and magnesium ions ($Ca^{2+}$ and $Mg^{2+}$) in water.
• Temporary Hardness: This type of hardness is caused specifically by the presence of calcium hydrogentrioxocarbonate (IV), also known as calcium bicarbonate, $Ca(HCO_3)_2$.
• The Process: When water containing calcium hydrogentrioxocarbonate (IV) is boiled, the heat causes the salt to decompose into insoluble calcium trioxocarbonate (IV) (calcium carbonate), water, and carbon (IV) oxide. • Equation: $Ca(HCO_3)_2(aq) \rightarrow CaCO_3(s) + H_2O(l) + CO_2(g)$
• The insoluble calcium trioxocarbonate (IV) ($CaCO_3$) precipitates out and sticks to the inner walls of the kettle, forming the hard, white crust known as "fur."

Step-by-Step Solution

1. Identify the cause of furring: Furring is the result of mineral deposits left behind after boiling water.
2. Evaluate the options: • A. Calcium hydrogentrioxocarbonate (IV): This is soluble in water and is the primary cause of temporary hardness. Upon heating, it decomposes to form the solid deposit. • B. Calcium trioxocarbonate (IV): This is the product formed after boiling, not the substance dissolved in the water that causes the furring in the first place. • C. Calcium tetraoxosulphate (VI): This causes permanent hardness, which cannot be removed by simple boiling. • D. Calcium hydroxide: This is "limewater" and is not typically responsible for the furring of kettles.
3. Conclusion: Since the question asks what is present in the water before it forms the fur, the correct substance is calcium hydrogentrioxocarbonate (IV).

Exam Tip

Remember the "Boiling Rule": If the hardness can be removed by boiling, it is temporary and caused by hydrogentrioxocarbonates. If it requires chemical treatment (like adding washing soda), it is permanent and caused by chlorides or tetraoxosulphates (VI) of calcium or magnesium.