[plasticfood]

...in a equilibrium constant expression? i know that their concentrations remain constant, but why is it that it's equal to 1?

[juanrga]

...in a equilibrium constant expression? i know that their concentrations remain constant, but why is it that it's equal to 1?

I think what you mean equilibrium expressions like (1)

$$ K_S = [X(aq)]  /$$

for

$$ X(s) \rightleftharpoons X(aq) /$$

But $$ K_S /$$ is not generally a true constant. Multiply both sides of (1) by the activity factor

$$ K_S \gamma = a_{X(aq)} /$$

and you obtain the true solubility constant

$$ K = a_{X(aq)} = \frac{a_{X(aq)}}{a_{X(s)}} /$$

the activity of a pure solid is 1 by definition.