Short Answer Type
Chemistry
Class 12 solid
state
1 :- Why are liquids and
gases categorised as fluids?
Ans :- The liquids and gases have a property to flow i.e. the
molecules can move past and tumble over one another freely. Hence, they have
been categorised as fluids.
2. Why are solids incompressible?
Ans :-The distance between the constituent particles (atoms,
ions, molecules etc.) is very less in solids. On bringing them still closer
repulsion will start between electron clouds of these particles. Hence, they
cannot be brought further close to each other.
3 Inspite of long range order
in the arrangement of particles why are the crystals usually not perfect?
Ans :-Crystals have long range repeated pattern of arrangement
of constitutent particles but in the process of crystallisation some deviations
from the ideal arrangement (i.e. defects) may be introduced, therefore,
crystals are usually not perfect.
4:-. Why does table salt, NaCl, some times appear yellow in colour?
Ans :-Yellow colour in sodium chloride is due to metal excess
defect due to which unpaired electrons occupy anionic sites. These sites are
called F-centres. These electrons absorb energy from the visible region for the
excitation which makes crystal appear yellow.
5:- Why is FeO (s) not formed in stoichiometric composition?
Ans :-In the crystals of FeO, some of the Fe2+
cations are replaced by Fe3+ ions. Three Fe2+
ions are replaced by two Fe3+ ions to make up for
the loss of positive charge. Eventually there would be less amount of metal as
compared to stoichiometric proportion
6 :-. Why does white ZnO (s) becomes yellow upon heating?
Ans :-On heating ZnO loses oxygen according to the following
reaction.
Ans :- On doping germanium with galium some of the positions of
lattice of germanium are occupied by galium. Galium atom has only three valence
electrons. Therefore, fourth valency of nearby germanium atom is not satisfied.
The place remains vacant. This place is deficient of electrons and is therefore
called electron hole or electron vacancy. Electron from neighbouring atom comes
and fills the gap, thereby creating a hole in its original position. Under
theinfluence of electric field electrons move towards positively charged plates
through these holes and conduct electricity. The holes appear to move towards
negatively charged plates.
8:-. In a compound, nitrogen atoms (N) make cubic close packed
lattice and metal atoms (M) occupy one-third of the tetrahedral voids present.
Determine the formula of the compound formed by M and N?
Ans :-M2N3
9:- Under which situations can an amorphous substance change to
crystalline form?
Ans :- Crystalline solids have a sharp melting point. On the
other hand, amorphous solids soften over a range of temperature and can be
moulded and blown into various shapes. On heating they become crystalline at
some temperature. Some glass objects from ancient civilisations are found to
become milky in appearance because of some crystallisation