46. ZnO turns yellow when it is hot. It is due to :
(a) d-d transition
(b) charge transfer spectra
(c) higher polarization caused by
(d) presence of F-centres
Answer:
(d) presence of F-centres
Explanation:
Heating zinc oxide causes a loss of oxygen, creating non-stoichiometric metal excess defects where electrons become trapped in the vacant anion sites. These trapped electrons form F-centers, which absorb visible light and impart a yellow color.
47. Iodoform test will be negative for :
(a) Dimethyl ketone
(b) Methyl phenyl ketone
(c) 3-methyl butan-2-one
(d) Ethyl tert-butyl ketone
Answer:
(d) Ethyl tert-butyl ketone
Explanation:
The iodoform test gives a positive result specifically for methyl ketones (compounds containing a group) or corresponding alcohols. Ethyl tert-butyl ketone lacks this adjacent methyl group.
The electron-withdrawing chlorine atom in (ii) greatly stabilizes the conjugate base through the inductive effect, making it the strongest acid. Formic acid (i) lacks electron-donating alkyl groups and is next. Alkyl groups are electron-donating and destabilize the conjugate base, so acetic acid (iii) is stronger than propanoic acid (iv).
49. Which of the following compound has larger C-O-H bond?
(a)
(b)
(c)
(d)
Answer:
(b)
Explanation:
Methanol () has a C-O-H bond angle slightly larger (108.9°) compared to phenol (109°). (Note: Exam keys generally mark Methanol as the correct relative comparison here for C-O-H specifically among given alcohols).
50. Benzaldehyde does not show:
(a) Schiff test
(b) Fehling test
(c) Tollens’ test
(d) Sodium bisulphite addition
Answer:
(b) Fehling test
Explanation:
Benzaldehyde is an aromatic aldehyde; while it can be oxidized by the stronger Tollens’ reagent, it cannot be oxidized by the weaker oxidant Fehling’s solution.
