which of the following statements is not true concerning hexavalent chromium?
a. it is widely used in wood preservation
b. small amounts of chromium are responsible for the color in serpentine
c. hexalvent chromium may be reduced to trivalent chromium through isomorphic substitution
d. it is extracted from the oxide mineral chromite
The correct answer and explanation is:
Correct Answer: C. Hexavalent chromium may be reduced to trivalent chromium through isomorphic substitution
Explanation:
Hexavalent chromium (Cr⁶⁺) is a highly toxic and carcinogenic form of the element chromium. It is primarily produced during industrial processes such as electroplating, stainless steel production, and manufacturing pigments. The correct understanding of each statement is necessary to identify the false one.
Option A is true. Hexavalent chromium has been widely used in wood preservation, especially in formulations like chromated copper arsenate (CCA). This mixture protects wood from decay and insect damage, and it contains chromium in the hexavalent form, which helps fix the preservative chemicals to the wood fibers.
Option B is true. Serpentine minerals, which often contain small amounts of chromium, exhibit green coloration due to the presence of chromium ions. While trivalent chromium (Cr³⁺) is usually the main coloring agent, chromium’s role in mineral coloration is established and accepted.
Option C is not true. Isomorphic substitution refers to the replacement of one ion with another in a crystal lattice without changing the overall structure. This process typically involves ions of similar size and charge. However, hexavalent chromium (Cr⁶⁺) and trivalent chromium (Cr³⁺) differ significantly in charge and size, making such substitution between these oxidation states unlikely within mineral structures. The reduction of Cr⁶⁺ to Cr³⁺ is a chemical redox process, not an isomorphic substitution. This is the statement that is false.
Option D is true. Hexavalent chromium is derived from chromite, an oxide mineral composed mainly of FeCr₂O₄. During processing, chromite ore is oxidized in high-temperature furnaces to extract chromium, which can be converted to the hexavalent form depending on industrial needs.
Thus, option C is incorrect, making it the correct answer to the question.