Chemical Tests for Water Purity

Steps:

• Recall that anhydrous copper(II) sulfate is white and turns blue upon hydration by water molecules.
• Evaluate option A: Distillation separates based on boiling points, targeting soluble impurities, not insoluble ones like sediment.
• Evaluate option B: Impure water still hydrates the compound, causing the color change.
• Evaluate options C and D: Compare against standard tests—cobalt paper turns pink with water, and impurities elevate boiling point above 100°C.

Why B is correct:

• Anhydrous CuSO₄ reacts with H₂O to form blue CuSO₄·5H₂O, and impurities do not prevent hydration in typical water samples.

Why the others are wrong:

• A: Distillation removes volatile or soluble impurities; insoluble ones require filtration.
• C: Cobalt(II) chloride paper is blue when dry and turns pink with water.
• D: Dissolved impurities raise the boiling point above 100°C via colligative properties.

Final answer: B