I have to do a lab investigating factors that affect the rate of reaction between Mg(s) and HCl(aq). I decided to change the concentration of HCl. I timed how long it took for the Mg to completely disappear when the HCl was in excess and the amount of Mg used is constant. Obviously the higher the concentration, the lower the time, the faster the rate of reaction. My question is that since I'm using solid Mg, I can't really talk about the concentration of that... so is it simply enough to say that the lower times means that the rate is faster? I can't say rate=Delta[Mg]/Delta(t) if there's no concentration of Mg. I'm just confused on how to correctly explain this without a concentration, what do I do?