Change
of state
Any matter around us can
be in three forms: solid, liquid and gasese forms of matter are called states
of matter. Depending upon the temperature, pressure and transfer of heat,
matter is converted from one state to another. The conversion of matter from
one state to another is called change of state in matter. The process of changing of
a substance
from one physical
state to another at a de nite temperature is de ned as change of state.
For example, water
molecules are in liquid state at normal temperature. When water is heated to
1000C, it becomes steam which is a gaseous state of matter. On reducing the
temperature of the steam it becomes water again. If we reduce the temperature
further to 00C, it becomes ice which is a solid state of water. Ice on heating,
becomes water again. us, water changes its state when there is a change in
temperature. ere are different such processes in the change of state in matter.
The gure 1.9 shows various processes of change state.
The process in which a
solid is converted to liquid by absorbing heat is called melting or fusion. The temperature at
which a solid changes its state to liquid is called melting point. The reverse of
melting is freezing. The process in which
a liquid is converted to solid by releasing heat is called freezing. The temperature at which a liquid changes
its state to solid is called freezing point. In the case of water, melting and
boiling occur at 0˚C.
The process in which a
liquid is converted to vapor by absorbing heat is called boiling or
vaporization. The temperature at which a liquid changes its state to gas is called boiling
point. The process
in which a vapor is converted to liquid by releasing heat is called
condensation. The temperature at which a vapour changes its state to liquid is
called condensation point. Boiling point as well as condensation point of water
is 100˚C.
Some solids like dry
ice, iodine, frozen carbon dioxide and naphthalene balls change directly from
solid state to gaseous state without becoming liquid. The process in which a
solid is converted to gaseous
state is called sublimation.
Various stages of
conversion of state of matter with heat with the corresponding change in
temperature is shown in Figure 1.10
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