Daily answer writing · Sat, 7 Nov, 2026
Zr and Hf are twins (lanthanide contraction); o-nitrophenol is steam-volatile but p-nitrophenol is not
Answer both parts (5 marks each).
(a) Zirconium (Z = 40) and hafnium (Z = 72) have almost identical atomic radii (about 160 and 159 pm) and are among the hardest pair of elements to separate. Explain this on the basis of the lanthanide contraction, and give two other consequences of the contraction.
(b) o-Nitrophenol is steam-volatile and less soluble in water than p-nitrophenol, and boils much lower (about 214 °C against about 279 °C). Explain.
Syllabus: Mixed day. (a) f-block elements, lanthanide contraction: CSE Paper I, IFoS Paper I, Geo-Scientist Mains (inorganic). (b) Hydrogen bonding and physical properties of organic compounds: CSE Paper I/II, IFoS Paper I/II, Geo-Scientist Mains Papers I and III.
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