<p>You said “I don’t think both the products and the reactants can increase in concentration, the substance that is less in moles relative to the other side will increase in concentration.”</p>
<p>If it was something like xA(g) + yB(g) ->/<- zC(g)
And the volume was reduced by x percent. This means that in order to have a decrease in concentration for let’s say, the product, then the shift in towards either side would need to cause the number of moles of the product would have to be small enough such that this new number will be 1/5 of the orignal number of moles or less in order to have a smaller concentration than before.</p>
<p>It is common that both the reactants and products will experience an increase or a decrease in concentration. Otherwise, how would the equilibrium constant stay constant?</p>