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An ideal gas refers to a gas where the particles constituting the gas are so small that their mass is approximately zero. The collisions between the particles is perfectly elastic; as a result the kinetic energy of the gas is conserved.
A real gas can be considered to behave like an ideal gas under low pressure and high temperature. As the temperature is decreased or the pressure increased, the behavior of the gas deviates from that of an ideal gas.
The conditions under which the behavior of a real gas deviates most from that of an ideal gas is under high pressure and low temperature.
Let say you have a REAL gas that is enclosed in a cylinder and subjected to a transformation at constant temperature (you compress the gas to smaller volumes by moving the piston of the cylinder keeping the temperature inside constant).
See the figure below.
First suppose the temperature T inside the cylinder is high enough (transformation 1 in the figure). The gas will obey the law of ideal gases PV= constant no matter how high the pressure P becomes inside by compressing the gas.
Second let us lower the temperature T inside (transformation 2 in the figure) and again, compress the gas inside. At the beginning again, the law of ideal gases will hold PV= constant By compressing further (at point A) inside the cylinder will form small drops of liquid. Now, from point A to point B by compressing all the gas inside will transform into liquid. The pressure will be the same on this portion of graph but the volume will keep becoming smaller and smaller. At point B (where there is only liquid inside, because the liquids are incompressible), a small decrease in volume will cause an infinite increase in the pressure.
This is how a REAL gas behaves. (In particular the curves presented in the figure are called the Andrews Isotherms and they were first studied around the year 1860).
You now have the answer to the initial question. The behavior of real gases deviates from ideal gases under high pressures P and small temperatures T (when the gas liquefies).
Correct answer is B)
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