Hydrogen Ques 2
2. At its melting point, ice is lighter than water because
[1992]
(a) $\mathrm{H}_2 \mathrm{O}$ molecules are more closely packed in solid state
(b) Ice crystals have hollow hexagonal arrangement of $\mathrm{H}_2 \mathrm{O}$ molecules.
(c) On melting of ice the $\mathrm{H}_2 \mathrm{O}$ molecule shrinks in size
(d) Ice froms mostly heavy water on first melting.
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Answer:
Correct Answer: 2.(b)
Solution: (b) In liquid state, the $\mathrm{H}_2 \mathrm{O}$ molecules are held together by intermolecular H-bonds.
When water freezes at atmospheric pressure it crystalises in normal hexagonal form. In it, each oxygen atom is surrounded by $4$ hydrogen atoms, two by strong covalent bonds and two by weak hydrogen bonds. Since hydrogen bonds are longer than covalent bonds. The molecules of water are not closely packed in crystal lattice. There exists a number of hollow spaces in the crystal lattice, which decreases the density of ice.
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