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2019 O Level Chem P1 Q26 — Transition Elements / Properties Solution

Transition Elements / Properties 2019 O Level Pure Chemistry · Paper 1 · Q26

Question

Vanadium has various oxidation states and forms coloured compounds. The aqueous VO²⁺ ion is blue while the aqueous VO₂⁺ ion is yellow. The aqueous V³⁺ ion is blue-green and the aqueous V²⁺ ion is violet in colour. Which statement is correct?

AWhen a vanadium compound in solution changes colour from violet to blue-green, reduction has taken place.
BVanadium has a high density.
CVanadium compounds are unlikely to be catalysts.
DVanadium has a low melting point.

Correct answer: B

Video Solution

Explanation

Answer: B (vanadium has a high density). Vanadium is a transition metal, and transition metals are dense, hard metals with high melting points; a density of about 6 g/cm³ is typical, so B is correct.

Option A is wrong: a change from violet V²⁺ to blue-green V³⁺ means the oxidation state rises from +2 to +3, which is a loss of an electron, so that is oxidation, not reduction. Option C is wrong: vanadium compounds are good catalysts — vanadium(V) oxide, V₂O₅, is the catalyst in the Contact process for making sulfuric acid. Option D is wrong: vanadium has a very high melting point (around 1900 °C), as expected for a transition metal, not a low one.

The variable oxidation states and coloured ions in the question are themselves clues that vanadium is a transition element with typically high density, melting point and catalytic activity, so the answer is B.

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