what is diff. between gas and vapour

There is a fundamental difference between
the state of gas and the state of vapor: substances as vapors can be liquefied, whereas
substances as gases can not be liquefied. Explanation
follows.

A real gas at high temperatures and low pressures
behaves exactly like an ideal gas. It means that the equation is
respected no matter how high the pressure is, given a sufficient high temperature. When
the temperature is lowered, initially at low pressures the equation will
hold, but by further increasing pressure, above low will fail to describe the behaviour
of the substance. Liquid drops will begin to form in the substance until all the gas
will be transformed to liquid by increasing the
pressure.

In this state it is said that the liquid is in
equilibrium with its vapors. Thus the name of vapor state.

In a diagram P-T (not P-V) of the states of matter there
will be four different phases: solid, liquid, vapor (which will meet all in a point
named the triple point of the substance) and gas.The gas and vapour phase will meet also
in the so called critical point. See the figure
attached.

Now you have the answer: if the substance is at a
temperature higher than the critical temperature it will be gaseous (can not be
liquefied by increasing pressure at whatsoever value), and will be vapor below the
critical temperature (when by increasing the pressure enough it will become all
liquid).

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