Chlorine gas is pale yellow-green, and both hydrogen and hydrogen chloride are colorless gases. According to the Le Chatelier's principle, the change in pressure will not affect the following equilibrium:

$$\ce{H2(g) + \color{green}{Cl2(g)} <=> 2HCl(g)}$$

Reduced volume will increase the partial pressures of gases. Will the color of the medium remain the same, or acquire a more saturated green tint due to increased concentration of chlorine gas?

  • 2
    $\begingroup$ If you evaporated a half volume of orange juice and it's color become more intense, it would not mean additional color substance were created. It would just be more concentrated. $\endgroup$
    – Poutnik
    Aug 20, 2022 at 8:58
  • $\begingroup$ Doesn’t equilibrium happens in closed system? $\endgroup$
    – nitsir
    Aug 20, 2022 at 9:39
  • $\begingroup$ Sure, it does, but it is not the point. It was analogy. Consider pure chlorine in a closed system, which is compressed to 1/2 volume and it's colour intensifies. Was there created extra chlorine? // You have to distinguish component mass and component concentration. $\endgroup$
    – Poutnik
    Aug 20, 2022 at 9:57
  • $\begingroup$ Isn't some high temperature (in some sense of high) presumed? What is the starting condition? - e.g. has a small spark been applied first...? Or what is the (potential) chemical energy? - Is some minimum energy assumed (say, after the said small spark). $\endgroup$ Aug 20, 2022 at 22:35

1 Answer 1


By (isothermal) compression of such a system, the equilibrium is not shifted, so the chlorine mass and molar amount stays constant (with possible tiny changes due non-ideal behaviour of gases).

But as the system volume decreased, concentration (or equivalently partial pressure) of components increased and the perceived chlorine color intensified.

If compression of the system were done by a linear way, like by transparent piston in a cylindrical vessel, the light absorption along the direction of this shortened path would be the same. As the shorter path is balanced by more intense absorption.

It means, the gas would be perceived in that direction as if colour did not change, but with more intense color in perpendicular directions.


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