A student investigates two unlabelled water samples, X and Y. Boiling sample X eliminates … — JAMB UTME

A student investigates two unlabelled water samples, $X$ and $Y$ . Boiling sample $X$ eliminates its soap-consuming property, allowing it to lather readily, whereas sample $Y$ continues to form scum with soap after boiling. Which solute present in sample $Y$ accounts for its persistent hardness?

Calcium tetraoxosulfate(VI)Answer
B
Calcium hydrogentrioxocarbonate(IV)
C
Magnesium hydrogentrioxocarbonate(IV)
D
Sodium chloride

Answer

Calcium tetraoxosulfate(VI) accounts for the persistent (permanent) water hardness in sample Y.

Permanent water hardness is caused by dissolved calcium tetraoxosulfate(VI) or magnesium chloride. These salts do not decompose upon heating, so boiling does not precipitate the metal ions responsible for consuming soap.

Step-by-Step Solution

Classify the type of hardness based on response to boiling.

Sample X exhibits temporary hardness because boiling removes its soap-consuming capability. Sample Y exhibits permanent hardness because boiling fails to remove its scum-forming property.

Temporary hardness is caused by hydrogen trioxocarbonates which decompose on boiling, whereas permanent hardness is caused by soluble sulfate or chloride salts of calcium or magnesium.

Identify the specific chemical species responsible for permanent hardness in sample Y.

Calcium tetraoxosulfate(VI) (

\text{CaSO}_4

) remains dissolved after boiling.

Tetraoxosulfate(VI) ions do not undergo thermal decomposition at boiling temperatures to form insoluble precipitates.

Key Concept

Temporary vs. Permanent Water Hardness

Estimated Time:1m 0s

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