Notes and Important Points on Solid State - JEE Main 2015 by ednexa.com

1. (1) Graphite is used as a lubricant. Explain.
Sol:
(1) In graphite, all the carbon atoms are sp2 hybridised.
(2) Each carbon atom is covalently bonded to three other sp2 hybridised
carbon atoms forming σ bonds, while fourth electron in 2pz orbitals is used in
the formation of π bond. This results in the formation of hexagonal rings in
two dimensions.
(3) In graphite, the layers consisting of hexagonal carbon network are held
together by weak van der Waals forces imparting softness.
(4) The layers in graphite can slide over another layer on applying a shearing
force; hence it is used as a lubricant and in lead pencils.
(2) Iron is strongly ferromagnetic. Explain.
Sol:
(1) Iron, 26Fe has electronic configuration, [Ar]18, 3d6, 4s2.
(2) Since Fe has four unpaired electrons, it is strongly ferromagnetic.
(3) Solid ice is lighter than water. Explain.
Sol:
(1) Ice has hexagonal three dimensional crystal structure formed due to
intermolecular hydrogen bonding which leave almost half the space vacant.
(2) The structures of liquid water and solid ice are almost identical.
(3) On melting of ice, some hydrogen bonds are broken and vacant spaces
are occupied by water molecules, and they accommodated more closely.

2. (4) In the solid ice, vacant space is more than in liquid water. Therefore the
density of ice is less than water
(5) Hence ice is lighter than water and ice floats on the surface of water.
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