Question

Zinc oxide (white) is heated to high temperature for some time. Observe the following statement regarding above process:
I. Zinc oxide colour changes to pale yellow
II. The type of defect formed is ‘metal deficiency’
III. Some \( Zn^{2+} \) and \( e^- \) are present in interstitial places

• A. I, II only
• B. I, III only
• C. II, III only
• D. I, II, III only

Hint:

When zinc oxide is heated, it forms a metal deficiency defect. This is associated with a color change from white to yellow due to the presence of \( Zn^{2+} \) vacancies and interstitial electrons.

Answer 1

The Correct Option is B

Step 1: Zinc Oxide and its Behavior upon Heating When zinc oxide (ZnO) is heated at high temperatures, it undergoes a color change from white to pale yellow. This color change occurs due to the formation of a metal deficiency defect. The high temperature causes zinc atoms to leave their lattice sites, creating vacancies. These vacancies are filled with electrons, giving the material its yellow color.
Step 2: Type of Defect Formed The defect formed is known as a 'metal deficiency' defect. In this defect, the zinc ions (\( Zn^{2+} \)) are absent from certain sites in the crystal lattice, creating vacancies. These vacancies are compensated by electrons that occupy interstitial sites, resulting in the yellow color of heated zinc oxide. Thus, statements I and III are correct.
Step 3: Conclusion - Statement I is correct: The color change to pale yellow is observed when zinc oxide is heated. - Statement III is correct: The presence of \( Zn^{2+} \) and \( e^- \) in interstitial places is what causes the color change. - Statement II is incorrect because the defect is 'metal deficiency', not a simple vacancy defect caused by heating. Therefore, the correct answer is option (2) I, III only.