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2.
States
of matter
Elements
and compounds can be in various states like solids, liquids or gaseous.
These are the three states of matter
The distance between neighboring atoms or molecules in a substance decides
how tightly the binding is. In a solid, the atoms or molecules are very
tightly bound. In comparison, in a liquid the binding is not very strong. In
a gaseous state the binding is even weaker.
Matter in a solid state has a definite shape or form. It is generally
defined in terms of the weight of the solid. On the other hand, matter in a
liquid state has no definite shape or form; the liquid takes the form of the
container in which it is placed. It may not fill the container completely
but the liquid can be defined in terms of volumes of liquid. Matter in a
gaseous state is neither in a definite shape nor volume. It is very
ephemeral. If put in a container, it fills the container completely! The
container volume in which it is placed defines a gaseous state of a
substance.
Temperature plays a very crucial
role in the type of binding that occurs between the atoms. Take little
amount of water in a container. A molecule of water is made up of two
hydrogen atoms and one oxygen atom. At ordinary temperatures, water is in
liquid form. If we heat the container to 100oC, the water inside
will boil and become steam. The binding between each molecule of water is
broken due to heat and therefore the distance between each molecule becomes
large. Steam is the gaseous state of water. If we reduce the temperature,
the steam will condense to become water again. Now, if we reduce the
temperature of the container to 0 oC, the water will solidify
into ice. Thus the three states of matter for a water molecule are : ice in
solid state, liquid water in liquid state and steam in gaseous state.
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Solid
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Liquid
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Gas
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Representation of molecules in solid, liquid and gaseous state
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We
have seen that matter can be quantified in terms of its weight and its
volume. If you take a piece of iron and wood of the same volume, you will
see that the wood weighs far less than the iron. You can intuitively say
that the iron is more densely packed with its atoms than the wood. We have
very easily crept the word density in our discussion. Density of a substance
decides whether it is in the solid, liquid or gaseous state.
Density
is defined as follows :
Mass of substance (in gm)
Density = 
Volume of substance (in cubic cm)
Generally,
if the density is high then the substance is in solid form. If the density
is low, then the substance is in liquid form. If the density is even lower,
then the substance is in gaseous state. It is imperative here, that density
is also a function of temperature. Density of iron is 7.86 gm/cc. The
density of iron in liquid form (at a temperature of 1537oC) is
approximately 7.23 gm/cc. Thus it can be seen that the solid iron volume
expands on heating and becomes liquid. Other than ice, all substances become
more dense when they solidify. Water is the only substance in nature whose
volume contracts on solidification. The density of ice is 0.93 gm/cc, where
as the density of water is 1 gm/cc. This is the reason why ice floats on
water. A piece of iron is
heavier i.e. more dense than water, so it will sink. Density of teakwood is said
to be 0.66 gm/cc, so it will float on water.
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Elements
and compounds can change their state from solid to liquid to gas, if
external heat is applied or removed. Some elements like Iodine can change
from solid state directly to gaseous state without the intervening liquid
state. It has to be remembered that if a compound while changing state is
also loosing its properties, then the picture is not that of change of
state, instead it is decomposition of compound.
In some cases a change of state can also be accomplished by
application of higher pressure or a combination of both change of
temperature and pressure. |
Interconversion of Matter
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