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The output of the following code is the same as the input.

waals = (P + a/V^2) (V - b) == R T;

ContourPlot3D[waals /. {a -> 1, b -> 1, R -> 1},
 {P, 0, 100}, {V, 0, 100}, {T, 0, 100}]

But

ContourPlot3D[(P + 1/V^2) (-1 + V) == T,
 {P, 0, 100}, {V, 0, 100}, {T, 0, 100}]

works fine.

What's wrong? Do I have to specify all the parameters in the equation?

share|improve this question
    
Related: mathematica.stackexchange.com/q/1731 – Michael E2 Aug 9 '13 at 4:02
up vote 4 down vote accepted

enter image description hereYou need to evaluate it before Plot to get placements done.

ContourPlot3D[
 Evaluate[waals /. {a -> 1, b -> 1, R -> 1}], {P, 0, 100}, {V, 0, 
  100}, {T, 0, 100}]

As suggested by @rcollyer, ContourPlot3d has HoldAll attribute, which can be seen as ,Attributes[ContourPlot3D] (*{HoldAll, Protected, ReadProtected}*)

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3  
The more complete answer is it has the HoldAll attribute. – rcollyer Aug 8 '13 at 19:22

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