So, one of my books said that partial pressure plays more significance in determining the gradient than concentration. So, it said it was possible for a gas to go from high partial pressure at point A to low partial pressure at point B, even though there is low concentration at point A and high concentration at point B.

How is this even possibly? I thought partial pressure and concentration were always related.

  • 1
    $\begingroup$ because of temperature. Temperature at A must be more than temperature at B $\endgroup$
    – manshu
    Commented Dec 1, 2015 at 14:33
  • $\begingroup$ This does not really answer my question. :( $\endgroup$ Commented Dec 19, 2015 at 15:45

1 Answer 1



At point A

The pressure according to ideal gas equation is $P_A = C_ART_A$. Here $C_A (= n/V)$ is concentration at A.

At point B

Similarly, the pressure according to ideal gas equation is $P_B = C_BRT_B$. Here again $C_B$ is concentration at B.

Now, $P_A$ > $P_B$

So, $C_ART_A$ > $C_BRT_B$

i.e. $C_A/C_B$ > $T_B/T_A$ .....equation 1

Now as the condition is that concentration at A is lower than the the concentration at B. Therefore we can say that

$C_A/C_B$ < $1$ .....equation 2

Now from equation 1 and 2 we have

$T_B/T_A$ < $1$

which gives us $T_A$ > $T_B$

So in the end, yes, your book was right. It is possible if the temperature at A is more than that at B.


Think of two containers A and B which are connected by a pipe. Both are connected with the help of a pipe which is closed (i.e. it doesn't let the gases exchange). Now we know that concentration of gas in A is lower than the the concentration of gas in B. If we want more pressure of gas in A then the velocity of its particles should be more. Now it is possible if it has more thermal energy. So the temperature should be more in A.


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