Matter is anything that has mass and occupies space.
There are three common states of matter i.e. solid,
liquid and gas. Solid has definite shape as well as
definite volume. Liquid has a definite volume but not
definite shape and it takes the shape of vessel in which
it is kept. Gas has neither definite shape nor definite
volume, it takes the shape of vessel in which it is kept
and occupies all the space available to it.
6.1.1. Kinetic Description of States of Matter
According to kinetic molecular theory, matter is
composed of very tiny particles, which are called as
molecules. These molecules are always in motion and they
possess kinetic energy. The three states of matter
depend upon the arrangements, motions and forces of
attraction between these particles. The difference
between solid, liquid and gas is explained as follows:
Solid State: In solid state molecules or
particles are tightly packed with one another and they
have only back and forth motion (Vibrational motion)
about their fixed positions. Therefore particles in
solid cannot slip or slide over one another and thus
they possess definite shape and definite volume.
Liquid State: In liquid state molecules are not
tightly packed with one another. Their positions are not
fixed and they can move in all direction, hence liquid
does not have any definite shape. But in liquid state
the kinetic energy of molecules is less than gaseous
state. Therefore intermolecular attractive forces are
more than gaseous state. Due to these intermolecular
attractive forces liquid has a fixed volume.
Gaseous State: In gaseous state molecules are
lying away from one another and they can move in all
direction easily. The cohesive forces in a gas are
negligible and the particles are free to move about in
all directions. The kinetic energy possessed by these
molecules are very high, therefore gases have neither
definite shape nor definite volume.
6.1.2 Inter Conversion of Three
Most of the important properties of liquids, solids and
gases depend upon the inter conversion of one state into
another state. For example when ice is warmed, it melts
into liquid state, water. When water is heated and
boiled, it changes into vapour (gas) state; some solids
upon heating instead of changing into liquid state are
converted directly into gaseous (vapour) state. This
process is called sublimation. The examples of such
substances are iodine, ammonium chloride and napthaline.
These chemical substances being in solid state, when
heated are sublimed directly kV vapours state. In
general, each state may be converted into another state
is explained as given below:
When a solid is heated, then the kinetic energy of
particles increases and it becomes hot. If heating is
continued then at certain temperature, the added energy
becomes enough to overcome the attractive forces holding
particles of solid in their fixed positions and it
starts melting. The temperature at which solid starts
melting is called melting point. At this point solid
particles lose their fixed positions as well as their
arrangements and thus solid is converted into liquid. At
melting point the temperature of solid and liquid
remains same until all the solid is melted though heat.
It means in this condition the rise in temperature is
used in change of state from solid to liquid and this
change is called fusion.
When liquid is heated, the kinetic energy of liquid
molecules increases. Due to increase of kinetic energy
certain molecules start escaping from
the surface of liquid and this escape of molecules is
called evaporation. At certain temperature the
vapour pressure of liquid becomes equal to external
pressure and at this temperature liquid starts boiling.
This temperature is called boiling point.
Actually boiling point is the temperature at which the
vapour pressure of a liquid becomes equal to external
pressure applied on the liquid. At this point bubbles of
vapours are able to form within the interior of liquid
and then rise to the surface, where they burst and
release vapours and thus liquid is converted into gas.
At boiling point the temperature of liquid remains same
until all of the liquid is evaporated or vapourized
though heat. It means in this condition the rise in
temperature is used in the change of state from liquid
to gas, and this change is called vapourization.