Question

Zinc oxide, which in colourless at room temperature, acquires yellow colour on heating due to

• A. Zn being a transition element
• B. paramagnetic nature of the compound
• C. trapping of electron at the site
• D. both (A) and (B)
  

Answer 1

Answer: (B)

paramagnetic nature of the compound

In ZnO, Zn2+ ions occupy the interstitial sites and electrons are trapped in the interstitial sites due to neutralization. Crystals showing such properties are usually colored. Zinc oxide is white in colour at room temperature but acquires yellow colour on heating because it loses oxygen reversibly at high temperature.

ZnO on heating gives Zn2+1/2 O2+ 2e-

In oxygen-deficient ZnO the free electrons which keep the compound electrically neutral are less tightly bound and may be promoted by absorption of photons in the violet region and one can observe the complementary colour of violet i.e. yellow