# Under what conditions does the behavior of real gases deviate most from that predicted by the ideal gas law?  (A)  low P, low T  (B)  high P, low T (C)  low P, high T  (D)high P, high T 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...

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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).

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